<%BANNER%>

Atmosphere Modification to Control Quality Deterioration during Storage of Fresh Sweetcorn Cobs and Fresh-Cut Kernels


PAGE 1

ATMOSPHERE MODIFICATION TO CO NTROL QUALITY DETERIORATION DURING STORAGE OF FRESH SWEETCORN COBS AND FRESH-CUT KERNELS By GAMAL RIAD A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2004

PAGE 2

Copyright 2004 by Gamal Riad

PAGE 3

This dissertation is dedicated to those in the world who care: my Mother, Hanaa Attia; my father, Samir Riad; my brother, Ah mad Samir; and all of my friends.

PAGE 4

ACKNOWLEDGMENTS First and foremost, I would like to express my deepest grateful and thanks to Allah, the God Almighty, for his continuous blessings. Then my deepest gratitude is due to my mother, father, and brother for their continuous unconditional love, support, encouragement, and guidance throughout my life, without which I wouldnt have been able to finish this work. Endless thanks and gratitude are due to my supervisory committee chair Dr. Jeffrey K. Brecht for contributing his time and effort in providing constructive criticism, advice and moral support throughout each step in my studies at the University of Florida. I am personally indebted to all my supervisory committee members, Dr. Khe V. Chau, Dr. Donald J. Huber, Dr. Steven A. Sargent, Dr. Charles A. Sims, and Dr. Stephen T. Talcott for their continuous help, valuable discussions, and support; all of them showed me the true meaning of the word advisor. I would also like to thank Dr. Stephen Talcott for his assistance and help with the phenolic measurements, and helpful discussions. I am grateful to my former committee in Egypt, Dr. Taha el-Shourbagy and Dr. Mordy Atta-Aly for their support and help before my travel to the United States. Special appreciation is due to Kim Cordasco and Abbie Fox for their help throughout this work, Special thanks go to Gary Harvey, produce manager of Publix supermarket warehouse (Jacksonville, Fla.) for his help in providing a continuous supply iv

PAGE 5

of sweetcorn during these experiments. Also I would like to thank Thermo King Corporation (Minneapolis, Minn.) for financial support through my assistantship. I would like to thank all my friends and colleagues in the Horticultural Sciences Department at the University of Florida; the National Research Center (Giza, Egypt); and Ain Shams University (Cairo, Egypt) for their help, encouragement, and support. Finally, I would like to thank my country, Egypt, for the financial support in the first 2 years of my study. v

PAGE 6

TABLE OF CONTENTS page ACKNOWLEDGMENTS.................................................................................................iv LIST OF TABLES...............................................................................................................x LIST OF FIGURES...........................................................................................................xi ABSTRACT.....................................................................................................................xiv CHAPTER 1 INTRODUCTION........................................................................................................1 2 LITERATURE REVIEW.............................................................................................4 Sweetcorn.....................................................................................................................4 Factors Optimizing Postharvest Preservation...............................................................6 Factors Optimizing Postharvest Preservation in Sweetcorn.........................................8 Controlled Atmosphere (CA) and Modified Atmosphere (MA) Technology............10 Beneficial Effects of CA and MA.......................................................................10 Detrimental Effects of CA and MA.....................................................................11 Modified Atmosphere Packaging (MAP)............................................................11 Active and Passive MA.......................................................................................15 CA and MA and Volatile Production..................................................................15 CA/MA and Sweetcorn Storage.................................................................................18 Fresh-Cut Fruits and Vegetables................................................................................22 Limitations of Fresh-Cut Fruit and Vegetable Production..................................23 Water loss.....................................................................................................23 Texture change and loss of tissue firmness..................................................25 Color change................................................................................................27 Microbial growth..........................................................................................31 Nutrient loss.................................................................................................36 Loss of flavor and aroma..............................................................................41 Fresh-Cut Sweetcorn Kernels.....................................................................................43 vi

PAGE 7

3 SWEETCORN TOLERANCE TO REDUCED O2 WITH OR WITHOUT ELEVATED CO2 AND EFFECTS OF CONTROLLED ATMOSPHERE STORAGE ON QUALITY........................................................................................45 Introduction.................................................................................................................45 Materials and Methods...............................................................................................47 Plant Material......................................................................................................47 Controlled Atmosphere Storage..........................................................................48 Parameters...........................................................................................................48 Respiration rate............................................................................................48 Sugar content................................................................................................49 Dimethyl sulfide (DMS) content..................................................................50 Visual appearance evaluation.......................................................................51 Statistical Analysis..............................................................................................51 Results and Discussion...............................................................................................51 4 PERFORATION MEDIATED MODIFIED ATMOSPHERE PACKAGING (PM-MAP) OF SWEETCORN...........................................................................................60 Introduction.................................................................................................................60 Materials and Methods...............................................................................................65 Plant Material......................................................................................................65 Perforation Mediated Modified Atmosphere Packaging Treatment...................66 Sweetcorn Storage...............................................................................................66 Parameters...........................................................................................................67 Gas composition...........................................................................................67 Respiratory quotient.....................................................................................67 Weight loss...................................................................................................67 Sugar content................................................................................................68 Ethanol and acetaldehyde content................................................................68 Statistical Analysis..............................................................................................69 Results and Discussion...............................................................................................69 5 SELECTION OF MATURITY STAGE, STORAGE TEMPERATURE, AND ATMOSPHERE FOR OPTIMUM POSTHARVEST QUALITY OF FRESH-CUT SWEETCORN KERNELS.........................................................................................78 Introduction.................................................................................................................78 Materials and Methods...............................................................................................80 Plant Material : .....................................................................................................80 Controlled Atmosphere Treatment......................................................................81 Fresh-Cut Kernel Storage and Cooked Sample Preparation...............................81 Parameters...........................................................................................................82 Respiration rate............................................................................................82 Sugar content................................................................................................82 Free amino acids...........................................................................................83 Statistical Analysis..............................................................................................84 vii

PAGE 8

Results and Discussion...............................................................................................84 6 REDUCED OXYGEN AND ELEVATED CARBON DIOXIDE TO PREVENT BROWNING OF FRESH-CUT SWEETCORN KERNELS AFTER COOKING....95 Introduction.................................................................................................................95 Materials and Methods...............................................................................................97 Experiment I : CA Effect on Fresh-Cut Sweetcorn Kernels................................97 Plant material................................................................................................97 Preparation of the fresh-cut sweetcorn.........................................................98 Storage and treatments.................................................................................98 Sugar content................................................................................................99 Total Soluble phenolics................................................................................99 Phenolic acids and 5-hydroxymethylfurfural (HMF).................................100 Microbial analysis......................................................................................101 After cooking browning.............................................................................101 Statistical analysis......................................................................................101 Experiment II : Microbial Load Effect on the After Cooking Browning...........102 Plant material..............................................................................................102 Preparation of the fresh-cut sweetcorn.......................................................102 Isolation of the microbes............................................................................102 Preparation of the inoculum.......................................................................102 Inoculation process.....................................................................................103 Controlled atmosphere treatment, storage, and sampling..........................103 After cooking browning measurement.......................................................103 Experiment III : Attempts to Identify the Browning Reaction Source...............104 Results and Discussion.............................................................................................105 Experiment I......................................................................................................105 Experiment II.....................................................................................................108 Experiment III...................................................................................................108 7 PHENOLICS PROFILE AND ANTIOXIDANT CAPACITY IN FRESH-CUT SWEETCORN DURING STORAGE......................................................................118 Introduction...............................................................................................................118 Materials and Methods.............................................................................................120 Plant Material....................................................................................................120 Controlled Atmosphere Treatment : ...................................................................121 Fresh-Cut Kernel Storage and Sample Preparation : ..........................................121 Extraction of Phenolic Compounds...................................................................122 Extraction of free phenolic compounds.....................................................122 Extraction of bound phenolic compounds..................................................122 Parameters.........................................................................................................122 Measurement of polyphenolic compound concentrations..........................122 Total soluble phenolics...............................................................................123 Quantification of antioxidant capacity.......................................................123 Statistical Analysis............................................................................................124 viii

PAGE 9

Results and Discussion.............................................................................................124 8 SUMMARY AND CONCLUSION.........................................................................134 Sweetcorn Cob Storage.............................................................................................134 Fresh-Cut Sweetcorn Kernel Storage.......................................................................136 After Cooking Browning, What Is It and What Is It Not?........................................140 After cooking browning, what is it?..................................................................140 After cooking browning, what is it not?............................................................141 LIST OF REFERENCES.................................................................................................143 BIOGRAPHICAL SKETCH...........................................................................................163 ix

PAGE 10

LIST OF TABLES Table page 3-1. Description of the visual quality ratings used for visual quality evaluation of sweetcorn after storage.............................................................................................55 4-1. Diameters and lengths of brass tubes used for creating PM-MAP with desired CO2 levels at different temperatures................................................................................73 6-1. Effect of atmosphere modification and storage period on the color parameters of cooked fresh-cut sweetcorn kernels stored in air or controlled atmosphere..........110 x

PAGE 11

LIST OF FIGURES Figure page 3-1. Effect of controlled atmosphere storage on respiration rate of sweetcorn stored in air or 2% O2 plus 0, 15, or 25% CO2 at 5 C.................................................................56 3-2. Effect of controlled atmosphere storage on the sugar content in sweetcorn kernels from cobs stored in different gas compositions.......................................................57 3-3. Effect of controlled atmosphere storage on the different attributes of sweetcorn visual quality after storage in different gas compositions at 5 C............................58 3-4. Effect of controlled atmosphere storage on dimethyl sulfide content in sweetcorn kernels from cobs stored at 5 C in different gas compositions at 5 C...................59 4-1. Gas composition in perforation-mediated MAP in sweetcorn packages stored for 10 d at 1 C....................................................................................................................74 4-2. Gas composition in perforation-mediated MAP in sweetcorn packages stored for 10 d at 10 C..................................................................................................................75 4-3. Effect of different levels of CO2 generated through PM-MAP on the respiratory quotient of sweetcorn stored for 10 d at 1 C or 10 C ...........................................76 4-4. Effect of temperature and different CO2 levels built up through PM-MAP on total sugars in sweetcorn stored for 10 d at 1 or 10 C....................................................77 5-1. The differences between the freshly harvested more mature and less mature sweetcorn cobs.........................................................................................................80 5-2. Effect of maturity stage, storage temperature and controlled atmosphere on respiration rate of fresh-cut sweetcorn.....................................................................87 5-3. Effect of maturity stage, storage temperature and controlled atmosphere on on sugar content in fresh-cut sweetcorn..................................................................................88 5-4. Effect of maturity stage, storage temperature and controlled atmosphere on on free amino acids in fresh-cut sweetcorn..........................................................................89 5-5. Fresh-cut sweetcorn kernels at day 0 (just after preparation).....................................90 5-6. Cooked fresh-cut sweetcorn kernels at day 0 (just after preparation)........................90 xi

PAGE 12

5-7. Fresh-cut sweetcorn kernels after storage for 10 d at 1 C in air...............................91 5-8. Cooked fresh-cut sweetcorn kernels after storage for 10 d at 1 C in air...................91 5-9. Fresh-cut sweetcorn kernels after storage for 10 d at 5 C in air...............................92 5-10. Cooked fresh-cut sweetcorn kernels showing after cooking browning after storage for 10 d at 5 C in air................................................................................................92 5-11. Fresh-cut sweetcorn kernels after storage for 10 d at 1 C in 2% O2 + 10% CO2....93 5-12. Cooked fresh-cut sweetcorn kernels after storage for 10 d at 1 C in 2% O2 + 10% CO2...........................................................................................................................93 5-13. Fresh-cut sweetcorn kernels after storage for 10 d at 5 C in 2% O2+10% CO2.....94 6-1. Effect of controlled atmosphere storage on the sugar content of raw and cooked fresh-cut sweetcorn kernels stored in air or CA at 5 C.........................................111 6-2. Typical phenolic acids profiles of fresh-cut sweetcorn kernels stored at 5 C for 10 d in airor CA..............................................................................................................112 6-3. Total soluble phenolics in raw and cooked fresh-cut sweetcorn kernels stored in air or CA for 10 d at 5 C............................................................................................113 6-4. Total aerobic microbial count of fresh-cut sweetcorn kernels stored in air or CA for 10 d at 5 C.............................................................................................................114 6-5. Raw and cooked fresh-cut sweetcorn kernels at day 0 ............................................114 6-6. Raw and cooked fresh-cut sweetcorn kernels stored in air for 7 d at 5 C...............115 6-7. Raw and cooked fresh-cut sweetcorn kernels stored in CA for 7 d at 5 C.............115 6-8. Raw (A) and cooked (B) fresh-cut sweetcorn kernels stored in air for 10 d at 5 C.116 6-9. Raw (A) and cooked (B) fresh-cut sweetcorn kernels stored in CA (2% O2+10% CO2) for 10 d at 5 C..............................................................................................116 6-10. HPLC chromatogram of the acetone-insoluble fraction of fresh-cut sweetcorn kernel juice after storing the kernels for 10 d at 5 C in air...................................117 7-1. HPLC chromatograms of free and bound polyphenolic compounds in sweetcorn extract on day 0 (before the storage treatment)......................................................127 7-3. HPLC chromatograms of free polyphenolic compounds in the sweetcorn extract after 4 d of storage in air (control) or CA.......................................................................128 xii

PAGE 13

7-4. HPLC chromatograms of bound polyphenolic compounds in the sweetcorn extract after 7 d of storage in air (control) or CA..............................................................129 7-5. HPLC chromatograms of free polyphenolic compounds in the sweetcorn extract after 7 d of storage in air (control) or CA.......................................................................129 7-6. HPLC chromatograms of bound polyphenolic compounds in the sweetcorn extract after 10 d of storage in air (control) or CA............................................................130 7-7. HPLC chromatograms of free polyphenolic compounds in the sweetcorn extract after 10 d of storage in air (control) or CA.....................................................................130 7-8. HPLC chromatogram of galacturonic acid in the free phenolic sweetcorn extract during 10 d of storage in CA..................................................................................131 7-9. Antioxidant capacity of free and bound phenolic fractions obtained from fresh-cut sweetcorn kernels as affected by storage in air or CA...........................................132 7-10. Total soluble phenolics (mgkg-1 FW) of free and bound fractions obtained from fresh-cut sweetcorn kernels as affected by storage in air or CA............................133 xiii

PAGE 14

Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy ATMOSPHERE MODIFICATION TO CONTROL QUALITY DETERIORATION DURING STORAGE OF FRESH SWEETCORN COBS AND FRESH-CUT KERNELS By Gamal Riad August 2004 Chair: J. K. Brecht Department: Horticultural Sciences Controlled atmosphere (CA) storage and modified atmosphere packaging (MAP) are beneficial tools for extending the postharvest life of fresh fruits and vegetables, but specific tolerance levels to gas composition must be determined in order to apply these techniques. Perforation-mediated modified atmosphere packaging (PM-MAP) for sweetcorn utilizing impermeable containers with a diffusion window was designed to establish 15, 20 or 25% CO2 atmospheres at 1 and 10 C. The desired CO2 concentrations were obtained at 1 C, but were about 3-5% lower than expected at 10 C. It took about 5 d to reach the equilibrium atmospheres at 1 C, and about 2 d at 10 C. Sweetcorn cobs in CA tolerated 2% O2 and up to 25% CO2 alone for 2 weeks at 5 C, but elevated respiration suggested that they may be damaged by the two gas levels in combination, although no significant ethanol or acetaldehyde production was detected in any CA or PM-MAP treatment. xiv

PAGE 15

The best atmosphere composition tested for maintaining sweetcorn quality was 2% O2 plus 15% CO2. The CA reduced sweetcorn respiration, maintained higher sugar concentrations, reduced loss of husk greenness, and improved silk and kernel appearance. This CA also maintained the highest concentration of dimethyl sulfide (DMS), the main characteristic aroma component in sweetcorn. The potential for storing and handling fresh-cut sweetcorn kernels was also examined. Fresh-cut sweetcorn kernels are extremely perishable. Successful handling requires low storage temperature and optimum maturity stage. Quality was maintained for 10 d in air at 1 C or in CA (2% O2 plus 10% CO2) at 5 C, but brown kernel discoloration after cooking limited shelf life in air at 5 C especially in the more mature kernels. The CA reduced fresh-cut sweetcorn respiration, inhibited sugar loss, and, most importantly, prevented after cooking browning. After cooking browning was not due to typical Maillard reaction (5-hydroxymethylfurfural was not present) nor due to changes in soluble phenolics. Higher aerobic microbe counts were associated with increased after cooking browning but not with a specific species. A water soluble brown pigment precursor was isolated from kernel juice but was not identified. xv

PAGE 16

CHAPTER 1 INTRODUCTION Sweetcorn (Zea mays L. rugosa) is widely used as a fresh or processed vegetable. It is sweeter than wild type (field) corn because it has a recessive mutant gene sugary-1 (su1) that restricts the conversion of sugar into starch (Creech, 1968; Laughnan, 1953; Wann et al., 1971). However, this conversion still occurs, and continues after harvest and during storage, resulting in rapid quality loss (Doehlert et al., 1993). Since consumer surveys have shown that most consumers prefer sweetcorn with higher natural sweetness (Showalter and Miller, 1962), one of the areas of genetic improvement of sweetcorn has involved the selection of mutant strains that produce high sugar levels in the seed endosperm (Courter et al., 1988; Garwood et al., 1976; Laughnan, 1953; Showalter and Miller, 1962; Wann et al., 1971). The most successful of these mutants is shrunken-2 (sh2), which blocks sucrose conversion to starch, and is found, alone or in combination with su1, in almost all modern commercial sweetcorn cultivars. Controlled atmosphere (CA) and modified atmosphere (MA), i.e., elevating carbon dioxide (CO2) levels and reducing oxygen (O2) levels around stored vegetables after harvest, can be useful supplements to maintenance of optimum temperature and relative humidity in maintaining postharvest quality of fresh fruits and vegetables. The main effects of CA/MA are reduced respiration and ethylene production; and, consequently, delayed ripening or senescence, reduced weight loss, and prolonged shelf life (Kader et al., 1989; Weichmann, 1986). The elevated CO2 used in CA/MA also competitively 1

PAGE 17

2 inhibits ethylene action (Burg and Thimann, 1959) and inhibits postharvest diseases (El-Gorani and Sommer, 1981; Daniels et al., 1985). Fresh sweetcorn is a perishable f ood product prone to rapid postharvest deterioration caused by kernel desiccation, loss of sweetness, husk discoloration, and development of decay. Keeping cobs in CA/MA with high CO2 and/or low O2 levels inhibits respiration; and, consequently, it reduces sugar loss and other metabolic reactions, and slows the grow th of pathogens; CA/MA also decreases husk yellowing by inhibiting chlorophyll degradation. Nowadays there is increased acceptance and demand for fresh-cut fruits and vegetables (sometimes called minimally pr ocessed or ready-to-e at produce) for many reasons such as their convenience, perceive d high nutritional value, and freshness. The flourishing of the fresh-cut industry in the last decade encourages the development of new fresh-cut products and there is now great er feasibility of sweetcorn kernels being developed as a fresh-cut product, but work is needed to determine the limiting factors in storing and handling such a value-added product. Study Objectives. The objectives of this work were as follows: Determine the specific tolerance levels of sweetcorn to low O2 and/or to high CO2 as an essential requirement to successf ully apply controlled and/or modified atmosphere techniques in sweetcorn storage. Determine the interactive effects of O2/CO2 combinations and temperature on induction of anaerobic respir ation in sweetcorn (using pe rforation-mediated MAP). Determine the effects of low O2 and high CO2 on levels of aroma volatiles and other quality factors in sweetcorn during storage. Introduce fresh-cut sweetcorn kernels and de termine its feasibility and value as a fresh-cut product.

PAGE 18

3 The last objective required investigating the following: The effect of sweetcorn maturity, storage temperature, and O2/CO2 levels on fresh-cut sweetcorn quality in order to determine the best postharvest treatments to reduce the occurrence of after-cooking browning of fresh-cut kernels. The cause of the browning of cooked fresh-cut kernels after storage, and testing the possibility of the Maillard reaction as its cause.

PAGE 19

CHAPTER 2 LITERATURE REVIEW Sweetcorn The quality of fresh sweetcorn [Zea mays L. rugosa (the old name was Zea mays L. saccharata)] depends to a large extent on the kernel texture and flavor, which are directly related to the sugar and polysaccharide content of the endosperm (Culpepper and Magoon, 1927; Flora and Wiley, 19974a). Sweetness in sweetcorn is closely related to kernel sucrose content (Reyes et al., 1982), which is the primary sugar in developing kernels (Cobb and Hannah, 1981). Texture and eating quality of sweetcorn consists of several factors, including pericarp tenderness (Bailey and Bailey, 1938), levels of soluble sugars and water-soluble polysaccharides (WSP) in the endosperm (Culpepper and Magoon, 1927; Evensen and Boyer, 1986), and moisture content (Wann et al., 1971). On the other hand, flavor and aroma, which are not as easily defined as sweetness and texture, are most often associated with kernel content of many volatiles, but mainly dimethyl sulfide (DMS) (Flora and Wiley, 1974b; Wiley, 1985; Williams and Nelson, 1973; 1974). Therefore, a major goal has been to develop sweeter sweetcorn and to find postharvest treatments to delay the depletion of this high level of sugars. The basic genetic difference between standard sweetcorn cultivars and starchy field corn is the presence of a recessive allele at the sugary-1 (su-1) locus on chromosome 4 in sweetcorn. This recessive gene conditions an 8to 10-fold increase in WSP, extreme starch reduction (about 50% or less than normal corn) and 2-fold increase in sugar over the normal corn (Creech, 1968; Laughnan, 1953; Wann et al., 1971). Doehlert et al. 4

PAGE 20

5 (1993) stated that the differences between normal kernels and su-1 kernels are that the su-1 kernels accumulate less weight, retain kernel moisture longer, have a thinner pericarp, and contain altered storage protein. Many other mutants have been introduced that improve the sugar content in sweetcorn, with one of the most important being the shrunken-2 (sh2) gene on chromosome 3, which blocks the conversion of sucrose to WSP and starch, resulting in accumulation of sugar (Laughnan, 1953). Cultivars containing this sh2 gene alone or in combination with the su1 gene (supersweet sweetcorn) usually contain more than twice the sugar content of the standard su1 sweetcorn (Courter et al., 1988; Showalter and Miller, 1962; Wann et al., 1971) and can retain higher sugar and moisture content for a longer time (Garwood et al., 1976); and consequently are preferred by consumers in taste tests (Evensen and Boyer, 1986; Showalter and Miller, 1962). There are many other genetic mutations in sweetcorn that have been introduced with limited success commercially, such as the endosperm mutants brittle-1 (bt1) and brittle-2 (bt2), which produce a high sugar content with a relatively low starch content in kernels, and which are adapted mainly for tropical climates. Another mutation is the sugary enhancer (se or se/su1) gene, which acts as an independent genetic modifier of the su1 gene. The se gene produces nearly double the sugar compared with su1; and unlike the watery endosperm of sh2 kernels, se varieties have a creamy endosperm due to production of WSP. The main disadvantage of the se varieties is that the sugar is rapidly converted to starch after harvest, unlike the sh2 varieties (Wily et al., 1989). Sweetcorn has a high respiration rate, which results in a high rate of heat evolution, and it loses sugars very rapidly after harvest (Brecht and Sargent, 1988; Evensen and

PAGE 21

6 Boyer, 1986; Wann et al., 1971). That is why fast cooling after harvest and keeping the cob temperature as close as possible to 0 C is the most important step in maintaining sweetcorn quality (Brecht, 2002). The sugar content, which so largely determines quality in sweetcorn, declines rapidly at room temperature and decreases less rapidly if the sweetcorn is kept at about 0 C. Early work by Appleman and Arthur (1919) showed that sugar loss is about four times as rapid at 10 C as at 0 C. At 30 C, 60% of the sugars in su1 sweetcorn may be converted to starch in a single day as compared with only 6% at 0 C (Brecht, 2002). While in sh2 varieties the sugar loss is actually at the same rate, the higher initial sugar in these cultivars helps in keeping it sweet tasting for a longer period (Brecht at al., 1990). Similar results were obtained by Olsen et al. (1990) who found that the sugar depletion rate was higher in sh2 than su1, but the sh2 still contained significantly more sugar after storage. These two types of sweetcorn lose sweetness and aroma during storage, but the main difference between them during storage is that su1 (and se) kernels tend to become starchy while sh2 varieties tend to taste more watery and bland (Brecht, 2002). Factors Optimizing Postharvest Preservation Harvested fruits and vegetables are highly perishable products. Over-ripening and senescence, mechanical injuries, trimming, water loss, and biological factors such as diseases and pests are the principal causes of postharvest losses. Postharvest preservation of these commodities is thus comprised of efficient techniques to reduce the tremendous amount of fresh produce losses, maintain produce quality, and extend shelf life throughout the postharvest chain, consequently increasing their commercial value.

PAGE 22

7 Postharvest deterioration can be controlled by primary and secondary factors. The primary factors to optimize preservation of a horticultural commodity (Kader et al., 1989) are as follows: Selecting varieties of the crop that have desirable postharvest and storage characteristics, Application of the ideal preharvest treatments, Harvesting at the optimum maturity stage, Minimizing mechanical injuries during harvesting and postharvest handling, Using proper sanitation procedures to reduce microbial infection, Providing the optimum temperature and relative humidity throughout the postharvest chain Temperature control (precooling and cold storage) has been identified as the most crucial factor for extending the shelf life of produce since biological reactions generally increase 2to 3-fold for every 10 C rise in temperature. Secondary factors to optimize preservation of a horticultural commodity include modification of O2 and CO2 concentrations in the atmosphere surrounding the commodity to levels different than normal air (Kader et al., 1989; Saltveit, 1989). Atmosphere modification is achieved either by reducing the concentration of O2 (which is required for respiration and for ethylene synthesis); or by increasing CO2 concentration (which inhibits respiration, ethylene action, and microbial growth) (Chinnan, 1989; Daniels et al., 1985; El-Gorani and Sommer, 1981; Kader, 1987; Kader et al., 1989; Labuza and Breene, 1989; Shewfelt, 1986). This is referred to as controlled atmosphere (CA) or modified atmosphere (MA) storage. Although the secondary factors are not as significant as the primary factors, their additive effect is important to preserve the overall postharvest quality of many commodities. It was also demonstrated that using secondary factors could solve some

PAGE 23

8 important postharvest problems in specific commodities. For example, the use of CA/MA technology on chilling sensitive produce may overcome the impact of low temperature injury (Forney and Lipton, 1990; Pesis et al., 2000; Wang and Qi, 1997) Factors Optimizing Postharvest Preservation in Sweetcorn Sweetcorn has one of the highest metabolic and respiration rates among vegetable crops, which makes it a very perishable product that requires special attention to the postharvest practices used in order to prolong its shelf life. The main sources of postharvest loss in sweetcorn are sugar loss, husk yellowing and drying, and kernel denting. Denting is caused by water loss, primarily from the husks, which in turn draw moisture from the cob and kernels, causing the latter to collapse, causing the dented appearance. It was estimated that a loss of only 2% moisture may result in objectionable kernel denting (Hardenburg et al., 1986). Mechanical injuries, which occur especially when sweetcorn is harvested mechanically or during trimming, can be a serious cause of postharvest losses by promoting water loss and decay. Sweetcorn is also affected by biological factors such as diseases and pests that increase postharvest losses. As discussed above, postharvest deterioration can be controlled by primary and secondary factors. The primary factors to optimize preservation of sweetcorn are Selection of varieties of the crop that have desirable postharvest and storage characteristics. For example, cultivation of sh2 varieties ensures high sugar content; and hence increases the tolerance for diverse postharvest conditions and extends the postharvest life of sweetcorn (Spalding et al., 1978). Application of the ideal preharvest treatments ensures high postharvest quality and long shelf life. It has been proven that preharvest factors have a great impact on postharvest quality (Kays, 1999) For example, use of the optimum nitrogen and sulfur fertilization rates increased the flavor quality in harvested sweetcorn due to increased dimethyl sulfide in the kernels (Wong et al., 1995).

PAGE 24

9 Harvesting at the optimum maturity stage. The early work by Rumpf et al. (1972) demonstrated that the highest levels of sugars were found when the cobs were harvested at the optimum degree of maturity. Minimizing mechanical injuries during harvesting and postharvest handling. Using proper sanitation procedures throughout the postharvest chain, to ensure reduced microbial infection, and thus reduce postharvest losses. Providing the optimum temperature and relative humidity throughout the postharvest chain. Similar to all perishable horticultural crops, temperature control (precooling and cold storage) is the most crucial factor in extending the shelf life of sweetcorn. It is well documented that fast precooling and storing at a low temperature (0-1 C) and high relative humidity (>90%) is the key factor in ensuring the highest postharvest quality in sweetcorn (Brecht, 2002; Brecht and Sargent, 1988; Evensen and Boyer, 1986). This is due to the reduction in metabolic rates at lower temperatures, which reduces the respiration rate and consequently the sugar consumption (high sugar content being the main quality factor in sweetcorn). Moreover, the low metabolic rate reduces the conversion of sugars to starch, which helps in keeping the high sugar content, which is very helpful in the genotypes that covert sugar to starch. Also, low temperature reduces water loss and subsequently reduces denting and/or husk drying of the sweetcorn cobs. On the other hand, low temperature also helps in reducing microbial growth and hence reduces pathological postharvest losses. The secondary factors to optimize preservation of sweetcorn as mentioned above also include using MA or CA, by reducing O2 levels and/or increasing CO2 levels (see discussion below). The MA and CA help in reducing sweetcorn metabolic rates, including respiration (Riad et al., 2002; Riad and Brecht, 2003) and carbohydrate metabolism (Risse and McDonald, 1990). They also reduce water loss from sweetcorn

PAGE 25

10 husks (Deak et al., 1987) because of the restricted gas exchange that is integral to MA and CA technology. Reduced O2 and more importantly elevated CO2 levels in MA and CA also reduce microbial growth on sweetcorn husks and kernels (Aharoni et al., 1996; Schouten, 1993). All these benefits help in reducing sweetcorn postharvest losses. Controlled Atmosphere (CA) and Modified Atmosphere (MA) Technology The technique of modification of the atmosphere surrounding perishable products is referred to as CA or MA. In CA, the atmosphere is created artificially and the gas composition is continuously monitored and adjusted to maintain the optimum gas concentration. On the other hand, in MA, the gaseous environment is modified naturally by the interplay among the physiology of the commodities and the physical environment, thus the control of the atmosphere in MA is less precise than CA. In MA, the respiration rate (O2 consumption and CO2 evolution) of the commodity being stored is in equilibrium with the O2 and CO2 concentrations in the surrounding environment. Several articles have been published on the benefits of CA/MA technology on the extension of perishable product shelf life (Anzueto and Rizvi, 1985; Nakashi et al., 1991; Zagory and Kader, 1988). Beneficial Effects of CA and MA Using CA and MA have a wide range of benefits (Kader, 1980; Kader et al., 1989) such as the following: The CA and MA conditions reduce the respiration rate (as long as the levels of O2 and CO2 are within those levels the commodity can tolerate and dont induce anaerobic respiration), which results in delayed ripening and senescence and better maintenance of the quality of the commodity. The CA and MA conditions reduce ethylene production and reduce sensitivity to ethylene (action) and this has many beneficial effects such as delaying fruit ripening and tissue senescence, delay of chlorophyll degradation, and maintenance of textural quality (decrease in lignification, etc.).

PAGE 26

11 The CA and MA conditions allow handling of chilling sensitive fruits (such as tomato, banana, and mangoes) at temperatures lower than the chilling threshold temperatures in normal air storage. Using CA and MA reduces the incidence and severity of some physiological disorders such as the disorders induced by ethylene or by chilling injury. Since delaying senescence, including fruit ripening, reduces the susceptibility to pathogens, CA/MA has a beneficial effect in decreasing postharvest diseases (Daniels et al., 1985; El-Goorani and Sommer, 1981). Levels of O2 below 1% and levels of CO2 above 10% can also have a significant direct effect on fungal growth. Carbon dioxide levels above 10-15% (in commodities that tolerate such levels) can be used to provide a fungistatic effect. Detrimental Effects of CA and MA Most CA/MA disadvantages are related to severe reductions of O2 and/or increases in CO2 that force the product into anaerobic respiration (Kader et al., 1989; Brecht, 1980). Anaerobic respiration causes many disorders such as Increased susceptibility to decay and shortening of the storage life. Irregular ripening. Accumulation of ethanol, acetaldehyde, and other compounds that produce off-flavors and other metabolic dysfunctions. Physiological disorders (such as brown stain in lettuce, internal browning and surface pitting in pome fruits). Activation of the growth of some anaerobic pathogens that are considered to be major health hazards. Modified Atmosphere Packaging (MAP) Modified atmosphere packaging (MAP) is an atmosphere control technique that relies on the natural process of respiration of the product and the gas permeability of the package holding the product. Due to respiration, there is a buildup of CO2 and a depletion of O2, and the package material helps to maintain the modified gas levels until the package reaches steady state because of restricted gas permeability. In the steady state

PAGE 27

12 condition, the O2 flow entering the package equals the O2 consumed by respiration, and the CO2 flow leaving the package equals the CO2 produced by respiration. Because of the limitation of CA storage to relatively large-scale systems, the MAP technique was developed to provide the optimal atmosphere; not for the entire storage facility, but for just the product, thus maintaining the desired atmosphere during almost all of the postharvest chain even during the retail display. The MAP can vary from a whole shipping container to a small retail package. As well as the benefits of modifying the O2 and CO2 levels, MAP has the additional benefits of water loss prevention, product protection, and brand identification. To achieve the desired atmosphere more rapidly, modification of the package atmosphere can be accelerated by using absorbents, or by using active modification instead of passive modification (i.e., initially replacing the package atmosphere with the desired one by gas flushing) (Kader et al., 1989) The development of MAP has faced several problems (Kader, 1987; Kader et al., 1989) such as Lack of commodity respiration data under several temperatures and gas compositions. Lack of permeability data for packaging materials at different temperatures and relative humidities. Lack of consistency in respiration data gathered for the same commodity. In medium and high-respiring commodities (like sweetcorn), using commonly available films such as low-density polyethylene (LDPE), polyvinyl chloride (PVC), and polypropylene is not ideal due to their low gas transmission rates, which may lead to respiration switching toward anaerobic respiration (Morales-Castro et al., 1994). Fonseca at al. (2000) summarized some of the limitations of using the flexible polymeric films that result from their structure and permeation characteristics such as

PAGE 28

13 Films are not strong enough for large packages. Film permeability characteristics change unpredictably when films are stretched or punctured. Some films are relatively good barriers to water vapor, causing condensation inside packages when temperature fluctuations occur and consequently increasing susceptibility to microbial growth. Film permeability may be affected by water condensation. The uniformity of permeation characteristics of films is not yet satisfactory. Film permeability is too low for high-respiring products. Products that require high CO2/O2 concentrations may be exposed to anaerobiosis because of the high ratio of the CO2 versus O2 permeability coefficients. Perforation-mediated MAP is a potential technique for postharvest preservation of fresh horticultural commodities. In this technique, instead of using the common polymeric films, a package is used in which the regulation of gas exchange is achieved by single or multiple perforations or tubes that perforate an impermeable package (Emond and Chau, 1990a,b; Emond et al., 1992). Using the perforation-mediated package has many potential advantages (Fonseca et al., 1997) such as The high values of mass transfer coefficients, implying that a reduced size and number of perforations for gas exchange are required, thus high-respiring produce can be packed in this system. A MAP using perforations can be adapted easily to any impermeable container, including large bulk packages. Polymeric films are not strong enough for packs much larger than those used for retail, but perforations can be applied to retail packages as well as shipping containers, because rigid materials can be used. Rigid packages also can prevent mechanical damage of the product. A flexible system is obtained due to the ability to change the gas transfer coefficients by selecting the adequate size and shape of the perforation. Commodities requiring high CO2 concentrations and relatively high O2 concentrations can be packed with this system.

PAGE 29

14 But on the other hand there are some limitations to this technique (Fonseca et al., 1997) such as Although it may be applied to products that could not be packed in conventional MAP, the range of products is not very wide, because the CO2/O2 transfer coefficient ratio averages 0.8, the ratio of the diffusion values of CO2 and O2 in the air. This may eventually be overcome by the use of perforation packed with materials with different affinities for CO2 and O2. Water loss in the product may become a problem, but packed perforations may solve this problem. Non-uniformity of concentrations inside the package due to gas stratification may also be a problem in large containers. More fundamental research and experimental validation is needed before its eventual commercial use. One commercially available film (Intellipac from Landec Corporation, Menlo Park, Calif.) is claimed by the manufacturer to be able to automatically adjust its permeability in response to temperature changes by a phase change in the film polymer structure (Clarke and De Moor, 1997). Permeability characteristics of this package are provided by using a highly permeable membrane over an aperture in the wall of the package. The membrane is made by coating a microporous substrate with a side chain crystallizable (SCC) polymer. In cold temperature, the SCC polymer exists in a crystalline solid phase; but when the temperature increases above the pre-selected switch temperature, the polymer changes to a more permeable liquid phase. Because this transformation involves a physical effect and not a chemical change, the metamorphosis is reversible. By changing the properties of the polymer and coating thickness, it is possible to obtain the permeability selectivity and the temperature switch required. Lange (2000) found that Intellipac-stored strawberries had ethanol and ethyl acetate levels similar to those of fresh

PAGE 30

15 fruit, while samples stored in regular perforated film had levels of these fermentative products 7-fold higher than those of fresh samples. Active and Passive Modified Atmosphere A MAP system maintains an adequate atmosphere within the package under steady state conditions through interaction of the respiration rate of the commodity and the package size and gas permeation of the package material (Kader et al., 1989). When atmospheres are modified passively by commodity respiration, it may require hours to days until the gas concentrations reach the steady state in the package, the required time being mainly a function of the package void volume (Ballantyne et al., 1988; Geeson et al., 1985). Using active or semi-active atmosphere modification i.e., removal or addition of a determined gas volume from the package, allows the desired atmosphere to be more quickly achieved, and thus further prolongs the storage life of the produce (Yahia and Gonzalez-Aguilar, 1998). This may be very important in sweetcorn, since addition of even a single day to its short storage life would be a significant benefit. Controlled and Modified Atmosphere and Volatile Production High CO2 and/or low O2 can induce anaerobic metabolism, resulting in accumulation of ethanol and acetaldehyde (Kader, 1989; Ke et al., 1993). Methanethiol production is also induced under low O2 atmospheres in Brassica crops (Forney at al., 1991); and production of other volatiles has also been shown to be enhanced under low O2 and/or high CO2 atmospheres (Hansen et al., 1992; Larsen and Watkins, 1995; Mattheis et al., 1991) The production of some volatiles may be modified if both O2 and CO2 are modified simultaneously. Obenland et al. (1995) found that low O2 levels enhanced methanethiol production in broccoli. However, the application of high CO2 levels in parallel with low

PAGE 31

16 O2 inhibited methanethiol production. Hence, the levels of both gases may be important in determining the production of volatiles. The critical O2 concentration that results in induction of anaerobic respiration is dependent on the character of the product in question (Gran and Beaudry, 1993). Also, temperature can influence the threshold of anaerobic induction. The O2 threshold for anaerobic induction increases with increasing temperature (Beaudry et al., 1992; Gran and Beaudry, 1993; Joles et al., 1994). For example, Cameron et al. (1994) found that packages designed for blueberries at 0 C reached the threshold for anaerobic induction at 5 C. Also Beaudry et al. (1992) reported that highbush blueberry fruit can tolerate ~1.8% O2 when stored at 0 C whereas at 25 C they require ~4% O2 to avoid elevating the respiratory quotient (RQ). They concluded that the risk of anaerobiosis within LDPE packages was increased by high temperature and the critical O2 level that induced anaerobic respiration was increased with increasing temperature. There are also several reports indicating that varieties of the same commodity have different potentials for accumulation of acetaldehyde and ethanol (Blanpied and Jozwiak, 1993; Folchi et al., 1995; Gran and Beaudry, 1993) Sulfur-containing volatiles are produced via free, sulfur-containing amino acids, peptides, thioglucosides, and thiophenes in plant tissue (Buttery, 1981; Richmond, 1973). The accumulation of many of these compounds is associated with off-odors and off-flavors (Di Pentima et al., 1995; Forney et al., 1991; Hansen et al., 1992). There are some reports indicating that hydrogen sulfide and allyl sulfide cause respiration enhancement over short periods of time (Hosoki et al., 1985). This could have a significant effect on

PAGE 32

17 MAP, since films used in these packages are selected on the basis of respiratory activity and film permeability (Toivonen, 1997). In natural ecosystems, plant tissue evolution of volatiles is influenced to a large degree by evapotranspiration (Charron and Cantliffe, 1995). The evolution of volatiles under low transpiration is much lower, and therefore the volatiles can accumulate in the tissues. A potential problem in CA/MA and especially MAP is that restricted ventilation may lead to accumulation of volatile compounds within the plant tissues that may be damaging or at least objectionable, from a quality standpoint. Among the compounds that may accumulate in CA and MA and cause off-odors during storage are those usually associated with anaerobic respiration such as ethanol and acetaldehyde, as well as acetone, dimethyl sulfide, hydrogen sulfide, methanethiol, and ethanethiol. By enhancing evapotranspiration in CA/MA storage, the accumulation of volatiles in the tissue can be reduced (Beaudry et al., 1993; Blanpied and Jozwiak, 1993). Toivonen (1997) reported that use of desiccants or a combination of desiccants and a volatile adsorbent in MAP significantly improved raspberry shelf life and quality at 10 C since it lowered the concentrations of ethanol and acetaldehyde. This treatment lowered the concentrations of ethanol and acetaldehyde in the fruit without much effect on their concentration in the headspace of the package, and increased acceptability from 42% to 75% with a slight increase in weight loss. These results agree with the previous suggestion that water movement from the product is important in lowering volatile levels in the tissue. Most of the odors present in fresh sweetcorn may be considered to contribute to its characteristic flavor; especially DMS, which is responsible for the characteristic aroma of

PAGE 33

18 cooked sweetcorn (Wiley, 1985; Williams and Nelson, 1973; 1974; Flora and Wiley, 1974b). It was noticed that the levels of these compounds are increased by canning or freezing (Flora and Wiley, 1974a). There is very little information about the effect of CA/MA storage and MAP on the levels of these compounds, or about the possible changes in their levels, or the levels of precursor compounds, during storage. CA/MA and Sweetcorn Storage Early work by Spalding et al. (1978) found that sweetcorn appearance and flavor were not significantly improved when stored for 3 weeks at 1.7 C under CA or low pressure. In those experiments, sweetcorn cobs were stored either in air, low-pressure atmosphere with the equivalent of 2% O2, or in CA containing 2 or 21% O2 plus either 0, 15, or 25% CO2. The increase in ethanol level was much higher with the increase of CO2 level over 15% than with the decrease of O2 level from 21% to 2%. Kernels from sweetcorn cobs stored in 2% O2 plus 25% CO2 were not significantly higher in sugars than air-stored sweetcorn and contained the highest amount of ethanol; but were the best treatment in terms of flavor rating, better than the air storage treatment, while storing in 2% O2 plus 15% CO2 was similar to air storage in sugar content and flavor rating. Also, in this experiment, the highest off-flavor levels detected were obtained when storing sweetcorn in 21% O2 plus either 15 or 25% CO2; but there was no significant difference between air storage and storing in 2% O2 plus either 15, or 25% CO2. Despite these results, it was concluded that sweetcorn appearance and flavor maintenance were not significantly improved by CA storage conditions; and that breeding of cultivars that better retain quality in combination with prompt precooling offers more potential for success than CA storage.

PAGE 34

19 On the other hand, there are many reports stating the beneficial effects of using MAP for sweetcorn. Deak et al. (1987) demonstrated that using MAP (shrink wrap film with moisture permeability of 0.1 gcm-2 h-1 and O2 permeability that varied from 0.4 to 40 mL cm-2 h-1) eliminated water loss and maintained beneficial CO2 and O2 levels within the package. These effects, together with low storage temperature (5 C), markedly reduced postharvest deterioration and hence resulted in an at least three-fold extension of shelf life (29 versus 8 d). But MAP treatment increased microbial growth due to the water-saturated atmosphere. Similar results were obtained by Risse and McDonald (1990) when sweetcorn was stored at 1, 4, or 10 C for 26 d unwrapped or wrapped in stretch or shrink wrap. It was concluded that film wrapping maintained freshness and reduced moisture loss better than the lack of wrapping. Also these authors recommended stretch wrap over shrink wrap since stretch wrap generated higher CO2 levels (4-9% versus 1-3%) and lower O2 levels (14-19% versus 18-21%), which resulted in higher total soluble solids retention. In that experiment also, an increase in microbial growth was experienced in the MAP treatments, especially in the presence of damaged husks or kernels, presumably due to the higher relative humidity within the MAP. However, reduced O2 and elevated CO2 have also been reported to reduce decay and maintain sweetcorn husk chlorophyll levels (Aharoni et al., 1996; Schouten, 1993). Aharoni et al. (1996) demonstrated that sweetcorn wrapped with PVC film benefited from the reduction of water loss, but the limiting factor affecting the shelf life of fresh sweetcorn in this case was the increase in pathogens on the trimmed ends of the cobs. Polyolefin stretch film has lower permeability rates for O2 and CO2 than PVC film, and consequently generated higher CO2 levels (~10% versus ~3%) and lower O2 levels (~7%

PAGE 35

20 versus ~15%) in the packages. This resulted in a significant reduction in the decay incidence and water loss and significantly better maintenance of the general appearance quality (Aharoni et al., 1996). These levels of O2 and CO2 did not trigger anaerobic respiration, and ethanol levels in the packages were very low until the packages were transferred to 20 C to simulate retail conditions. Upon transfer to 20 C, there was a sudden increase in CO2 (20-25%) and decrease in O2 levels (2-4%), which increased the ethanol concentration significantly in the sweetcorn in those packages. Nevertheless, these high levels of ethanol had little effect on the general quality of the sweetcorn, since off-odor occurred only in two types of packages, out of the six different combinations used in the experiment. Aharoni et al. (1996) suggested that the increase in microbial growth in MAP reported by Deak et al. (1987) and Risse and McDonald (1990) was probably due to relatively low CO2 levels within their packages, a consequence of the high ratio of the CO2 versus O2 permeability coefficients of plastic films as mentioned previously. Schouten (1993) demonstrated that CA storage with higher CO2 (2% O2 plus 10% CO2) retained higher sugar content than air storage and CA was more beneficial when used at a higher temperature (5-6 C compared with 1-2 C). The CA storage significantly reduced respiration and pathological breakdown, and also resulted in an increase in ethanol content; but the higher content of sugars had a more positive influence on the taste than the negative influence of the ethanol. On the other hand, Rumpf et al. (1972) clearly proved that the loss in sucrose content, the decisive factor in determining the taste quality of sweetcorn, may be delayed by both low temperature (0 C versus 5 or 10 C) and low O2 content, since 1% O2 was the best treatment in retaining the sucrose levels after 11 d

PAGE 36

21 of storage at 5 C followed by 2% O2 then 4% O2 and finally air storage. All the CA treatments in this experiment had 0% CO2. The main difficulty in using MAP for sweetcorn is that the film permeability is usually designed to maintain a desirable atmosphere during storage, but a rapid depletion of O2 may occur in the retail display where the temperature is higher, resulting in a shift toward anaerobic metabolism and causing rapid deterioration and quality loss. Recently Silva et al. (1999b) introduced the idea of using MAP designed for the retail display temperature and selecting the surrounding atmosphere inside a CA container during transport to overcome the negative effect of changes in surrounding temperatures during the postharvest chain. A similar idea was used by Rodov et al. (2000) who used a nested sweetcorn package i.e., a film wrap for retail packages (PVC film-wrapped trays containing two cobs of sweetcorn) suitable for the display temperature (20 C) along with a master carton liner that provided the desirable atmosphere during shipping and storage temperature (2 C). The low O2 levels (10-15%) and the high CO2 levels (5-10%) obtained in both cases were beneficial in inhibiting mold growth but werent severe enough to increase the ethanol levels. Due to the lack of work on CA/MA effects on sweetcorn, there are no specific limits to cite for O2 and/or CO2 levels or the threshold of sweetcorn sensitivity to reduced O2 or elevated CO2 for more than a few specific temperatures and storage times, however some work has shown that sweetcorn can tolerate up to 25% CO2 and down to 0.5% O2 without damage for 2 weeks at 1 C, scoring the best in flavor ratings (Riad and Brecht, 2003; Spalding et al., 1978). The usual recommended CA combination for sweetcorn is a more conservative 2% O2 plus 15% CO2, which is assumed to show benefits and not

PAGE 37

22 cause damage over the likely commercial range of temperatures and storage times (Brecht, 2002; Saltveit, 1989). Fresh-Cut Fruits and Vegetables Fresh-cut fruits and vegetables (sometimes called minimally processed or lightly processed fruits and vegetables) represent a relatively new and rapidly developing segment of the fresh produce industry. Fresh-cut processing involves preparing fresh produce to be ready to eat or cook by the final consumer. According to the International Fresh-Cut Produce Association (IFPA), fresh-cut produce is defined as any fresh fruit or vegetable or any combination thereof that has been physically altered from its original form, but remains in a fresh state. Regardless of commodity, it has been trimmed, peeled, washed and cut into 100% usable product that is subsequently bagged or prepackaged to offer consumers high nutrition, convenience, and value while still maintaining freshness (IFPA, 2002). Sales of fresh-cuts have grown from about $5 billion in 1994 to $10-12 billion in 2002, which is about 10% of total produce sales (includes retail and foodservice sales). Packaged salads alone topped the $1.6 billion mark in retail sales in 1999, marking a 15.9% increase from the previous year (IFPA, 2002). Fresh-cut fruit and vegetable products are different from traditional, intact vegetables and fruits in terms of their physiology and their handling requirements. Fresh-cut produce is essentially purposely-wounded plant tissue that must subsequently be maintained in a viable, fresh state for extended periods of time. Fresh-cut vegetables deteriorate faster than intact produce as a direct result of the wounding associated with processing, which leads to a number of physical and physiological changes affecting the viability and quality of the produce (Brecht, 1995; Saltveit, 1997).

PAGE 38

23 Limitations of Fresh-Cut Fruit and Vegetable Production Water loss Plant tissues are mainly composed of water and any small changes in water content may have a large impact on produce quality and could cause a variety of negative characteristics such as limpness, flaccidity, shriveling, wrinkling, and/or tissue desiccation. Sams (1999) demonstrated that losing a small amount of water content like 3% or 5% of the water content in spinach or apple, respectively, render these commodities unmarketable. On the other hand, crispness, an important characteristic of fresh produce that is related to water turgor pressure in the tissue, could be easily lost due to water loss. The loss in crispiness results in softening and flaccidity. Leafy vegetables are particularly susceptible to desiccation because of their large surface-to-volume ratios; moreover loose leaves, such as spinach, are more prone to desiccation than a compact head, such as a whole lettuce head (Salunkhe and Desai, 1984). Therefore, as a consequence of water loss, appearance changes such as wilting and reduced crispness may occur. In case of fresh-cut products, the possibilities to undergo severe water loss are much higher than in the case of intact produce since during fresh-cut preparation the produce goes through peeling and cutting or shredding, slicing, etc. Losing the skin has a critical effect on many fresh-cut products because the skin is a protective waxy coating, highly resistant to water loss, and thus peeled, fresh-cut fruits and vegetables are more perishable and more susceptible to turgor loss and desiccation. On the other hand, the mechanical injury brought on by cutting, shredding and/or slicing, directly exposes the internal tissues to the atmosphere, promoting the evaporation of intercellular water and

PAGE 39

24 hence starting desiccation of the tissue. Furthermore, fresh-cut preparation increases the relative product surface area per unit mass or volume, which also increases the water loss. Another factor affecting the water loss affects products that require rinsing after cutting, since this is frequently followed by centrifugation. If accelerated centrifugation speed or long centrifugation times are applied, increased desiccation can result, as reported for cut lettuce (Bolin and Huxsoll, 1989). The control of water loss in fresh-cut fruits and vegetables is possible through the use of appropriate handling techniques, including temperature and relative humidity control, which can greatly help minimize the rate of water loss. Reduction of water loss can be achieved basically by decreasing the capacity of the surrounding air to hold water, which can be achieved by lowering the temperature and/or increasing the relative humidity. On the other hand, applying edible coatings to the fresh-cut commodity can greatly reduce the water loss since such coatings provide an additional barrier to moisture movement (Baldwin et al., 1995a). When a thin layer of protective material is applied to the surface of the fruit or vegetable as a replacement for the natural protective tissue (epidermis, peel), it can have several possible effects (Baldwin et al., 1996; Baldwin et al., 1995a,b; Li and Barth, 1998; Nisperos and Baldwin, 1996). The coating may act as a barrier to water loss or, alternatively, hygroscopic coating materials may serve to maintain a moist surface appearance of the fresh-cut product. The coating may also work as a semipermeable barrier that helps in reducing gas diffusion and hence increase internal CO2 levels and reduce O2 levels, which results in a MA-like effect. This can help reduce respiration, as well as associated senescence processes such as color and texture changes, and may help to retain aroma volatiles. Coatings containing antimicrobial

PAGE 40

25 compounds or with low pH may be used to reduce microbial growth. For example, the primary parameter affecting fresh-cut celery quality is water loss because small reductions in water content (2.5%) may lead to flaccidity, shriveling, wrinkling, and pithiness. A significant increase in moisture retention by celery sticks was obtained with the application of a caseinate acetylated monoglyceride coating (Avena-Bustillos et al., 1997). Texture change and loss of tissue firmness During fruit ripening, one of the most notable changes is softening, which is related to biochemical alterations at the cell wall, middle lamella, and membrane levels, although significant roles in the softening process have been attributed to the pectic enzymes, polygalacturonase, and pectin methylesterase (PME). Fresh-cut fruits and vegetables may be considered to undergo a ripening-like process due to wound-induced ethylene production and the subsequent increase in respiration, which results in loss of tissue firmness. Textural changes in fresh-cut vegetables and fruits are minimized at low temperatures. Thus, temperature management is the first step to maintain the initial, fresh textural quality of these products. Several treatments have been proven to be effective in reducing fresh-cut firmness loss. A common treatment used to improve tissue firmness is to spray or dip fruit or vegetable pieces in aqueous calcium salts, as described for shredded carrots, zucchini slices, and fresh-cut pears, strawberries, kiwifruit, nectarines, peaches, and melons (Agar et al., 1999; Gorny et al., 1999; 2002; Izumi and Watada, 1994; 1995; Luna-Guzman et al., 1999; Luna-Guzman and Barrett, 2000; Main et al., 1986; Morris et al., 1985; Rosen and Kader, 1989). This is a result of the pivotal role played by calcium in maintaining the textural quality of produce due to its effect of

PAGE 41

26 rigidifying cell wall structure by cross-linking ester groups and also preserving the structural and functional integrity of membrane systems (Poovaiah, 1986). The use of CA or MA and MAP also retards senescence, lowers respiration rates, and consequently slows the rate of tissue softening (Kader, 1992). Rosen and Kader (1989) demonstrated that texture loss in strawberry fruit slices was reduced significantly by using CA. Strawberry slices kept under CA for a week had comparable firmness to whole and to freshly sliced fruit. Also, the best treatment to retain firmness of peach halves was a combination of a dip in 2% calcium chloride and 1% zinc chloride, followed by packaging with an O2 scavenger and storage at 0 to 2 C (Bolin and Huxsoll, 1989). Heat treatment also appears to have the potential to beneficially affect product texture. Heat treatment of whole apple fruit resulted in firmer fresh-cut products when compared with non-heated fruit (Kim et al., 1993). However, while heat treatment of whole apples improved apple slice firmness, the storage temperature of the whole fruit after heating also had a significant effect on product firmness. Mild heat treatment (45 C for 1.75 h) of whole fruit prior to cutting retained greater fresh-cut apple firmness during storage for 21 d at 2 C (Kim et al., 1994). Heat treatment could be applied prior to fresh-cut preparation, for example, apples that were kept at 38 C for 6 d immediately after harvest, then cold stored for 6 months prior to slicing and dipping in calcium solution, produced slices that were firmer than slices from control fruit (Lidster et al., 1979). Also heat treatment could be used during the calcium dipping treatment, as Luna-Guzman et al. (1999) demonstrated for fresh-cut muskmelons cylinders dipped in 2.5% calcium chloride solution for 1 minute at different temperatures (60, 40, or 20 C). The texture was firmer in samples treated at higher dip

PAGE 42

27 temperatures, perhaps due to the activation of PME, which is known to be activated in the 55-70 C range, resulting in reduced pectin methyl esterification. The de-esterified pectin chains may crosslink with either endogenous calcium or added (exogenous) calcium to form a tighter, firmer structure. However, there is a problem noticed when calcium chloride is used as a firming agent, which is that it may result in undesirable bitter flavor of the product. Fresh-cut cantaloupe cylinders dipped in calcium lactate solutions resulted in a textural improvement similar to calcium chloride-treated fruit cylinders. Sensory evaluation indicated that results were better since less bitterness and a more detectable melon flavor were perceived (Gorny et al., 2002; Luna-Guzman and Barrett, 2000). Color change Most fresh-cut fruits and vegetables experience some kind of color change after preparation. There are many reasons and degrees of this color change. The most common color change is the browning discoloration found in many fresh-cut commodities due to oxidation of colorless phenolic compounds to produce brown phenolic pigments. Browning. Brown discoloration is one of the most limiting factors in the shelf-life of fresh-cut products. The main reason for brown discoloration is enzymatic browning. During the preparation stages, produce is subjected to operations where cells are broken, causing enzymes to be liberated from tissues and put in contact with their substrates. Enzymatic browning is the discoloration that results from the action of a group of enzymes called polyphenol oxidases (PPO), which have been reported to occur in all plants, and exist in particularly high amounts in mushroom, banana, apple, pear, potato, avocado, and peach. Enzymatic browning must be distinguished from non-enzymatic browning, which results upon heating or storage after processing of foods; types of non

PAGE 43

28 enzymatic browning include the Maillard reaction, caramelization, and ascorbic acid oxidation. Enzymatic browning is a complex process, which can be subdivided into two parts. The first reaction is mediated by PPO, resulting in the formation of o-quinones (slightly colored), which, through non-enzymatic reactions, lead to the formation of complex brown pigments. The o-quinones are highly reactive and can rapidly undergo oxidation and polymerization. The o-quinones react with other quinone molecules, other phenolic compounds, aromatic amines, thiol compounds, ascorbic acid, and the amino groups of proteins, peptides and amino acids (Nicolas et al., 1993; Whitaker and Lee, 1995). Usually, brown pigments are formed, but in addition, reddish-brown, blue-gray, and even black discolorations can be produced on some injured plant tissues. Color variation among products of enzymatic oxidation is related to the phenolic compounds involved in the reactions (Amiot et al., 1997), and both color intensity and hue of pigments formed vary widely (Nicolas et al., 1993). Consequences of enzymatic browning are not restricted to discoloration, since undesirable-tasting chemicals can also be produced and loss of nutrient quality may result (Vamos-Vigyazo, 1981). Heat treatments can be used to control fresh-cut produce browning. Brief exposures to temperatures in the range of 40 to 60 C can redirect plant tissue metabolism toward production of heat shock proteins, which can, in some cases, prevent undesirable metabolic processes from occurring (Saltveit, 2002). For example, the synthesis of wound-induced enzymes such as the phenolic biosynthesis enzyme polyphenol oxidase (PAL) can be prevented by giving lettuce tissue a brief heat shock (e.g., immersion in 45 C water for 90 s) after processing (Saltveit, 2000). While this technique is very

PAGE 44

29 effective at preventing browning in plant tissue with constitutively low levels of phenolic compounds (e.g., celery and lettuce), it is ineffective in tissue with constitutively high levels of phenolic compounds (e.g., artichokes and potatoes). Also there are different chemical treatments that help in controlling fresh-cut product browning such as using acidulants, reducing agents, or chelating agents. Polyphenol oxidase, which catalyzes the formation of o-quinones from o-diphenols, beginning the sequence of reactions leading to formation of brown phenolic pigments in vegetable and fruit tissue, has a pH optimum of 6.0 to 6.5 and shows little activity below pH 4.5. Therefore, using citric, ascorbic, or erythorbic acids as acidulants helps by reducing the pH and hence inhibiting PPO activity. The use of reducing agents such as ascorbic acid or its isomer erythorbic acid helps in inhibiting the active oxidation reactions and helps prevent brown pigment formation. Also, the use of chelating agents such as ethylenediamine tetraacetic acid (EDTA) can reduce browning by chelating the copper required for the PPO active site and thus inhibiting its activity (Brecht et al., 2004). Fresh-cut onions and sweetcorn kernels sometimes develop a brown or black discoloration of unknown cause after cooking (after-cooking browning). This discoloration, which limits the shelf life of these products, increases with longer storage time of the fresh tissue at higher temperature, and is reduced by lowered O2 and/or elevated CO2 (Blanchard et al., 1996; Langerak, 1975; Riad and Brecht, 2001; Riad et al., 2003). Wounding before storage is apparently a prerequisite for sweetcorn after-cooking browning because it does not occur if sweetcorn kernels cut from stored cobs are cooked or when intact kernels are separated from the cob before storing (Brecht, 1999).

PAGE 45

30 White blush. Another common color change is white blush or white bloom of carrots, in which the bright orange color of fresh carrots disappears in stored fresh-cut products, particularly when abrasion peeling is used. Fresh-cut carrots with white blush develop a white layer of material on the peeled surface, giving a poor appearance to the product. Upon peeling, the protective superficial layer (epidermis) of carrots is removed, generally by abrasion, leaving cell debris and an irregular surface, which while moist presents the natural orange color of carrots. The sequence of disruption of surface tissues followed by dehydration and white blush formation was confirmed by scanning electron microscopy, when comparing carrots peeled with a knife and a razor sharp blade. Knife-peeled carrot surfaces appeared severely damaged, compressed, and separated from underlying tissue, therefore prone to dehydration. Razor-peeled carrot surfaces were cleaner and apparently only a thin layer of cells had been removed, resulting in a product that upon drying did not acquire the whitish appearance (Tatsumi et al., 1991). It has been suggested that phenolic metabolism may be activated by peeling, inducing increases in lignin production, which result in the color change (Cisneros-Zevallos et al., 1995; Howard and Griffin, 1993). Consumers perceive white blush carrots as aged or not fresh, and using edible coatings was very successful in treating the white blush in carrots (Sargent et al., 1994). Sensory results showed preference for carrots coated with edible, cellulose-based coating, due to a fresh appearance (Howard and Dewi, 1995; 1996). Yellowing. Yellowing of plant tissue due to chlorophyll degradation and exposure of the preexisting yellow carotenoid pigments is a normal process in ripening or senescence of many fruits and vegetables and this change can be accelerated by ethylene. In fresh-cut products, the stress of the wounding during preparation results in increased

PAGE 46

31 ethylene production, and hence increases the yellowing discoloration in green tissues. Yellow discoloration was observed during storage of leafy and other green fresh-cut products. Shredded Iceberg lettuce darkens (develops brown discoloration) during storage, particularly at high temperatures. Simultaneously, loss of green pigmentation is observed (Bolin et al., 1977; Bolin and Huxsoll, 1991). In a study of cabbage processed into coleslaw, the color changed from green to a lighter white color during the cold storage period as a result of chlorophyll degradation and because cabbage lacks yellow pigments (Heaton et al., 1996). The use of MAP (10% O2 plus 8% CO2) was beneficial in maintaining green color in broccoli and reduced the yellowing of the florets at 10 C compared to an unpackaged control, which lost about 10% of its initial chlorophyll within 3 d of storage at 10 C (Barth et al., 1993). Microbial growth Prior to harvest, plants are covered with a protective layer (the skin or epidermis), which protects the plant cells from microbial attack. Due to tissue damage and the fact that these tissues lose their protective skin during fresh-cut preparation, fresh-cut products are more prone to increased microbial growth. This increase is also promoted as a result of the release of cells fluids that microorganisms can use as nutrients (King and Bolin, 1989). Cell sap released from cut cells floods adjacent intercellular spaces and, when it comes in contact with bacteria, a suspension is formed that allows bacterial cells to move into the intercellular space. Bacterial cells that contact the cell sap become suspended, can move in the flooded intercellular spaces, and become protected from surface treatments. Bartz et al. (2001) demonstrated that within 5 seconds of application to a cut surface, cells of Erwinia carotovora became located in sites within tomato fruit

PAGE 47

32 tissue that could not be successfully disinfected with 1.34 mM chlorine at pH 6.0. Furthermore, the passage of a knife through plant tissues during preparation can drag bacteria into contact with damaged cells (Lin and Wei, 1997). The growth of bacteria, fungi, and/or yeasts may directly limit the life of fresh-cut vegetables and fruits by causing changes in the appearance and texture of the products, or through production of off-flavors and slimes that make them inedible. Many bacterial, fungal, and yeast species have been found to limit shelf-life of fresh-cut fruits and vegetables (Bartz and Wei, 2002; Farr et al., 1989; Heard, 1999; King and Bolin, 1989; Korsten and Wehner, 2002; Robbs et al., 1996). In general, bacteria and certain fungi cause decays in vegetables, including fruit vegetables, whereas fungi cause most of the decays in fruits. This is because fruit tissues are low pH (3.5-4.5), which favors mold and yeast growth, while vegetable tissues are nearer neutral pH (6.0-6.5), which favors bacteria growth. The major reasons for the separation between classes of microbes and plant hosts are not only the low pH of true fruits, but also the quantity and nature of the acidulants responsible for the low pH (Brecht et al., 2004). Microbial contamination and growth on or in fresh-cut vegetables and fruits is a major concern for the industry (Beuchat, 1996; Fain, 1996; Francis et al., 1999; Hurst, 1995; Nguyen-the and Carlin, 1994; Zink, 1997) especially the possibility that certain human pathogens also can grow or survive on fresh-cut vegetables and fruits. These microbes ordinarily have no direct effect on the life of the product, but their presence turns the fresh-cut product into an unacceptable product regardless of other quality factors (Brecht et al., 2004). The major consideration in fresh-cut safety is inoculation of the nutrient-rich flesh of vegetables and fruits with human pathogens during preparation.

PAGE 48

33 The presence of human pathogens is of particular concern with fresh-cut products because they are almost always consumed raw (i.e., without a heat treatment). In addition, it has been suggested that the elimination of spoilage microbes without elimination of human pathogens may extend shelf-life of a fresh-cut product to the point that safety is compromised because the human pathogens may be more likely to proliferate (Brackett, 1994; Hintlian and Hotchkiss, 1986). Contamination of fruits and vegetables with human pathogens can occur during growth in the fields, during harvesting and postharvest handling, in the course of processing, and during transport (Beuchat, 1996). Human pathogens that have been found on produce include bacteria, viruses, and parasites. These pathogens make contact with the produce by cross contamination or by being naturally present in the environment. That environment can include fields, air, and dust within a processing or packing facility. In the field, produce is subjected to irrigation, fertilization, and animal contact. Irrigation water is often not potable water and may contain pathogens (Sadovski et al,. 1978). Beuchat and Ryu (1997) pointed out that soil contact can lead to accidental contamination by immature compost or environmentally present pathogens. They also stated that animal removal and control should be monitored frequently and, if possible, all animals should be eliminated from entering the premises of vegetable and fruit production and processing facilities. There are many methods to control microbial growth in fresh-cut fruits and vegetables such as sanitation, temperature control, modifying storage atmosphere (CA/MA), and irradiation. Irradiation has the potential to eliminate vegetative forms of bacterial pathogens as well as parasites and extend shelf life (Chervin and Boisseau,

PAGE 49

34 1994; Farkas et al., 1997; Foley et al., 2002; Gunes et al., 2000; Hagenmaier and Baker, 1997; 1998; Molins et al., 2001; Prakash et al., 2000). However, irradiation doses required to eliminate some microorganisms may cause vitamin C losses, negative textural changes (Gunes et al., 2001), and enzymatic browning (Hanotel et al., 1995) in some vegetable and fruit tissues. Irradiation levels of 1.5-20 kGy are necessary to destroy yeasts and molds, which may exist as spores, and these levels are damaging to plant tissues (Brackett, 1987; Kader, 1986a). Sanitation of the produce, the washing tanks, and the fresh-cut preparation facility is critical in controlling microbial growth in fresh-cut produce. Use of chemical sanitizers has been successful in preventing contamination of food products by maintaining low levels of microorganisms in the processing environment. Rigorous sanitation of preparation areas reduces the level of microbial contamination, while chemical treatments and low temperatures restrict microbial growth during storage and marketing. Sanitation is usually done using cold (0-1 C) chlorinated water (0.67-2.7 mM free chlorine at pH 7 or less). Application of chlorine is not very effective at reducing microbial levels on contaminated tissues, but rather primarily acts to reduce microbial loads in the water and prevent cross-contamination (Hurst, 1995). The chlorine rinse also removes cellular contents at cut surfaces that may support microbial growth as well as promote browning, and may also directly inhibit some browning reactions (Brecht et al., 2004). Heated water may also be useful alone or as a supplement to sanitizer treatment in reducing microbial populations on fresh-cut products. Delaquis et al. (1999) demonstrated a 3-log reduction in microbial (mainly pseudomonad) levels on fresh-cut lettuce washed in chlorinated

PAGE 50

35 (1.34 mM NaOCl) water at 47 C for 3 minutes compared with a 1-log reduction using 4 C chlorinated water. Nowadays, there are many other alternatives sanitizers to chlorine that have been used or proposed for use in fresh-cut plants include chlorine dioxide (ClO2), bromine and iodine compounds, hydrogen peroxide (H2O2), peroxyacetic acid, and ozone (Beuchat, 2000). On the other hand, several chemicals can be used by the fresh-cut produce industry to control microbial growth during storage including acidulants such as sorbic acid and benzoic acid, which lower pH to levels unfavorable to microbes and thus inhibit their growth during storage (Chipley, 1993; Sofas and Busta, 1993). Although the actual mechanism of action of sorbic acid against bacteria is not known, there are some theories as to the action which include the possibility that sorbate inhibits amino acid uptake resulting in either destruction or disruption of the membrane There is also the theory the sorbate effects enzyme activity by the accumulation of beta unsaturated fatty acids preventing the function of dehydrogenase inhibiting metabolism and growth and the last possibility states that sorbate potentially inhibits respiration by competitive action with acetate in acetyl coenzyme A formation. (Davidson 2001). Beside their other benefits, edible coatings help retain acidulants and antimicrobials on cut surfaces (Baldwin et al., 1995a,b). Incorporating the antimicrobials potassium sorbate and sodium benzoate into edible coatings on fresh-cut apple and potato improved their effectiveness compared with aqueous dips (Baldwin et al., 1996). Also, one of the important factors in controlling microbial growth on fresh-cut products is management of temperature and relative humidity (Babic and Watada, 1996;

PAGE 51

36 Omary et al., 1993; Riad and Brecht, 2003). Maintenance of low temperature throughout the postharvest chain plays a pivotal role in controlling microbial growth either by retarding the microbes activity or by enhancing the produce quality by delaying ripening and senescence (Heard, 1999) and delaying tissue senescence. On the other hand, CA/MA storage helps in controlling microbial growth directly and indirectly. The direct effect came from the effect of CA/MA on the microorganisms and the indirect effect is a result of the effect of CA/MA on the plant physiology. It is known that CA/MA reduces the respiration rate and delays ripening and senescence and, since delaying senescence (including fruit ripening), reduces susceptibility to pathogens, CA/MA has a beneficial effect in decreasing postharvest diseases (Daniels et al., 1985; El-Goorani and Sommer, 1981). Levels of O2 below 1% and levels of CO2 above 10% can have a significant inhibitory effect on fungal growth. Carbon dioxide levels of 10-15% (in commodities that tolerate such levels) can be used to provide a fungistatic effect (Kader, 1986b; Kader et al., 1989). It is well known that sweetcorn can tolerate very high CO2 levels, as high as 25%, without any symptoms of injury (Riad et al., 2003; Spalding et al., 1978) Nutrient loss Nowadays there is more awareness of the importance of fruits and vegetables as a great source of antioxidant compounds such as polyphenolics, vitamin C, vitamin E, -carotene, and other carotenoids. It has been suggested that these phytonutrients have long-term health benefits and may reduce the risk of diseases such as cancer and heart disease. Most of these antioxidant compounds are known to inhibit cellular and DNA damage caused by reactive oxygen species and free radicals that may lead to degenerative

PAGE 52

37 diseases (Hu et al., 2000; Lagiou et al., 2004; Marrow, 1998; Tapiero et al., 2004). Vegetables and fruits are the primary source of these antioxidant compounds in our daily diet and there is a lot of epidemiological work that shows strong correlations between the delay or suppression of certain diseases and consumption rates of fruits and vegetables (Block and Langseth, 1994; Gershoff, 1993; Steinmetz and Potter, 1996; Ziegler, 1991). Retaining maximum bioactivity of these phytonutrients in fresh-cut fruits and vegetables is a very important goal and this could be achieved through better understanding of the effect of fresh-cut processing, packaging, and storage on bioactivity of these compounds. Many investigations with fresh-cut fruits and vegetables have demonstrated that concentrations of vitamins and other phytochemical compounds are reduced following fresh-cut operations as a result of wounding and are affected by conditions of handling, packaging, and storage (Kader, 2002; Klein, 1987). But, on the other hand, stress associated with processing may also initiate biosynthesis of many compounds that affect antioxidant content and product quality. The synthesis of wound ethylene after fresh-cut operations can stimulate a variety of physiological responses including loss of vitamin C and chlorophyll and induction of polyphenolic metabolism (Kader, 1985; Saltveit, 1999; Tudela et al., 2002a,b). Carotenoids. Carotenoids are important compounds in vegetables and fruits for their excellent antioxidant properties and the diversity of color they provide. Human daily consumption of carotenoids is mainly from vegetables and fruits (Goddard and Matthews, 1979). Carotenoids have diverse roles in the biological functioning of both plants and humans. They possess provitamin A and antioxidant activity, modulate detoxifying enzymes, regulate gene expression, aid in cellular communication, and augment immune

PAGE 53

38 functions (Clevidence et al., 2000). Due to their role as antioxidant compounds, as well as the color characteristics they impart to vegetables and fruits, exploration of techniques to retain carotenoids is vital for nutritional and sensory quality characteristics. Disruption of plant tissues by mechanical means or during senescence can also lead to rapid destruction of carotenoids through the action of oxidase enzymes, and may be prevented by the use of reducing agents or MA (Biacs and Daood, 2000; Simpson et al., 1976). Vitamin C. Ascorbic acid (vitamin C) is a water-soluble antioxidant long associated with inhibition of oxidative reactions and is a key marker compound for determining the extent of oxidation in fresh-cut vegetables and fruits (Barth et al., 1993). Ascorbic acid is easily destroyed during fresh-cut operations and levels are affected by cutting technique (Barry-Ryan and OBeirne, 1999), gas composition (Gil et al., 1998a; 1999), package design (Barth and Zhuang, 1996), water loss and storage time/temperature (Nunes et al., 1998; Lee and Kader, 2000), light intensity, heat, oxidase enzymes, and pro-oxidant metals (Albrecht et al., 1991; Lee and Kader, 2000). Polyphenolics. Polyphenolics are a major category of antioxidant compounds present in vegetables and fruits that encompass thousands of individual compounds in various commodities and concentrations. Recent findings have increased the interest in polyphenolic compounds present in fresh and fresh-cut vegetables and fruits due to their elevated antioxidant capacity. Polyphenolics, along with carotenoids and ascorbic acid, constitute a significant portion of the overall antioxidant capacity of vegetables and fruits; therefore maintaining their level in fresh-cut produce is critical for optimal human health. In previous reviews, the nutritional content was believed to decrease in fresh-cut as compared with intact

PAGE 54

39 vegetables and fruits, especially levels of vitamin C (Klein, 1987; McCarthy and Matthews, 1994). Following tissue wounding and exposure to light and air, antioxidant phytochemicals may be lost to enzymatic and oxidative action at the site of cellular disruption, in secondary or coupled oxidation reactions with lipids, in reactions with wound ethylene, from exposure to chlorinated sanitizers, or from mild desiccation (Barth et al., 1990; Nunes et al,. 1998; Park and Lee, 1995; Wright and Kader, 1997a,b). Therefore, developing postharvest treatments to alleviate phytonutrient loss following fresh-cut operations is vital to insure that maximum levels of phytonutrients reaching the consumer. Several treatments are used in the fresh-cut industry to achieve the goal of maintaining maximum nutritional value in fresh-cut products. For example, proper temperature control is among the most critical factors influencing nutrient retention in fresh-cut vegetables and fruits as enzymatic and oxidative reactions occur more rapidly at elevated storage temperatures. Temperature control serves to reduce microbial populations and slow chemical reactions that affect sensory characteristics and phytochemical concentrations. Also, different physical and chemical treatments have been investigated in fresh-cut vegetables and fruits as means to maintain fresh-like characteristics and nutritional quality. Mild heat treatments or surface acidification have been used as means to inhibit oxidative enzymes and serve to protect some nutrient compounds, as long as a high water-activity is maintained (Dorantes-Alvarez and Chiralt, 2000). Maintaining a high RH during storage was shown to be effective in retaining antioxidant compounds (Jiang and Fu, 1999). Ascorbic acid is commonly applied to cut surfaces through edible coatings or dips to prevent browning on cut surfaces: acting both

PAGE 55

40 as an acidulant and a reducing agent, ascorbic acid can reduce quinones back to colorless phenolic compounds. Combinations of reducing agents and acids were effective in prevention of surface browning and retention of sugars and organic acids in fresh-cut apples and their effect may be enhanced when combined with additional preservation techniques such as MAP or proper temperature control (Buta et al., 1999). On the other hand, the use of MAP has proven to be an effective means to reduce enzymatic and autooxidative reactions affecting phytonutrients in fresh-cut vegetables and fruits by reducing concentrations of O2 and increasing CO2. Modified atmospheres were used to maintain higher levels of provitamin A and vitamin C in fresh-cut broccoli (Barth et al., 1993; Barth and Zhuang, 1996; Paradis et al., 1996) and jalapeno peppers (Howard and Hernandez-Brenes, 1998), but had little effect on provitamin A concentrations in peach and persimmon slices (Wright and Kader, 1997a), and was ineffective in ascorbic acid retention in sliced strawberry or persimmon (Wright and Kader, 1997b). Extreme CO2 concentrations (>20%) may actually cause greater degradation or suppressed synthesis of vitamin C (Agar et al., 1997; Tudela et al., 2002b; Wang, 1983) and anthocyanins (Gil et al., 1997; Holcroft et al., 1998; Holcroft and Kader, 1999; Tudela et al., 2002a), while certain CO2 levels may induce biosynthesis of provitamin A carotenoids (Weichmann, 1986). Modified atmospheres did not affect flavonoid content of Swiss chard, but significantly reduced levels of ascorbic acid (Gil et al., 1998a), while flushing packages of fresh-cut lettuce with 100% N2 retained higher ascorbic acid concentrations than passive MAP and air controls (Barry-Ryan and OBeirne, 1999). In fresh-cut spinach, flavonoid content remained constant during storage in air or MAP, but spinach in MAP contained higher dehydroascorbic acid

PAGE 56

41 concentrations that resulted in lower antioxidant activity compared with air-stored spinach (Gil et al., 1999). However decreases in flavonoids were observed in Lollo Rosso lettuce stored in MAP (Gil et al., 1998b), further indicating a commodity-specific role of gas composition on overall phytonutrient retention. Furthermore, the application of food-grade compounds into wash water or on the surface of fresh-cut vegetables and fruits as coatings has an advantage of immediate benefits at the active site of phytonutrient deterioration. The benefits of these edible coatings include decreased respiration rate, browning inhibition, and retention of various quality factors by creating a barrier to O2, which influences enzymatic and nonenzymatic oxidation rates (Li and Barth, 1998). Edible coatings are a common method to extend the fresh-like appearance and quality characteristics of many vegetables and fruits (Baldwin et al., 1995a,b; 1996). Fresh-cut carrots with cellulose-based edible coatings retained greater provitamin A levels during storage in one study (Li and Barth, 1998), but another coating had no effect (Howard and Dewi, 1996). Loss of flavor and aroma Flavor quality of fresh-cut vegetables and fruits is critical to their acceptance and appreciation by consumers. Sensory attributes such as sweetness and characteristic aroma may be the most important indicators of shelf life from the consumers point of view. The challenge in fresh-cut vegetable and fruit handling is to maintain the taste and aroma attributes of the original whole product. Due to the short shelf life of fresh-cut fruits and vegetables, starting with produce at its optimum maturity or ripeness stage and using the highest quality standards is extremely important in maintaining flavor shelf life (Beaulieu and Gorny, 2002).

PAGE 57

42 Taste and aroma together make up flavor, which contributes to the recognizable nature of a food. Taste refers to detection of nonvolatile compounds on the tongue while aroma is related to volatile compounds detected in the nose. These two aspects of flavor are inextricably linked as it has been shown that perception of aroma can be influenced by levels of taste components, and vice versa (Beaulieu and Baldwin, 2002). The taste component in fruits and vegetables mainly depends on the sugar and organic acid levels and the relation between them. As a result of increased respiration due to wounding during preparation of fresh-cut produce, there is a depletion of sugar levels in the commodity and therefore organoleptic quality is reduced, especially in products like melons whose quality depends on high levels of sugars. This problem tends to be worse in fruits such as tomato or melon that have very limited capacity to replenish soluble sugars lost to accelerated respiration during storage or ripening, in comparison with fruits such as banana and apple that have a reserve of starch and can convert it to sugars during ripening. Organic acids are one of the major respiratory substrate and the increase in tissue pH in fresh-cut apples has been attributed to utilization of organic acids in respiration (Kim et al., 1993). Depletion of acids also can have negative organoleptic effects in fruits like apple, peach, and mango for which the balance of sweetness (sugars) and tartness (acids) is an important flavor attribute. The second component of flavor, aroma, is related to synthesis of volatiles during the growth and development of fruits or vegetables, but the most dramatic production coincides with fruit ripening. These volatiles include alcohols, aldehydes, esters, ketones, lactones, and other compounds (Baldwin, 2002). Little is known about the effects of different storage temperatures and atmospheres on normal aroma volatile production in

PAGE 58

43 fresh-cut fruits and vegetables other than the inhibition that occurs as a consequence of chilling injury (Buttery et al., 1987; Maul et al., 2000). Brecht et al. (2004) stated in their review that fresh-cut fruit taste becomes bland during the course of extended storage due to the loss and/or the reduced production of aroma volatiles. Recently there is an interest in designing edible coatings to retain volatile flavor compounds within the tissue as a method to overcome this problem (Baldwin et al., 1998; Miller and Krochta, 1997). Fresh-Cut Sweetcorn Kernels There is not much information about sweetcorn as a fresh-cut commodity. A few European retailers prepare fresh-cut sweetcorn kernels onsite and dont store them due to the excessive perishability of the product. It is well documented that sweetcorn is a very perishable product as a result of its very high respiration rate, which results in a quick loss of the sweetness (the most important characteristic of sweetcorn) unless it was rapidly cooled and stored at a low temperature (as close to 0 C as possible). Supplementation of proper temperature management by gas modification (CA or MA) greatly helps in maintaining high quality through reducing the high respiration rate (Brecht, 2002; Riad and Brecht, 2001; Riad et al., 2003). As a result of fresh-cut processing (cob trimming, de-husking, and kernel separation), the respiration rate increases and also there may be increased microbial load due to the open wounds in the kernels. This makes fresh-cut sweetcorn kernels a very delicate product, yet the early work by Brecht (1999) showed that there is great potential for sweetcorn kernels as a fresh-cut product. The main problem in fresh-cut sweetcorn kernels storage is the formation of brown pigment in kernels when they are cooked. This after-cooking browning is more pronounced as kernel maturity advances, increases with longer storage period and higher storage temperature, and is also higher in cut kernels than whole

PAGE 59

44 kernels (Brecht, 1999). Riad et al. (2003) found that the browning starts to appear after 7 d of storage at 5 C and was prevented during 2 weeks storage at 0 C. Also, storing the cut kernels in CA (2% O2 plus 10% CO2) prevented this after cooking browning even when the kernels were stored at 5 C. It has been suggested (Brecht, 1999, Riad and Brecht, 2001) that sweetcorn after-cooking browning may be caused by the Maillard reaction, a non-enzymatic reaction usually associated with food processing due to a reaction between free sugars and basic amino acids at elevated temperatures, forming a brown pigment. Sweetcorn has a high background of soluble sugars (Courter et al., 1988) and amino acids (Grunau and Swiader, 1991). More work is needed to determine the exact cause for sweetcorn after-cooking browning.

PAGE 60

CHAPTER 3 SWEETCORN TOLERANCE TO REDUCED O2 WITH OR WITHOUT ELEVATED CO2 AND EFFECTS OF CONTROLLED ATMOSPHERE STORAGE ON QUALITY Introduction Storing fruits or vegetables in controlled (CA) or modified atmosphere (MA) enriched with high CO2 and/or utilizing low O2 levels could be a very beneficial tool in maintaining product quality and extending shelf life. Atmosphere modification is achieved either by reducing the concentration of O2, which is required for respiration, or by increasing the CO2 concentration, which inhibits respiration and ethylene action. Reducing respiration consequently retards sugar loss and the growth of pathogens; and it also decreases yellowing through decreasing chlorophyll degradation. Several articles have been published on the benefits of CA/MA technology on the extension of perishable product shelf life (Chinnan, 1989; El-Goorani and Sommer, 1981; Kader, 1987; Kader et al., 1989; Shewfelt, 1986; Zagory and Kader, 1988). On the other hand, increasing CO2 above, or decreasing O2 below the commodity tolerance level may result in product deterioration due to anaerobic respiration and off-flavor and odor development. These tolerance levels may be affected by the physiological condition of the commodity, the storage temperature, and the storage duration. One of the challenges in CA storage is to determine the tolerance levels that result in the maximum benefits with the minimum possibility of injury to the stored commodity. Fresh sweetcorn (Zea mays L. rugosa) is a very perishable food product and is prone to rapid postharvest deterioration caused by kernel desiccation, loss of sweetness, 45

PAGE 61

46 husk discoloration, and loss of taste and aroma. Most of these problems are mainly due to its high respiration rate, which suggested that using CA/MA could be beneficial in sweetcorn storage. The early work by Spalding et al. (1978) found that sweetcorn appearance and flavor were not significantly improved using CA storage and they concluded that breeding cultivars that better retain quality, in combination with prompt pre-cooling, offers more potential for success than MA or CA. On the other hand, there are many other reports stating the beneficial effects of using gas modification either by CA or MA for sweetcorn. Deak et al. (1987) demonstrated that using modified atmosphere packaging (MAP) eliminated water loss and maintained beneficial CO2 and O2 levels surrounding the sweetcorn. These effects together with lower storage temperature markedly reduced postharvest deterioration and extended the shelf life. But MAP treatment increased microbial growth due to the water-saturated atmosphere. Similar results were obtained by Risse and McDonald (1990). However, reduced O2 and elevated CO2 have also been reported to reduce decay and maintain husk chlorophyll levels (Aharoni et al., 1996; Schouten, 1993). Aharoni et al. (1996) suggested that the increase in microbial growth reported by Deak et al. (1987) and Risse and McDonald (1990) was probably due to relatively low CO2 levels during storage. The main benefit of CA/MA is that the loss in sucrose content, the decisive factor in determining the taste quality of sweetcorn, was delayed by high CO2 and/or low O2 content (Riad et al., 2002, Risse and McDonald, 1990; Rumpf et al., 1972; Schouten, 1993). Generally, the quality of fresh sweetcorn depends to a large extent on the kernel sweetness, texture, and flavor. Sweetness in sweetcorn is closely related to kernel sucrose

PAGE 62

47 content (Reyes et al., 1982). Texture and eating quality of sweetcorn consists of several factors, including pericarp tenderness, levels of water-soluble polysaccharides (WSP) in the endosperm, and moisture content (Bailey and Bailey, 1938; Culpepper and Magoon, 1927; Wann et al., 1971). On the other hand, flavor and aroma, which are not as easily defined as sweetness and texture, are most often associated with kernel content of many volatiles that are present in fresh sweetcorn and considered to contribute to its characteristic flavor, especially dimethyl sulfide (DMS), which is responsible for the characteristic aroma of cooked sweetcorn (Flora and Wiley, 1974a,b; Wiley, 1985; Williams and Nelson, 1973; 1974). Despite its importance in the overall quality of sweetcorn, there is no or very little information on the effect of CA on the flavor and aroma and specifically on the levels of DMS during storage. The objective of this work was 1) to give a better idea about the tolerance level of sweetcorn to reduced levels of O2 with and without elevated levels of CO2; 2) to determine the best gas composition that results in the best quality after storage; and 3) to determine the effects of CA storage on the different quality attributes of sweetcorn. Materials and Methods Plant Material Fresh harvested sweetcorn (Zea mays L. rugosa var. Prime Time) cobs were harvested at a local farm in Florida (Hugh Branch Inc., Pahokee) on 26 November 1999 precooled, and delivered overnight by refrigerated truck to a local grocery chains distribution facility in Jacksonville (Publix Distribution Center, Jacksonville, Fla.) on 27 November, where it was picked up, transferred to the postharvest lab (University of Florida, Gainesville, Fla.), and the experiment started on the same day. As soon as the sweetcorn arrived at the lab, the cobs were sorted, trimmed, and the outermost leaves

PAGE 63

48 were removed. A window of about 3-4 cm width was opened in the husk to show 3-4 rows of kernels. The sweetcorn cobs were kept at 5 C for about 3 h until they were distributed among the treatments at the start of the CA application. Controlled Atmosphere Storage A flow-through CA system was used. Four sweetcorn cobs were placed in each of three, 3.8-L glass jars per treatment in a 5 C storage room, then the jars were closed and the CA gas mixture was introduced to the jars. Six gas compositions were used in this study, namely air, air plus 15 or 25% CO2 (which resulted in O2 concentrations of 17.7 and 15.6%, respectively), 2% O2, and 2% O2 plus 0, 15, or 25% CO2. The balance in all CA treatments was made up with N2. The CA gas mixtures were established using a system of pressure regulators, manifolds, and needle valve flowmeters to blend air, N2, and CO2 from pressurized cylinders. The CA was a flowing system with flow rates set to maintain respiratory CO2 accumulation under 0.5%. The gas flow was humidified by bubbling the gas mixture through water before introducing it to the containers. Gas composition in CA was monitored daily for 14 d using a Servomex O2 and CO2 analyzer (Servomex, Norwood, Mass.). The O2 analyzer measures the paramagnetic susceptibility of the sample gas by means of a magneto-dynamic type measuring cell, while CO2 is measured by infra-red absorption using a single beam. The analyzer was calibrated using N2 and certified O2 and CO2 standards. Parameters Respiration rate Respiration rate was measured daily using a static method in which the storage containers were closed and incubated for 2 h. Carbon dioxide concentration was measured before and after incubation, and respiration rate was calculated.

PAGE 64

49 Carbon dioxide levels were measured using a Gow Mac, series 580 gas chromatograph (Gow Mac Instruments Co., Bridgewater, N.J.) equipped with a thermal conductivity detector (TCD) and a Hewlett Packard Model 3390A integrator (Hewlett and Packard Co. Avondale, Pa.). Column, detector, and injector temperatures were 40, 28, and 28 C, respectively. Detector current was 90 mA during the analysis. Carrier gas was He with a 30 mLmin-1 flow rate at 276 KPa (40 psi). A 1-mL sample was withdrawn from the storage container headspace using a 1.0 mL plastic syringe and 0.5 mL was injected in the gas chromatograph. Calibration was done prior to sample analysis using a certified standard mixture. Sugar content Total soluble sugars were measured in raw and cooked kernels using the phenol-sulfuric method described by Dubois et al. (1956). In this experiment, 15 g of sweetcorn kernels were blended in a commercial blender with 85 mL of 95% ethanol for 2-3 min then were heated for 20 min in an 85 C water bath. Then the samples were stored overnight at -20 C to precipitate ethanol-insoluble materials. The samples were then filtered through Whatman #2 filter paper in a Buchner funnel attached to a side-arm Erlenmeyer flask. A vacuum pump was connected to the side arm to expedite the extraction. The collected filtrates were volumetrically adjusted to 200 mL using 95% ethanol and 20 mL portions of the extracts were stored in scintillation vials at -20 C until the measurements were performed.

PAGE 65

50 To prepare samples for measurement of total soluble sugars, 0.5 mL of 5% phenol (w/w) was added to 0.5 mL of diluted (1:250) ethanol extract in test tubes, the mixture was vortexed, then 2.5 mL concentrated sulfuric acid was added and the mixture vortexed again. The test tubes were let to stand for 10 min then were placed in a 25 C water bath for another 10 min to stop the reaction. The total soluble sugars were measured by reading the absorbance at 490 nm using a Shimadzu UV-1201 spectrophotometer (Shimadzu Scientific Instruments, Inc.; Tokyo, Japan). The absorbance values were compared to a standard curve of glucose. Dimethyl sulfide (DMS) content Dimethyl sulfide was measured chromatographically using a modified method from Ren et al. (2001) in which a water bath was used instead of a microwave oven. A 10-g sample of sweetcorn kernels was placed in a 40-mL glass vial fitted with a septum under the screw cap (Fisher Scientific, Pittsburgh, Pa., Cat. No. 05-727-6) then the vial was placed in a water bath (70 C) for 2 h. A 1-mL sample of the vial head space was withdrawn with a 1.0-mL plastic syringe and injected on a HP 5890 Series II gas chromatograph (Hewlett Packard Co. Avondale, Pa.) equipped with a flame ionization detector and a Hewlett Packard Model 3390A integrator. A 3 mm by 2 m stainless steel column with 20% OV-101 on 80/100 CWHP packing (Alltech, Deerfield, Ill.) was used and the carrier gas was N2 (30 mLmin-1). Oven, injector, and detector temperatures were 70, 150, and 200 C, respectively. Calibration was done prior to sample analysis using standards that were prepared from the authentic compound (Sigma-Aldrich Co. St. Louis, Mo.; Cat. No. 47,157-7)

PAGE 66

51 Visual appearance evaluation Husk, silk, and kernel appearance, and kernel denting were measured using a scale from 1 to 9, where a score of 9 represents excellent quality and a score of 1 represents the lowest quality level as described in Table 3-1. Statistical Analysis The experimental design was a completely randomized design with three replicates consisting of four sweetcorn cobs in each of three jars per treatment. The data were analyzed using the Statistical Analysis System (SAS). Analysis of variance (ANOVA) was used to determine the effect of treatments on the dependent variables. Means were separated by the least significant difference (LSD) test at P<0.05. Results and Discussion Controlled atmosphere storage is known for its beneficial effects on fresh fruits and vegetables. Sweetcorn was expected to benefit from this technique during storage since CA can reduce the high respiration rate of sweetcorn and consequently reduce sugar, water, and flavor losses. In this work, sweetcorn was stored in different gas compositions in a flow-through CA system in which fresh sweetcorn cobs were stored in air or 2% O2 plus 0, 15, or 25% CO2 at 5 C to determine the best atmosphere composition for maintaining quality and in order to determine the tolerance levels of sweetcorn to low O2 and/or high CO2. Respiration rate (Fig. 3-1) was significantly affected by CA treatment. Storing sweetcorn in 2% O2 plus either 15 or 25% CO2 significantly increased the respiration rate compared with air storage, which might be a result of the sweetcorn metabolism switching to anaerobic respiration. There was no significant effect of 2% O2 plus 0% CO2 on respiration, and storage in either 15 or 25% CO2 in air (17.7 or 15.6% O2,

PAGE 67

52 respectively) significantly reduced respiration compared with the air control. These results suggest that sweetcorn is more sensitive to reduced O2 plus elevated CO2 together than to either alone. Riad et al. (2002) reported that no ethanol or acetaldehyde was detected in the atmosphere surrounding sweetcorn stored in perforation-mediated modified atmosphere packaging (PM-MAP) even with steady state CO2 levels as high as 25% plus O2 levels as low as 1%, but de-husked sweetcorn was used in those experiments, which might have affected the tolerance levels to high CO2 and low O2. Also, these results are not with agreement with Morales-Castro et al. (1994), who reported that elevated CO2 had a negligible effect on respiration rate in the range from 0 to 15% while reduced O2 had a large influence on respiration rate in the range from 21 to 0%. While all CA treatments had significantly higher levels of sugars than the air control (Fig. 3-2), which is the main quality parameter for sweetcorn (Evensen and Boyer, 1986; Showalter and Miller, 1962), the highest sugar content was found in the two CA treatments that resulted in reduction of respiration rate (Fig. 3-1). These results are in agreement with the results obtained by Riad et al. (2002) who reported that higher sugar content was maintained in PM-MAP treatments with CO2 concentrations up to 25%. Also, in the early work by Rumpf et al. (1972), higher sugar levels were obtained with all CA treatments over air storage, even with O2 concentration as low as 1%. Schouten (1993) observed that even when CA treatment resulted in a significantly higher ethanol production, which might indicate fermentative metabolism, there was no effect on the sugar content. On the other hand, these results are not in agreement with the results

PAGE 68

53 obtained by Spalding et al (1978) who found that CA treatment resulted in no or negative impact on sugar content. Controlled atmosphere treatments had a positive impact on the visual quality of stored sweetcorn (Fig. 3-3). Elevated CO2 significantly reduced loss of greenness and maintained the fresh appearance of the husks. The CA also improved silk appearance (reduced discoloration) over the air control, but it had no effect on kernel denting, which might be due to the minimal water loss that occurred in all treatments since all the gas mixtures were humidified before being introduced to the storage containers. Kernel appearance was also improved in all CA treatments, especially air plus 15% CO2 and 2% O2 plus 15% CO2. Storage in 2% O2 plus 15% CO2 resulted in the best score in all visual quality parameters. These results are in agreement with Aharoni et al. (1996) who demonstrated that sweetcorn stored in MAP with high CO2 level had better general appearance quality due to better green color maintenance of the husk, less denting (due to the reduced denting as a result of the reduction in weight loss), and also reduced decay incidence. Dimethyl sulfide is the main characteristic volatile component in sweetcorn and is responsible for the characteristic aroma of cooked sweetcorn, providing the corny smell during cooking (Flora and Wiley, 1974b; Wiley, 1985; Williams and Nelson, 1973; 1974; Wong et al., 1994). It was noticed that the levels of this compound is increased by canning or freezing (Flora and Wiley, 1974a) but there is no information about the effect of storage in CA storage on the levels of DMS. There were significantly greater concentrations of DMS in CA-stored sweetcorn compared with the air control at the end of the 14-d storage period (Fig. 3-4). Storing sweetcorn in 2% O2 plus 15% CO2 resulted

PAGE 69

54 in the highest DMS content followed by air or 2% O2 plus 25% CO2. The results suggest that elevated CO2 was more efficient than reduced O2 in maintaining higher levels of DMS after 14 d of storage at 5 C. In conclusion, CA storage was beneficial in maintaining most sweetcorn quality parameters during 14 d of storage at 5 C. Storage in 2% O2 plus 15% CO2 gave the best result in terms of quality maintenance since it preserved the highest sugar level, reduced deterioration in sweetcorn visual quality to the greatest extent, and maintained the highest DMS content and thus presumably the highest flavor and aroma. Sweetcorn had a high tolerance level to either 2% O2 or 25% CO2 alone for 2 weeks, but the tolerance was apparently less when the two gases were combined since 2% O2 plus 25% CO2 provided less benefits compared with using 2% O2 plus 15% CO2.

PAGE 70

Table 3-1. Description of the visual quality ratings used for visual quality evaluation of sweetcorn after storage. Parameter Rating Description Husk appearance Silk appearance Kernel appearance Kernel denting 9 Field fresh Field fresh, turgid appearance Light color, fresh, and turgid Fresh with bright, shiny color and very turgid No denting with turgid kernels and bright appearance 7 Good Reasonably fresh,leaves still green and flexible Not that fresh but still had a light color and turgid texture Still noticeably bright color and turgid No denting with slightly dull kernel appearance 5 Fair Drying and yellowingapparent, some leaves brittle Starting to show some brown color and some drying Dull appearance with no denting A few kernels showing denting and more dull kernels 3 Unmarketable Portion of the leaves show dry appearance or water soaking Obvious discoloration and dryness with limp texture Severe dull appearance and some brown or white color 10-30% of the kernels showing denting and dull appearance 1 Unusable Completely dried out Limp texture or watery decay Kernels showing severe denting or decay More than 30% of the kernels dented 55

PAGE 71

56 204060801001201401234567891011121314Days after storageRespiration rate (ml CO2 /kg-h) air (Control) air + 15% CO2 air + 25% CO2 2% O2 + 0% CO2 2% O2 + 15% CO2 2% O2 + 25% CO2 Storage duration (d) Fig. 3-1. Effect of controlled atmosphere storage on respiration rate (mL CO2kg-h-1) of sweetcorn stored in air or 2% O2 plus 0, 15, or 25% CO2 at 5 C. Bars indicate 1 SD value (n=3).

PAGE 72

57 dccabb01234567891011air(Control)air + 15% CO2air + 25% CO22% O2 + 0% CO2 2% O2 + 15% CO22% O2 + 25% CO2 TreatmentSugar content (%) Fig. 3-2. Effect of controlled atmosphere storage on the sugar content (%) in sweetcorn kernels from cobs stored for 14 d in different gas compositions (air or 2% O2 plus 0, 15, or 25% CO2) at 5 C. Columns with the same letter are not statistically different at the 5% level by LSD.

PAGE 73

58 daacab012345678Husk appearance baaaaa012345678Silk appearance babbabc012345678air(Control)air + 15%CO2air + 25%CO22% O2 + 0% CO2 2% O2 +15% CO22% O2 +25% CO2 TreatmentKernels appearance aaaaaa012345678air(Control)air + 15%CO2air + 25%CO22% O2 + 0% CO2 2% O2 +15% CO22% O2 +25% CO2 TreatmentKernels denting Fig. 3-3. Effect of controlled atmosphere storage on the different attributes of sweetcorn visual quality after 14 d of storage at 5 C in different gas compositions (air or 2% O2 plus 0, 15, or 25% CO2). Husk, silk, and kernel appearance and kernel denting were evaluated using a scale from 1 to 9, where a score of 9 represents the best quality and a score of 1 represents the lowest quality. Columns with the same letter are not statistically different at the 5% level by LSD.

PAGE 74

59 abbcde01020304050607080air (Control)air + 15% CO2air + 25% CO22% O2 + 0% CO2 2% O2 + 15% CO22% O2 + 25% CO2 TreatmentDimethyl Sulfide content (mg/L) Fig. 3-4. Effect of controlled atmosphere storage on dimethyl sulfide content (mgL-1) in sweetcorn kernels from cobs stored for 14 d in different gas compositions (air or 2% O2 plus 0, 15, or 25% CO2) at 5 C. Columns with the same letter are not statistically different at the 5% level by LSD.

PAGE 75

CHAPTER 4 PERFORATION MEDIATED MODIFIED ATMOSPHERE PACKAGING (PM-MAP) OF SWEETCORN Introduction The beneficial effect of atmosphere modification during fresh fruit and vegetable storage is well documented (Kader et al., 1989; Kader, 1986b; Weichmann, 1986). Because of the limitation of CA storage to relatively large-scale systems, the modified atmosphere packaging (MAP) technique was developed to provide the optimal atmosphere, not for the entire storage facility, but for just the product, thus maintaining the desired atmosphere during almost all of the postharvest chain even during the retail display. Modified atmosphere packaging is an atmosphere control system that relies on the natural process of respiration of the product along with the gas permeability of the package holding the product. Due to respiration, there is a build up of CO2 and a depletion of O2 within the package and the package material helps to maintain these modified gas levels until the package reaches the steady state because of restricted gas permeability. The steady state levels of O2 and CO2 are a function of the mass of product, its respiration rate, the package permeability properties, the area of the package surface available for gas diffusion and temperature. In addition, the time to reach steady state conditions is largely a function of the void volume in the package. In the steady state, the O2 diffusing into the package equals the O2 being consumed by respiration, and the CO2 diffusing out of the package equals the CO2 being produced by respiration. Besides the benefits of modifying the O2 and CO2 levels in lowering respiration rate, MAP has the 60

PAGE 76

61 additional benefits of water loss prevention, produce protection, and brand identification. (Kader et. al., 1989) The early work by Spalding et al. (1978) found that sweetcorn appearance and flavor were not significantly improved under modified storage conditions (CA and low pressure) and concluded that breeding cultivars that retain quality, in combination with prompt pre-cooling, offers more potential for success than modified atmospheres. In that work, sweetcorn cobs were held in 2 or 21% O2 plus 0, 15, or 25% CO2 atmospheres for 3 weeks at 1.7 C. On the other hand, there are many reports stating the beneficial effects of using MAP for sweetcorn. Deak et al. (1987) demonstrate that using MAP (shrink wrap film with moisture permeability of 0.1 g cm-2 h-1 and O2 permeability varying from 0.4 to 40 mL cm-2 h-1) at 10 C eliminated water loss and maintained beneficial CO2 and O2 levels of ~14-18% O2 plus ~4-5% CO2 within the package. These effects together with lower storage temperature (10 C versus 20 C) markedly reduced postharvest deterioration and extended the shelf life. But MAP treatment increased microbial growth due to the water-saturated atmosphere. Similar results were obtained by Risse and McDonald (1990) who stored sweetcorn at 1, 4, or 10 C for 26 d. either unwrapped or wrapped in stretch or shrink wrap. It was concluded that film wrapping maintained freshness and reduced moisture loss better than the lack of wrapping. Also these authors recommended stretch wrap over shrink wrap since stretch wrap generated higher CO2 levels (4-9% versus 1-3%) and lower O2 levels (14-19% versus 18-21%), which resulted in maintenance of higher total soluble solids. Reduced O2 and elevated CO2 have been reported to reduce decay of sweetcorn and maintain husk chlorophyll levels (Aharoni et al., 1996; Schouten, 1993). Aharoni et al.

PAGE 77

62 (1996) demonstrated that sweetcorn wrapped in either polyvinyl chloride (PVC) or polyolefin stretch films benefited from the reduction of water loss, but there was more pathological breakdown on the trimmed ends of the cobs in the case of the PVC film. Polyolefin stretch film has lower permeability rates for O2 and CO2 than PVC film and consequently generated higher CO2 levels (~10% versus ~3%) and lower O2 levels (~7% versus ~15%) in the packages and this resulted in a significant reduction in the decay incidence and water loss and a significantly better maintenance of the sweetcorns general appearance (Aharoni et al., 1996). These levels of O2 and CO2 did not trigger anaerobic respiration, and ethanol levels in the packages were very low until the packages were transferred to 20 C to simulate retail conditions. Aharoni et al. (1996) suggested that the increase in microbial growth reported by Deak et al. (1987) and Risse and McDonald (1990) was probably due to relatively low CO2 levels within their packages, a consequence of the high ratio of the CO2 versus O2 permeability coefficients of plastic films as mentioned previously. Schouten (1993) demonstrated that using CA storage (2% O2 plus 10% CO2) retained higher sugar content than air storage and CA was more beneficial when used at a higher temperature of 5-6 C compared with 1-2 C. Similar results were obtained in the work of Rumpf et al. (1972), which clearly proved that the loss in sucrose content, the decisive factor in determining the taste quality of sweetcorn, may be delayed by both low temperature (0 C versus 5 or 10 C) and low O2 content since 1% O2 was the best treatment in retaining the sucrose levels after 11 d of storage at 5 C followed by 2% O2, then 4% O2, and finally air storage. All the CA treatments in this experiment had 0% CO2.

PAGE 78

63 The development of MAP has faced several problems such as the lack of commodity respiration data under several temperatures and gas compositions, lack of permeability data for packaging materials at different temperatures and relative humidities, and also the lack of consistency in respiration data gathered for the same commodity (Kader, 1987; Kader et al., 1989). In medium and high respiring commodities, like sweetcorn, using commonly available films such as low density polyethylene (LDPE), PVC, and polypropylene is not ideal due to their low gas transmission rates, which may lead to anaerobic respiration (Morales-Castro et al., 1994). Fonseca at al. (2000) summarized some of the limitations of using the flexible polymeric films that result from their structure and permeation characteristics such as 1) Films are not strong enough for large packages; 2) The film permeability characteristics change unpredictably when films are stretched or punctured; 3) Film permeability may be affected by water condensation; 4) Film permeability is too low for high respiring products: 5) Products that require high CO2/O2 concentrations may be exposed to anaerobiosis in film packages because of the high ratio of the film CO2 and O2 permeability coefficients. On the other hand, perforation-mediated modified atmosphere packaging (PM-MAP) is a potential technique for postharvest preservation of fresh horticultural commodities that overcomes several of the disadvantages associated with films. In this technique, instead of using the common polymeric films, a package is used in which the regulation of gas exchange is achieved by single or multiple perforations or tubes that perforate an impermeable package. (Emond and Chau, 1990a,b; Emond et al., 1992; Silva et al., 1999a).

PAGE 79

64 For sweetcorn, the recommended atmosphere is 2% O2 plus 10-20% CO2 (Kader at al., 1989; Riad and Brecht, 2003; Schouten, 1993; Spalding, 1978) and, to get such an atmosphere, a package with a value (the ratio between CO2 and O2 mass transfer coefficients) of around 1 is required. Using PM-MAP could be very beneficial in obtaining such a steady state atmosphere since the value of a perforation is around 1, which is better than the common polymers used for film formulation. Most of such films have values ranging from 2 up to 10 (Exama et al., 1993; Kader et al., 1989), which, for sweetcorn, would likely lower the package O2 level and increase the CO2 level to the critical levels that induce anaerobic respiration and accompanying increased ethanol, acetaldehyde, and off-flavor production. Using the PM-MAP technique has many potential advantages (Fonseca et al., 1997) such as 1)The low values of mass transfer coefficients implying that high-respiring produce can be packed in this system; 2) MAP using perforations can be adapted easily to any impermeable container, including large bulk packages; polymeric films are not strong enough for packs much larger than those used for retail, but perforations can be applied to retail packages as well as shipping boxes because rigid materials can be used; rigid packages also can prevent mechanical damage to the product; 3) It is a flexible system due to the ability to change the gas transfer coefficients by selecting the adequate size and shape of the perforation; 4) Commodities requiring high CO2 concentrations along with relatively high O2 concentrations can be packed with this system. The respiratory quotient (RQ) is the ratio between the respiration rate expressed in terms of CO2 production and respiration expressed as O2 consumption. The RQ has been observed to depend on both temperature and gas composition. It increases with a decrease

PAGE 80

65 in O2 level or an increase in CO2 level and also with an increase in the temperature (Beaudry et al., 1992; Joles et al., 1994). Joles et al., (1994) reported that in red raspberry RQ was around 1 at 0 C and increased to about 1.3 at 10 or 20 C. The RQ is assumed to be around 0.7 when the respiratory substrate is lipid, 1.0 when it is sugar, and up to 1.3 when it is acid. Under stress conditions, when the O2 level decreases or CO2 increases beyond the critical level and anaerobic respiration takes place, the RQ usually is much greater than one. In grated carrots and mushrooms, the RQ reached high levels of up to 5-6 when anaerobic respiration started (Beit-Halachmy and Mannheim, 1992; Carlin et al., 1990; Mannapperuma et al., 1989). The general objectives of this study were to examine the validity and the effectiveness of PM-MAP to create the previously predicted atmosphere and also to study the effect of this treatment on the visual and chemical quality of sweetcorn. Materials and Methods Plant Material Fresh harvested sweetcorn (Zea mays L. rugosa var. Prime Time) cobs were harvested from a local farm in Florida (Hugh Branch Inc., Pahokee, Fla.) on 24 January 2000 precooled, and delivered overnight by refrigerated truck to a local grocery chains distribution facility in Jacksonville (Publix Distribution Center, Jacksonville, Fla.) on 25 January. The sweetcorn was shipped to Gainesville, Fla. on 26 January where it was then picked up and transferred to the postharvest lab (University of Florida, Gainesville, Fla.) and the experiment started on the same day. As soon as the sweetcorn arrived at the lab, the cobs were de-husked and stored at 5 C until they were distributed among the PM-MAP treatments (within 6 h).

PAGE 81

66 Perforation Mediated Modified Atmosphere Packaging Treatment The PM-MAP was established using 3-L glass wire bail jars with glass lids (Village Kitchen, Redding, Calif.). Two holes were drilled in the glass lids; one of them was fitted with an appropriate size brass tube and the other fitted with a rubber serum stopper to facilitate gas sample withdrawal. The brass tube size was determined using a computer program (Chau, 2001) that depends on the equations of Fonseca et al. (2000). This program uses respiration rate, RQ, commodity weight, and tube diameter and length. In this program, tube design can be based on the desired O2 or CO2 concentrations. In this experiment, all the calculations were made based on the final desired CO2 level in the containers; also, RQ = 1 was used in all these calculations. Brass tubes with the diameters and lengths shown in Table 4-1 were cut from commercial brass pipes. From previous work by the author, three gas compositions were chosen (15, 20, and 25% CO2) and two storage temperatures (1 and 10 C). The average respiration rate in the previous experiments was used to calculate the appropriate dimensions of the brass tubes for the PM-MAP. Sweetcorn Storage Three cobs of sweetcorn (about 500 g) were placed in each of three replicate containers per treatment and stored at 1 or 10 C for 10 d. Kernel samples for sugar measurement were taken after storage by removing the kernels from the cob using a sharp stainless steal knife. Kernel samples (about 100 g) were placed in plastic freezer storage bags and stored at -20 C until analysis.

PAGE 82

67 Parameters Gas composition Oxygen and CO2 treatment levels were measured each 12 h for 10 d using a PBI Dansensor Checkmate O2 and CO2 analyzer (Topac, Hingham, Mass.) with zirconium and infrared detectors, respectively. The unit is designed to measure package atmospheres and automatically withdraw samples through a tube fitted with a needle that is inserted in the package or through a septum in a jar lid in this experiment and gives a reading every 5 s. Respiratory quotient Respiratory quotient was calculated for each gas composition measurement using the following formula, RQ= CO2 jar / (O2 air O2 jar) Where: RQ = Respiratory quotient CO2 jar = Carbon dioxide concentration in the jar O2 air = Oxygen concentration in the air outside the jar O2 jar = Oxygen concentration in the jar Weight loss Cob weight was recorded initially before starting the experiment and also after the storage period, and percentage of weight loss was calculated.

PAGE 83

68 Sugar content Total soluble sugars were measured in raw and cooked kernels using the phenol-sulfuric method described by Dubois et al. (1956). In this experiment, 15 g of sweetcorn kernels were blended in a commercial blender with 85 mL of 95% ethanol for 2-3 min then were heated for 20 min in an 85 C water bath. Then the samples were stored overnight at -20 C to precipitate ethanol-insoluble materials. The samples were then filtered through Whatman #2 filter paper in a Buchner funnel attached to a side-arm Erlenmeyer flask. A vacuum pump was connected to the side arm to expedite the extraction. The collected filtrates were volumetrically adjusted to 200 mL using 95% ethanol and 20 mL portions of the extracts were stored in scintillation vials at -20 C until the measurements were performed. To prepare samples for measurement of total soluble sugars, 0.5 mL of 5% phenol (w/w) was added to 0.5 mL of diluted (1:250) ethanol extract in test tubes, the mixture was vortexed, then 2.5 mL concentrated sulfuric acid was added and the mixture vortexed again. The test tubes were let to stand for 10 min then were placed in a 25 C water bath for another 10 min to stop the reaction. The total soluble sugars were measured by reading the absorbance at 490 nm using a Shimadzu UV-1201 spectrophotometer (Shimadzu Scientific Instruments, Inc.; Tokyo, Japan). The absorbance values were compared to a standard curve of glucose. Ethanol and acetaldehyde content Ethanol and acetaldehyde were measured in the storage container headspace. Measurements were done using an HP 5890 Series II gas chromatograph equipped with flame ionization detector and a Hewlett Packard Model 3390A integrator (Hewlett Packard, Avondale, Pa.). A 3 mm by 2 m column of 5% Carbowax 20M on

PAGE 84

69 80/120 Carbopack B (Supelco, Bellefonte, Pa.) was used and the carrier gas was N2 (30 mL.min-1). Oven, injector, and detector temperatures were 80, 110, and 300 C, respectively. A 1-mL sample of the container headspace gas was withdrawn using a 1.0 mL plastic syringe and injected on the gas chromatograph. Calibration was done before sample analysis using standards prepared from authentic compounds. Statistical Analysis The experimental design was a completely randomized design with three replicates consisting of three sweetcorn cobs in each of three jars per treatment. The data were analyzed using the Statistical Analysis System (SAS). Analysis of variance (ANOVA) was used to determine the effect of treatments on the dependent variables. Means were separated by the least significant difference (LSD) test at P<0.05. Results and Discussion Using PM-MAP could be a helpful tool in storing sweetcorn. The high respiration rate of sweetcorn makes the use of a package with a low value very beneficial in order to reduce the possibility of the sweetcorn switching to anaerobic respiration. In this work, the use of PM-MAP to store sweetcorn cobs for 10 d was very successful. After 10 d of storage, the visual quality was very high in all treatments with no sign of microbial growth or deterioration even at the higher temperature (10 C), which might be due to the fungistatic effect of high CO2 (El-Goorani and Sommer, 1981). Similar results were obtained in a study by Aharoni et al. (1996) with sweetcorn MAP using polyolefin stretch film in which the high CO2 and low O2 levels resulted in a significant reduction in both decay and water loss and hence maintained significantly better appearance quality than that of unpackaged controls.

PAGE 85

70 The weight loss was very slight in all treatments, which is a common observation for MAP treatment of sweetcorn. Many researchers have demonstrated that using film wrapping reduces water loss compared with unwrapped sweetcorn and furthermore, using films with lower water permeability further reduces water loss compared with films with high water permeability (Aharoni et al., 1996; Deak et al., 1987; Risse and McDonald 1990). In this work, weight loss ranged between 0.05 and 0.08% at 1 C with no significant differences among the MAP treatments, while at 10 C it ranged between 0.12 and 0.22% with significant differences among MAP treatments that corresponded to tube size (i.e., treatments utilizing smaller tubes lost less water). The higher weight loss at the higher temperature was also probably due to the larger brass tubes used for PM-MAP at 10 C than at 1 C. The low weight loss at both temperatures might be one of the factors that helped in maintaining the high quality of the sweetcorn with shiny kernels and no denting, both of which are affected by low humidity levels around the sweetcorn (Wiley et al., 1989). The expected gas composition was successfully obtained in all treatments at 1 C (Fig. 4-1) while at 10 C the levels obtained were less than expected by about 3-5% CO2. This might have been due to overestimation of the respiration rate while designing the tubes for PM-MAP, which led to steady state CO2 levels around 10, 17, and 22% CO2 instead of 15, 20, 25% CO2, respectively (Fig. 4-2). Sweetcorn stored at 1 C reached the expected CO2 level in about 5 d with slightly more time required for the higher concentrations (Fig. 4-1). The sweetcorn stored at 10 C was faster in reaching the steady state (about 2 d.) and this was expected due to higher CO2 production at the higher temperature (Fig. 4-2).

PAGE 86

71 There was a noticeable change in the RQ during storage that was most noticeable at 1 C. There was a gradual increase in the RQ during the storage period and the RQ was generally higher at 10 C (Fig. 4-3). Similar results were observed in blueberry and red raspberry, for which the RQ values varied between 1.0 and 1.3 and there was also an increase with increasing storage temperature. Also, when the blueberry and raspberry packages reached a very low steady state O2, the RQ increased to ~4-5 (Beaudry et al., 1992; Joles et al., 1994). In this experiment, RQ values never exhibited such a sudden increase, which usually indicates the switching to anaerobic respiration, but rather ranged between 0.9 and 1.4. Ethanol production was very low in all treatments, around 1-3 ppm, and there were no significant differences among the treatments. This is in agreement with Aharoni et al. (1986) and Rodov et al. (2000) who reported that no significant production of ethanol was detected when sweetcorn was stored in MAP at 1 or 2 C although higher ethanol production (200-500 ppm) was detected when the MAP packages were later transferred to 20 C. Sugar content was higher in the sweetcorn stored at the lower temperature, but there were no significant differences among the treatments at 1 C (Fig. 4-4). The only difference in sugar concentrations that was observed was in the sweetcorn stored at 10 C with 15% CO2, which had significantly lower sugar content than sweetcorn stored in 20 or 25% CO2. This finding is in agreement with previous work with CA or MAP storage of sweetcorn in which it was reported that there were higher sugar concentrations in kernels stored in high CO2 and/or low O2 (Risse and McDonald, 1990; Rumpf et al., 1972; Schouten, 1993)

PAGE 87

72 This work showed that using PM-MAP could be a very useful tool in sweetcorn storage at different storage temperatures. Perforation mediated modified atmosphere packaging was mostly successful in generating the desired and expected gas compositions and maintaining them during a 10-d. storage period. The PM-MAP system was also successful in maintaining sweetcorn quality for 10 d of storage. On the other hand, using active gas modification could help reduce the time required to reach equilibrium, especially at the lower temperature. More work needs to be done to collect improved respiration data for different temperatures and also to determine the switching point from aerobic to anaerobic respiration in sweetcorn, which will lead to a better understanding of possible O2 and CO2 levels that could be used for sweetcorn storage.

PAGE 88

73 Table 4-1. Diameters and lengths of brass tubes used for creating PM-MAP with desired CO2 levels at different temperatures. Temperature (C) Desired CO2 (%) Diameter (mm) Length (mm) 15 4 27.1 20 4 43.8 1 25 4 63.7 15 8 19.4 20 8 31.4 10 25 8 45.7

PAGE 89

74 0510152025 O2 CO2A 0510152025Concentration (%)B 05101520253000.511.522.5344.555.566.577.588.599.510Storage duration (d)C Fig. 4-1. Gas composition in perforation-mediated MAP originally designed to generate 15% CO2 (A), 20% CO2 (B) or 25% CO2 (C) in sweetcorn packages stored for 10 d at 1 C. Bars indicate 1 SD (n = 3).

PAGE 90

75 75 10 d at 10 C. Bars indicate 1 SD (n = 3). 0510152025 O2 CO2A 0510152025Concentration (%)B 05101500.511.522.5344.555.566.577.588.599.510Storage duration (d) 2025C Fig. 4-2. Gas composition in perforation-mediated MAP originally designed to generate 15% CO2 (A), 20% CO2 (B) or 25% CO2 (C) in sweetcorn packages stored for

PAGE 91

76 0.80.91.01.11.21.31.41.5 15% 20% 25%A 0.80.91.01.11.21.30.511.522.5344.555.566.577.588.599.510Storage duration (d)B 1.41.5 atory Quotient (RQ) Respir quotient (RQ) of sweetcorn stored for 10 d at 1 C (A) or 10 C (B). Fig. 4-3. Effect of different levels of CO2 generated through PM-MAP on the respiratory

PAGE 92

77 abaaaa4.05.06.07.08.09.010.011.01C10CStorage temperatureTotal Sugars (%) 15% CO2 20% CO2 25% CO2 -antly Fig. 4-4. Effect of temperature and different CO2 levels built up through perforationmediated MAP on total sugars (%) in sweetcorn stored for 10 d at 1 or 10 C. Means with the same letter in each storage temperature are not significdifferent at P <0.05 by LSD.

PAGE 93

CHAPTER 5 SELECTION OF MATURITY STAGE, STORAGE TEMPERATURE, AND ATMOSPHERE FOR OPTIMUM POSTHARVEST QUALITY OF FRESH-CUT SWEETCORN KERNELS Introduction Shelf life of perishable fruits and vegetables can be extended by a variety of techniques. The most widely used techniques are refrigeration and controlled atmosphere (CA) storage or modified atmosphere packaging (MAP; Chinnan, 1989; Kader et al., 1989). For sweetcorn, the consensus recommendation is to keep it as close to 0 C as possible during transportation and storage (Brecht, 2002) and this may be of even more importancewith regaretable shelf life are well documented (Anzueto and Rizvi, 1985; Brecht, 1980; Nakashi et al., 1991; Zagory and Kader, 1988), including sweetcorn (Aharoni et al., 1996; Deak et al., 1987; Risse and McDonald, 1990; Schouten, 1993) Sweetcorn shelf life is one of the shortest among fruits and vegetables. This is due to its high metabolic and respiration rates, which lead to severe sugar loss and rapid quality deterioration. Although sweetness is the most important quality characteristic in sweetcorn (Evensen and Boyer, 1986; Showalter and Miller, 1962), the importance of other flavor components in consumer acceptance is also well documented (Flora and Wiley, 1974; Wiley, 1985; Williams and Nelson, 1973). These other flavor components include texture and aroma. for fresh-cut sweetcorn. On the other hand, CA storage and MAP benefits d to extension of fruit and veg 78

PAGE 94

79 With the increasing acceptability and demand for fresh-cut or ready-to-eat products, fresh-cut sweetcorn kernels could be a valuable new product. Previous work on fresh-cut sweetcorn showed that intact kernels (prepared by splitting the ears lengthwise andere prepared similarly to the canntting the whole kernels from the cob ) in maintaining higher sugar content and better flavor. Intact kernels showed little tration of free amino acids, which could increase the probability of occurrence of Maill then pushing the kernels off the cob) were much better than cut kernels (which wed or frozen sweetcorn by cu or no after cooking brown discoloration but were very difficult to prepare (Brecht, 1999). Browning during cooking is a common occurrence during the preparation of many foods. One of its causes is the non-enzymatic browning reaction usually associated with thermal processing that is known as the Maillard reaction. In this reaction, brown pigments are generated from the reaction between free sugars and basic amino acids at elevated temperatures (Bayindirli et al., 1995; Gogus et al., 1998). Sweetcorn has a high background of sugars (Courter et al., 1988) and amino acids (Grunau and Swiader, 1991). Protein degradation in sweetcorn kernels may also occur during storage, increasing the concen ard reaction. Unlike many other foods in which browning during cooking is a desirable occurrence, browning of fresh-cut sweetcorn kernels is a negative quality factor that would make the product unsalable. The main objective of this work was to determine the effects of maturity stage, storage temperature, and atmosphere modification on the quality of fresh-cut sweetcorn in order to develop recommendations for commercial development of this as a fresh-cut product.

PAGE 95

80 Materials and Methods Plant Material Fresh harvested sweetcorn (Zea mays L. rugosa var. Prime Time) cobs were harvested from a local farm in Florida (Wilkinson-Cooper Produce Inc., Belle Glade, Fla.) on 11 January 2002 precooled, and delivered overnight by refrigerated truck to a local grocery chains distribution facility in Jacksonville (Publix Distribution Center, Jacksonville, Fla.) on 12 January where it was picked up and transferred to the postharv est lab (University of Florida, Gainesville, Fla.) and the experiment started on the sa me day. As soon as the sweetcorn arrived at the lab, the cobs were de-husked and divided into two groups according to the maturity stage. The more mature sweetcorn had larger kernels and the kernels were more compact on the cob, while the less mature kernels were smaller and there were still some spaces between the kernel rows (Fig. 5-1). A B Fig. 5-1. The differences between the freshly harvested more mature (A) and less mature (B) sweetcorn cobs.

PAGE 96

81 The cobs were held at 5 cut. The kernels were cut from harp stainless steel knife. The kernels in the first row were popped out first uand CO2 from pressurized cylinders. The CA was a flowing system with a flow rate set to maintain respiratory CO2 accumulation under 0.5%. The air-flow was humidified by bubbling through water before introducing it to the containers. Three replicates from each treatment were prepared by putting 300 g of cut kernels in 1.2-L plastic containers (Tupperware; Orlando, Fla.) fitted with 2 plastic connections that attached to the plastic tubing and served as in and out ports for the gas flow. Cut kernels were exposed to three gas compositions: Air, 2% O2 plus 10% CO2, or 2% O2 plus 20% CO2. Gas composition was monitored daily using a Servomex O2 and CO2 analyzer (Servomex, Norwood, Mass.). Fresh-Cut Kernel Storage and Cooked Sample Preparation The cut kernels were stored at 1 or 5 C for 10 d with subsamples taken at 4 and 7 d. The other treatments (maturity stage and CA) were distributed randomly in each C while the kernels were the cobs using a s sing the edge of the knife and discarded, then the adjacent rows were cut from the cob near the kernel base and the cut kernels were collected. Special care was taken during cutting in order to minimize damage to the kernels. After cutting the kernels, they were dipped in chlorinated (1.34 mM NaOCl), 5 C water at pH 7 for 1 min. for sanitation and kept covered at 5 C until distributed among the treatments. Controlled Atmosphere Treatment Controlled atmosphere treatment was used to determine the effect of atmosphere modification on fresh-cut sweetcorn quality and the potential application of MAP technology for fresh-cut sweetcorn handling. The CA mixtures were established using a system of pressure regulators, manifolds and needle valve flowmeters to blend air, N2,

PAGE 97

82 storagN.J.) equipped with thermal ewlett Packard Model 3390A integrator (Hewlett and Pand cooked kernels using the phenoth. Then the samples were erials. The samples were then f e room. After the storage period, the cut kernels were cooked by placing 50 g of kernels in 250 mL boiling water for 5 min. Both fresh and cooked samples were stored at -20 C until analysis. Parameters Respiration rate Carbon dioxide levels were measured daily using a Gow Mac series 580 gas chromatograph (Gow Mac Instruments Co., Bridgewater, conductivity detector (TCD) and a H ackard Co. Avondale, Pa.). The respiration rate was measured using the static method in which the gas flow was blocked using metal clips and the first sample was withdrawn. The containers were then incubated for 2 h before taking the second sample and removing the metal clips. The difference in CO2 concentration in the samples withdrawn before and after the incubation period was used to calculate the respiration rate. The gas samples were withdrawn from the container headspace using 1.0 mL BD plastic syringes and a 0.5-mL sample was injected on the gas chromatograph. Calibration was done before sample analysis using a certified standard mixture. Sugar content Total soluble sugars were measured in raw l-sulfuric method described by Dubois et al. (1956). In this experiment, 15 g of sweetcorn kernels were blended in a commercial blender with 85 mL of 95% ethanol for 2-3 min then were heated for 20 min in an 85 C water ba stored overnight at -20 C to precipitate ethanol-insoluble mat iltered through Whatman #2 filter paper in a Buchner funnel attached to a side-arm Erlenmeyer flask. A vacuum pump was connected to the side arm to expedite the

PAGE 98

83 extraction. The collected filtrates were volumetrically adjusted to 200 mL using 95% ethanol and 20 mL portions of the extracts were stored in scintillation vials at -20 C until the measurements wer e performed. are samples for measurement of total soluble sugars, 0.5 mL of 5% phenol to 0.5 mL of diluted (1:250) ethanol extract in test tubes, the mixture was vninhydrin in 100 mL ffer) with 1.2 mL glycerol and 0.2 mL 0.5 M citrate buffer (0.5 M citric acid a To prep (w/w) was added ortexed, then 2.5 mL concentrated sulfuric acid was added and the mixture vortexed again. The test tubes were let to stand for 10 min then were placed in a 25 C water bath for another 10 min to stop the reaction. The total soluble sugars were measured by reading the absorbance at 490 nm using a Shimadzu UV-1201 spectrophotometer (Shimadzu Scientific Instruments, Inc.; Tokyo, Japan). The absorbance values were compared to a standard curve of glucose. Free amino acids The ethanol extracts prepared for soluble sugar measurements were used to measure free amino acids colorimetrically using the ninhydrin method described by Lee and Takahashi (1966). In this method, a 0.1-mL aliquot of the ethanol extract was added to 1.9 mL of reaction mixture [0.5 mL 1% ninhydrin solution (1 g 0.5 M citrate bu nd 0.5 M sodium citrate)], mixed well, heated in a boiling water bath for 12 min, then cooled in a water bath at room temperature. Samples were mixed once more then measured at 570 nm on a Shimadzu UV-1201 spectrophotometer (Shimadzu Scientific Instruments, Inc.; Tokyo, Japan). Total free amino acid concentration was calculated from a standard curve of tryptophan (Sigma Chemical Co.; St. Louis, Mo.).

PAGE 99

84 Statistical Analysis The experimental design was a completely randomized design with three replicates. The data were analyzed using the St atistical Analysis System (SAS). Analysis of variance (ANOcorn kernels had a respiration rate that was about 50% lower than the less m the storage period than kernels stored at 5 C (Fig. 5-3). Storing sweetcorn kernels in 2% O2 plus 10% or 20% CO2 maintained about 30% higher sugar content than air storage (Fig. 5-3). VA) was used to determine the effect of treatments on the dependent variables. Means were separated by the least significant difference (LSD) test at P<0.05. Results and Discussion Fresh-cut sweetcorn is a potential product for the fresh-cut produce industry. Fresh-cut sweetcorn was stored up to 10 d without severe loss in visual quality even at 5 C. Also, dipping the kernels in chlorinated water (1.34 mM NaOCl) for 1 min after cutting was apparently effective in controlling decay since there was no visual appearance of any microbial growth in these experiments. Mature sweet ature kernels (Fig. 5-2). Also, the respiration rate of kernels stored at 5 C was about 25% higher than those stored at 1 C (Fig. 5-2), which is in agreement with previous work by Brecht (1999). Using CA with 2% O2 plus 10% CO2 significantly reduced the respiration rate by about 25%, but using 2%O2 plus 20% CO2 nearly doubled the respiration rate through most of the storage period (Fig. 5-2), which may have been due to initiation of anaerobic respiration. Despite the increase in the respiration rate when kernels were held in 2% O2 plus 20% CO2, this treatment did not seem to result in a significant difference in kernel taste compared with the air control (personal assessment by the author, no taste panel conducted). Kernels stored at 1 C had 28% higher sugar content at the end of

PAGE 100

85 On the other hand, le ss mature kernels had about 25% more sugar at the beginning of the experch reports indicating that temperature contfactor in the preservation fruits and vNakashi et al., 1991; Zagory and Kader, 1988) includrnels stored in air at 5 C, it did not occur in cooke iment but levels decreased more rapidly than in the mature kernels such that there was no significant difference between the two maturities by the end of the experiment. This may have been due to the higher respiration rate of the less mature kernels, which led to a greater sugar loss. These results are in agreement with several resear rol is the most important egetables generally and in sweetcorn specifically due to its high metabolic rate, which makes sweetcorn more prone to rapid deterioration (Brecht, 2002; Brecht and Sargent, 1988; Evensen and Boyer, 1986). It is also in agreement with work on the importance of gas modification (CA and/or MA) in maintaining quality in fruits and vegetables (Anzueto and Rizvi, 1985; ing sweetcorn cobs (Aharoni et al., 1996; Deak et al., 1987; Risse and McDonald, 1990; Schouten, 1993) Visual quality was not greatly affected by any of the storage treatments (Fig. 5-5, 7, 9, 11, and 13). The main problem in all treatments was loss of flavor and the kernels becoming more watery with a bland taste which is in agreement with Kader (2002) that the post-cutting life based on flavor is shorter than that based on appearance. Fresh-cut sweetcorn stored in air at 5 C did not show a severe loss in visual quality after storage for 10 d (Fig. 5-9), but there was severe browning of the kernels after cooking (Fig. 5-10) compared with the appearance of kernels cooked before storage (Fig. 5-6). While after cooking browning appeared in cooked ke d kernels after storage in air at 1 C (Fig. 5-8), and did not appear in the kernels stored in CA at either 1 or 5 C (Fig. 5-12 and 14). This browning also was more severe

PAGE 101

86 in the kernels from the more mature cobs. More mature kernels and kernels stored at 5 C or in air had higher content of free amino acids at the end of storage than less mature kernels and the kernels stored at 1 C or in CA (Fig. 5-4), supporting the possibility of involvement of Maillard reaction in the after cooking browning. An increase in free amino acids during storage due to protein degradation, a symptom of deterioration and senescence, may have predisposed those kernels to undergo the browning reaction. This experiment indicated clearly that temperature is the most important factor in maintaining fresh-cut sweetcorn kernel quality. All quality factors evaluated were better maintained during storage at 1 C compared with 5 C storage. Also, using a secondary preservation factor such as CA was beneficial in maintaining the quality of fresh-cut sweetcorn by maintaining sweetness and preventing after cooking brown discoloration. After cooking browning that occurred in 5 C air-stored kernels was completely inhibited by storage in 2% O2 plu s 10% CO2 at 5 C. The tolerance of fresh-cut sweetcorn kernels to ele vated CO2 appears to be quite high as well since, in addition to preventing after cooking browning, exposure to 20% CO2 for 10 d caused no negative quality changes. This suggests that using MAP could be a useful technique for commercial handling of fresh-cut sweetcorn. On the other hand, more detailed work is needed on the effect of these treatments on different aspects of the flavor and aroma of fresh-cut sweetcorn.

PAGE 102

87 Fig. 5-2. Effect of maturity stage (M1, less mature and M2, more mature kernels), storage temperature (1 C or 5 C), and controlled atmosphere (air control, 2% O2 plus 10 or 20% CO2) on respiration rate of fresh-cut sweetcorn. Bars indicate the mean +SD (n=3). 10154012345678910Days after storage air 2:10 2:20 203540 2530 M1 M2 1512345678910 101525303540espation rat (ml O2/kg-) 204512345678910RireC h. 1C 5C 202530354550 Storage duration (d)

PAGE 103

88 plus 10 or 20% CO2) on sugar content in fresh-cut sweetcorn. Bars indicate the mean +SD (n=3). 56789101112131404710 Days after storage air 2:10 2:20 678910111213141504710 M1 M2 6789101112131404710% sugar 1C 5C S torage duration (d) Fig. 5-3. Effect of maturity stage (M1, less mature and M2, more mature kernels), storage temperature (1 C or 5 C), and controlled atmosphere (air control, 2% O2

PAGE 104

89 Fig. 5-4. Effect of maturity stage (M1, less mature and M2, more mature kernels), storage temperature (1 C or 5 C), and controlled atmosphere (air control, 2% O2 plus 10 or 20% CO2) on free amino acids in fresh-cut sweetcorn. Bars indicate the mean +SD (n=3). 5791113151704710Days after storage air 2:10 2:20 3579111304710 15 M1 M2 678910111204710Free amino acids (mg/100g F.W.) 1C 5C Storage duration (d)

PAGE 105

90 Fig. 5-5. Fresh-cut sweetcorn kernels at day 0 (just after preparation). M1 = kernels from less mature cobs while M2 = kernels from more mature cobs. Fig. 5-6. Cooked fresh-cut sweetc 0 (just after preparation). M1 = kernels from less mature cobs while M2 = kernels from more mature cobs. M 1 M 2M 1 M 2 orn kernels at day

PAGE 106

91 Fig. 5-7. Fresh-cut sweetcorn kernels after storage for 10 d at 1 C in air. M1 = kernels from less mature cobs while M2 = kernels from more mature cobs. M 1 M 2M 1 M 2 Fig. 5-8. Cooked fresh-cut sweetcoage for 10 d at 1 C in air. M1 = kernels from less mature cobs while M2 = kernels from more mature cobs. rn kernels after stor

PAGE 107

92 ig. 5-9. Fresh-cut sweetcorn kernels after storage for 10 d at 5 C in air. M1 = kernels from less mature cobs while M2 = kernels from more mature cobs. F M 1 M 1 M 2 M 2 F ig. 5-10. Cooked fresh-cut sweetcorn kernels showing after cooking browning after storage for 10 d at 5 C in air. M1 = kernels from less mature cobs while M2 = kernels from more mature cobs.

PAGE 108

93 M 1 M 2 Fig. 5-11. Fresh-cut sweetcorn kernels after storage for 10 d at 1 C in 2% O2 plus 10% CO. M1 = kernels from less mature cobs while M2 = kernels from m 2ore mature cobs. M 1 M 2 Fig. 5-12. Cooked fresh-cut sweetcorn kernels after storage for 10 d at 1 C in 2% O2 plus 10% CO2. M1 = kernels from less mature cobs while M2 = kernels from more mature cobs.

PAGE 109

94 Fig. 5-13. Fresh-cut sweetcorn kernels after storage for 10 d at 5 C in 2% O2+10% CO2. M1 = kernels from less mature cobs while M2 = kernels from more mature cobs. M 1 M 2M M 2 1 Fig. 5-14. Cooked fresh-cut sweetcorn kernels showing no after cooking browning after storage for 10 d at 5 C in 2% O2+10% CO. M1 = kernels from less mature cobs while 2M2 = kernels from more mature cobs.

PAGE 110

CHAPTER 6 REDUCED OXYGEN AND ELEVATED CARBON DIOXIDE TO PREVENT BROWNING OF FRESH-CUT SWEETCORN KERNELS AFTER COOKING Introduction Fresh-cut sweetcorn is a very perishable product since the kernels have a very high respiration rate that increases the rate of deterioration compared to intact sweetcorn. Previous work with fresh-cut sweetcorn showed that this product can be stored for 10 d in temperature (0-1 C) (Brecht, 1999; Riad and Brecht, 2001). Using atmosphere odification (2% O plus 10% CO) enabled storage even at higher temperature (5 C) without muated loss of flavor after preparation of the fresh-cut product, hich is associated with reduced concentrations of soluble sugars and dimethyl sulfide, the main component of sweetcorn aroma. The second is browning that can occur after cooking when the fresh-cut kernels are stored in air at higher than optimum temperatures (Riad and Brecht, 2001). Blanchard et al. (1996) described a similar after cooking browning problem in air-stored diced onion, which was eliminated by storage in an atmosphere containing air plus 10% CO2. r-cooking browning as a result of aillard reaction (a non-enzymatic ng) is comm -hydroxymethylfurfural (HMF) (Bayindirli et al., 1995; Gogus et al., 1998). In the case f sweetcorn, the possibility of Maillard reaction would seem to be very high since low m 22ch loss in visual or chemical quality. There are two main problems associ with this product. The first is w Afte the M reaction between sugars and free amino acids usually associated with thermal procession in the food processing industry. This reaction produces 5 o 95

PAGE 111

96 sweetcorn has high levels of free amino acids and sugars (Courter et al., 1988; Grunau and Swiader, 1991). The free amino acid content would be expected to increase in response to wounding and as the kernels senesce during storage due to protein degradation. Also, the brown color doesnt develop until the kernels are cooked. Another possibility for cause of this browning is autooxidation of phenolic compounds that causes polymerization of polyphenolic compounds (Talcott and Howard, 1999). It is well known that phenolic acids increase after wounding (Babic et al., 1993a,b; Howard and Griffin, 1993; Ramamurthy et al., 1992). Brecht (1999) ed that the browning is severe in cut nels but there is no browning in intact kels, which also suggests that the cutting or role in the afFresh-cut products may have high microbial loads (circa 105 to 107 CFUg-1) and these populations may increase during storage and reach high levels (e.g., 108 CFUg-1) in commercial soup mixes (Manzano et al., 1995). Using disinfectants such as chlorine can be very beneficial in reducing the microbial load on fresh-cut commodities. Adams et al. (1989) found that using 100 ppm free chlorine (pH 5) in washing lettuce leaves resulted in a 6.22 log reduction in the microbial population count. Generally using chlorine as a disinfectant in the washing water results in a 1-2 log reduction in the microbial 995). Heard (2002) summarized the different symptoms of microbial spoilage in fresh-ut fruits and vegetables as follows: Soft rot, which is the result of maceration of the tissue caused by enzymatic degradation of the plant cell wall by pectinolytic enzymes. notic k erern injured tissues in the kernels might result in elevated levels of phenolics that may play a ter cooking browning. p opulation in fruit and vegetable (Cherry, 1999). A high microbial load can negatively affect the sensory quality of fresh cut products (Abbey et al., 1988; Manzano et al., 1 c

PAGE 112

97 Off-odors and off-flavors production due to the activity of microbial enzymes and also due to the fermentative reactions that increase the levels of acids, alcohols, and CO2. Loss of texture and wilting as a result of vascular infections. the This work was carried out to icause of after cooking browning of fresh-ing browning Expe Production of brown discoloration since the polyphenol oxidase activity of microflora may contribute to the brown discoloration. nvestigate the cut sweetcorn using a CA treatment previously shown to be optimal for reducing the losses of sugar and aroma in fresh-cut sweetcorn. The effect of CA storage on the concentrations of phenolics and HMF in fresh-cut sweetcorn was quantified, as well as its effect on microbial growth on the sweetcorn kernels. Materials and Methods Three experiments were done to study the after cooking browning in fresh-cut sweetcorn. In the first experiment, the effect of controlled atmosphere on the sugar content, the soluble phenolic content, and microbial load of fresh-cut sweetcorn kernels was measured and related to the after cooking browning. In the second experiment, microbes isolated from the first experiment were used to inoculate fresh-cut sweetcorn kernels to determine the effect of the microbial load on the after cooking browning. In the last experiment several attempts were made in an effort to isolate and identify the compound(s) responsible for the after cook riment I: CA Effect on Fresh-Cut Sweetcorn Kernels Plant material Fresh harvested sweetcorn (Zea mays L. rugosa var. Prime Time) cobs were harvested from a local farm in Florida (Hugh Branch Inc., Pahokee, Fla.) on 6 February 2001 precooled, and delivered overnight by refrigerated truck to a local grocery chains

PAGE 113

98 distribution facility in Jacksonville (Publix Distribution Center, Jacksonville, Fla.) on 7 February. The sweetcorn was shipped to Gainesville, Fla. on 8 February where it was then picked up and transferred to the postharvest lab (University of Florida, Gainesville, Fla.) and the experiment started on the same day. Preparation of the fresh-cut sweetcorn As soon as the sweetcorn arrived at the lab, the cobs were de-husked and the kernels were cut from the cobs using a sharp stainless steel knife. The kernels in the first row were popped out first using the edge of the knife and discarded, then the adjacent rows were cut from the cob near the kernel base and the cut kernels were collected. Special care was taken during cutting in order to minimize damage to the kernels. After cutting the kernels, they were dipped in ch lorinated (2.7 mM NaOCl), 5 C water at pH 7 for 2 min for sanitation and kept at 5 C until distributed among the treatments. Storage and treatments Fresh-cut sweetcorn kernels were stored in either air or CA (2% O2 plus 10% CO2). This was the best atmosphere for fresh-cut sweetcorn as determined by Riad and Brecht (2001). The CA mixture was established using a system of pressure regulators, manifolds, and needle valve flowmeters to blend air, N2, and CO2 from pressurized cylinders. The CA was a flowing system with the flow rate set to maintain respiratory CO2 accumulation under 0.5%. The gas flow was humidified by bubbling through water before introducing it to the containers. Four replicates of 300 g of sweetcorn kernels per treatment were stored at 5 C for 10 d.

PAGE 114

99 Sugar content Total soluble sugars were measured in raw and cooked kernels using the s experiment, 15 g of 2-3 min then were heated for 20 min in an 85 C water bath. Then the samples were stored overnight at -20 C to precipitate ethanol-insoluble materials. The samples were then filtered through Whatman #2 filter paper in a Buchner funnel attached to a side-arm Erlenmeyer flask. A vacuum pump was connected to the side arm to expedite the extraction. The collected filtrates were volumetrically adjusted to 200 mL using 95% ethanol and 20 mL portions of the extracts were stored in scintillation vials at -20 C until the measurements were performed. To prepare samples for measurement of total soluble sugars, 0.5 mL of 5% phenol (w/w) was added to 0.5 mL of diluted (1:250) ethanol extract in test tubes, the mixture was vortexed, then 2.5 mL concentrated sulfuric acid was added and the mixture vortexed again. The test tubes were let to stand for 10 min then were placed in a 25 C water bath for another 10 min to stop the reaction. The total soluble sugars were measured by reading the absorbance at 490 nm using a Shimadzu UV-1201 spectrophotometer (Shimadzu Scientific Instruments, Inc.; Tokyo, Japan). The absorbance values were compared to a standard curve of glucose. added to a 20 mL test tube and 0.5 mL of 0.25N Folin-Ciocalteu phenol reagent phenol-sulfuric method described by Dubois et al. (1956). In thi sweetcorn kernels were blended in a commercial blender with 85 mL of 95% ethanol for Total Soluble phenolics The Folin-Ciocalteu assay was used to measure total soluble phenolics in previously prepared ethanol extracts from raw and cooked kernels as gallic acid equivalents (Howard et al., 1996). In this experiment, 0.5 mL of the ethanol extract was

PAGE 115

100 (Sigma-Aldrich Co. St. Louis, Mo.) was added, the tube then vortexed and left to stand for 3 min to allow reduction of phosphomolybdic acid by phenolic compounds. Next, 0.5 mL of 1N sodium carbonate was added and the samples left to stand for 7 min before addition of 3.5 mL DI-water. Finally, the samples were m ixed and left to stand for 30 min were then centrifuged for 5 min at 5040 gn in ordertrifuged 8 C, filtered, and analyzed by HPLC for individual phenolic acids.tion was prepared with a raised pH (1N NaOH was used to obtain pH 10) and, when development of the brown color occurred, the solution was neutralized (using 1N HCl) and then it was injected onto the HPLC. before measuring. After 30 min, the tubes to remove the suspended starch molecules. The visible blue chromophores were then measured colorimetrically using a Shimadzu UV-1201 spectrophotometer (Shimadzu Scientific Instruments, Inc.; Tokyo, Japan) at 726nm. Phenolic acids and 5-hydroxymethylfurfural (HMF) Phenolic acids and HMF were extracted by blending a 5-g sample of kernels with 10 mL of methanol acidified with HCl (1000:1) for 1 min. The homogenate was filtered through Miracloth (Calbiochem, San Diego, Calif.). A 3-mL portion was then cen at 2500 gn for 5 min at 1 Separation was conducted on a 100 4.6 mm Spherisorb ODS C18 column (Alltech Associates, Inc., Deerfield, Ill.) connected in series to a 150 3.9 mm Nova-Pak C18 column (Waters Corporation, Milford, Mass.), and peaks were monitored using a Waters 996 photodiode array detector at 280 nm. Spectral characteristics of phenolic acids were compared to those of external standards (Sigma Chemical Co., St. Louis, Mo.) for identification and quantification. A model Maillard reaction was performed by boiling glucose (1 gL-1) with tryptophan (0.25 gL-1) in acidified medium to produce HMF, which was analyzed as described above. The solu

PAGE 116

101 Microbial analy sis. ing a reflectance colorimeter (Model CR200; Minolta Came to determine the effect of treatments on the ns were separated by the least significant difference (LSD) test at P< Total aerobic plate counts were performed for each treatment after 0, 4, 7, and 10 d of storage using Petrifilm plates (3M Company, St. Paul, Minn.). A 5-g sample of kernels from each treatment was placed into a 50-mL disposable sterile centrifuge tube containing 45 mL phosphate buffer solution (PBS), which was then placed in a rotary shaker for 20 min. Serial dilutions were plated on Petrifilm plates and incubated at 30 C for 3 d. Aerobic microbial counts were reported as logarithm of colony forming units per gram (log10 CFUg-1). After cooking browning. A 50-g sample of kernels was added to 300 mL of boiling water for 10 min. After cooking browning was evaluated us ra Co. Ltd., Japan). Color was measured using the CIE L*, a*, b* scale. The a* and b* values were converted to hue angle (tan b*/a*) and chroma ((a*2+b*2)1/2). Cooked samples (25 g) were blended with 25 g of water, the resulting slurry was poured into a Petri dish, and the color of the slurry was measured. Statistical analysis The experimental design was a completely randomized design with four replicates per treatment. The data were analyzed using the Statistical Analysis System (SAS). Analysis of variance (ANOVA) was used dependent variables. Mea 0.05.

PAGE 117

102 Experiment II: Microbial Load Effect on the After Cooking Browning Plant material Fresh harvested sweetcorn (Zea mays L. rugosa var. Prime Time) cobs were harvested from a local farm in Florida (Hugh Branch Inc., Pahokee, Fla.) on 17 April 2001, precooled, and delivered overnight by refrigerated truck to a local grocery chains distribution facility in Jacksonville (Publix Distribution Center, Jacksonville, Fla.) on 18 April where it was picked up and transferred to the postharvest lab (University of Florida, Gainesville, Fla.). The sweetcorn was kept at 5 C and th e experiment started on PrepaOn the day of inoculation, three plates per unique microbe isolate were washed with sterilized water and the rinsate placed on a magnetic stirrer to obtain a suspension of the microbes. Cell concentrations were measured using a spectrophotometer, and then the following day, 19 April. ration of the fresh-cut sweetcorn Fresh-cut sweetcorn was prepared as described for experiment I. Isolation of the microbes From the last experiment (Experiment I) aerobic microbes were transferred from 3M Petri films to Petri dishes containing either nutrient agar (NA; to isolate bacteria) or acidified potato dextrose agar (APDA; to isolate yeasts and molds). The NA Petri dishes were incubated at 30 C for 2 d and the APDA Petri dishes were incubated at room temperature (approximately 24 C) for 3 d. The resulting colonies were then transferred to new plates with a total of three different yeasts and five different bacteria being isolated. Preparation of the inoculum

PAGE 118

103 10 L of microbe susp ension was prepared for each microbe and adjusted to contain 106 ceicrobe suspension. atment, storage, and sampling t, 1945) in which the percentage of le affected by the browning were scored using a scale from 1 to 10. The s field plots. Because the severity of after cooking browning increased as the incidence of brown kernels increased, higher scores necessarily imply both increased incidence and severity of browning. lls for the bacteria and 105 cells for the yeasts. Inoculation process Cheesecloth sacks were prepared to contain about 2.5 kg of fresh-cut sweetcorn. Two buckets (10 L) were filled with 5 L of each inoculum and a sweetcorn sack was dipped in each bucket for 5 min then it was removed and the excess inoculum was drained. About 75 g of kernels from each sack were placed in four, 125-mL glass flasks with sponge plugs per m Controlled atmosphere tre The glass flasks were placed in two sealed metal chambers with Plexiglas front covers in a 5 C storage room for 10 d. The chambers were ventilated with either air (control) or a CA of 2% O2 plus 10% CO2 prepared as described in Chapter 3. After cooking browning measurement After cooking browning was evaluated in the cooked fresh-cut sweetcorn kernels using the Horsfall-Barratt scale (Horsfall and Barrat the kernels in a samp core is based on 50% as a mid-point and the grades differ by a factor of two in either direction as follows: 1=0, 2= 6, 3= 12, 4 = 25, 5=50, 6= 75, 7= 87, 8= 94, 9=97, and 10= 100 The Horsfall-Barratt scale was originally developed for evaluating disease incidence in

PAGE 119

104 Experiment III: Attempts to Identify the Browning Reaction Source Fresh-cut sweetcorn kernels were stored for 10 d in air at 5 C to induce after cookiter in another test tube. Both tubes were placed in a boiling ing the dark pigment resulting from boiling the re-dise kernels washed to remove the pigment. 6 photodiode array (PDA) detector. Individual compounds were separated with ified water (98:2, water: acetic acid) and (68:30:2, water: aceto ng browning. After storage, the kernels were crushed in a mortar and pestle then pressed using cheesecloth to separate the kernel juice from the cell wall materials and each portion placed in a 20 mL test tube with sealed plastic cap, then each of those two components were placed in a boiling water bath for 5 min. In the next step, the kernel juice was centrifuged and the juice solids were precipitated using acetone. The resulting supernatant was collected in a test tube and the pellet was re-dissolved in wa water bath for 5 min. The extracts contain solved pellets was added to crushed raw kernels for 1 min then th with either water or ethanol The last attempt was to run this unidentified brown pigment on the HPLC using the same protocol that was used to measure phenolic compounds concentration. The unidentified brown pigment was measured in both extracts using a Waters Alliance 2690 HPLC (Waters Corp., Milford, Mass.) equipped with a dual column system using a Supelcosil LC-18 column, 250 x 4.6 mm, and a Waters Spherisorb C-18 ODS (5 m) column, 4.6 x 250 mm. Polyphenolic compounds were detected using a Waters 99 gradient mobile phases of acid nitrile: acetic acid) at a flow rate of 0.8 mLmin-1 with PDA detection at 280 nm. (Talcott et al., 2000).

PAGE 120

105 Results and Discussion Experiment I Fresh-cut sweetc orn is a very perishable product with a high respiration rate, which makegar content was significantly higher in raw due to leaching of soluble solidss the after cooking browning, which causes the appeaFig. 6-6, and 8). This after cooking browning was controlled by use at 5 C (Fig.6ness) was also higher in the air storage after 7 and 10 d as an indication of a color change toward brown color in the air-stored treatment s storage at low temperatures, as close to 0 C as possible, critical to ensure quality retention (Riad and Brecht, 2001). In this work, the effect of gas modification as a secondary method of quality preservation was used at a relatively high storage temperature (5 C). The sugar content was significantly higher in the kernels from CA storage than from air storage. Also, the su kernels than in cooked kernels (Fig. 6-1), which might be from the cut kernel surfaces during boiling. After 10 d at 5 C, the sugar content on a FW basis was about 9% in CA storage and about 6.5% in air storage, both of which are very acceptable levels for sugars in sweetcorn. The main problem in this product i rance of a dark brown pigment in the kernels that appears to be attached to the cell wall materials. As a result, the pigment responsible for the after cooking browning was not soluble in water or ethanol. The after cooking browning occurred mainly in kernels stored in air (Fig. 6-5, 6, and 8). The browning first occurred after 7 d of storage in air and was severe after 10 d ( ing CA storage with 2% O2 plus 10% CO2 even after 10 d of storag -7, and 9). The L value or lightness of the sweetcorn slurry did not change during the storage period, however, the a* value (as a measure of redness) was significantly higher in the air storage after 7 and 10 d of storage (with the start of the browning), and the b* value (as a measure of yellow

PAGE 121

106 compared to the light yellow color of the CA-stored treatment (Table 6-1 ). There was no signifis browning could be due to a MaillF was successfully measured in a reactifrom airor CA-shas been shown to cause formation of brown color during thermal processing in other food products such as carrot puree (Talcott et al., 2000). icant difference in hue angle between treatments until after 10 d of storage, when the hue of air-stored kernels dropped sharply, while the chroma value was significantly higher in air storage after both 7 and 10 d. In the process of studying the cause of the after cooking browning of fresh-cut sweetcorn kernels, Riad and Brecht (2001) suggested that th ard reaction since higher concentrations of free amino acids were found in the treatments with more severe browning, and very high soluble sugar concentrations were maintained in those treatments. In this work, no HMF, the intermediate of the Maillard reaction, was found in any of the sweetcorn samples before or after cooking (Fig. 6-2), which indicates that the Maillard reaction is not responsible for the after cooking browning in fresh-cut sweetcorn. However, HM on model consisting of boiled glucose plus tryptophan solution. There were no differences found in the phenolic profiles of extracts tored sweetcorn kernels (Fig. 6-2) and there also was no trend or significant difference in their total soluble phenolics contents (Fig. 6-3), but there was a significant reduction in the levels of soluble phenolics noticed after cooking (Fig. 6-3). This could mean that soluble phenolic compounds in fresh kernels were converted to insoluble compounds during cooking or the soluble phenolics might have leached out in the boiling water and been diluted. More work needs to be done to identify the individual polyphenolic compounds that might be responsible for the after cooking browning via autooxidation, which

PAGE 122

107 The initial microbial coun and increased only slightly in storage during the first 4 d of storage at 5 C (Fig. 6-4). There was a >2-lome product of micro t was about log 2 CFUg-1 either air or CA g increase in microbial count in air-stored sweetcorn kernels between days 4 and 7 of storage, and a further almost 1-log increase between days 7 and 10 (to log 5.5 CFUg-1) (Fig. 6-4), which corresponds to the appearance and worsening severity, respectively, of after cooking browning in those kernels. Controlled atmosphere significantly reduced the increase in total aerobic microbial count during storage (Fig. 6-4), but there was still about a 2-log increase in microbial count in the CA-stored kernels between days 4 and 10 of storage. It is interesting to consider the possibility that the higher microbial load on the air-stored kernels might play a role in the after cooking browning. It may be possible that the browning is related to a direct response of the sweetcorn tissue to the microorganisms, or the browning may alternatively be associated with so bial enzyme activity. In conclusion, fresh-cut sweetcorn kernels can be successfully stored for 10 d in CA with little loss of visual or sensory quality. After cooking browning can be totally controlled by storage in 2% O2 plus 10% CO2, even at a higher than recommended temperature (5 C versus 0-1 C). Although high levels of free amino acids and sugars are found in sweetcorn kernels that develop after cooking browning, the Maillard reaction is not the cause of the after cooking browning since no HMF was detected in the browned kernels. More work needs to be done regarding the identification of different phenolic-protein or phenolic-amino acid complexes that might play a role in the after cooking browning. Also, more work is needed to determine the relationship, if any, of the microbial load with the after cooking browning.

PAGE 123

108 Experiment II In this experiment, the microbial load on inoculated fresh-cut sweetcorn reached 108 within 2 d of storage at 5 C, but did not increase more than that during the rest of the storage period. After cooking browning s tarted earlier in the air controls than in the CA-st all the microbe treatmid not cause browning in the kernel residue while the juice started to brown within 1-2 min. When the tored treatments and, after 4-5 d, all the air control kernels turned brown after cooking. The CA storage significantly reduced the microbial load, but it was not effective in reducing the after cooking browning. This is in contrast to the obvious effectiveness of CA in reducing after cooking browning in non-inoculated sweetcorn kernels and may have been due the initial high microbial load already present at the beginning of storage in this experiment due to inoculation. There was no specific effect of any of the isolated microbes used to inoculate the sweetcorn kernels since all the inoculated treatments had severe browning compared to the non-inoculated controls. The appearance of after cooking browning was advanced by 1-2 d for some microbes, bu ents had 100% browning after about 4-5 d. These results indicate that after cooking browning is not caused directly by the presence of a specific microorganism. The microbe load could be responsible for after cooking browning in an indirect way such as by increasing the kernel deterioration rate. Alternatively, microbial activity may act on the sweetcorn substrate to release or induce the formation of a browning precursor, either alone or through reaction with one or more other compounds. This possible precursor could then result in the browning when the kernels are heated. Experiment III In this experiment, boiling the separated kernel juice and cell wall materials d

PAGE 124

109 kerned brown pigment was run on HPLC it was found that it is a very w l juice was centrifuged to remove suspended material and the juice dissolved solids were precipitated using acetone, the resulting supernatant did not brown, but when the acetone-insoluble pellet was re-dissolved in water and the solution was boiled, the solution quickly turned brown within seconds (the reaction started after 20 s and the solution was very dark brown to nearly black after 60 s). When the solution with dark pigment that was obtained from boiling the re-dissolved pellet of acetone-insoluble material was added to crushed raw kernels it stained the tissue with a brown color that was not subsequently washable. This supports the idea that the brown compound is a water-soluble brown pigment that is developed during boiling and then reacts with (i.e., binds to) cell wall material and becomes insoluble. This could be the reason why we were unable to isolate this pigment from cooked sweetcorn kernels using water, ethanol, or acetone. When this unidentifie ater-soluble compound(s). It was eluted very quickly, before any known authentic compounds we have, and it was absorbed in the entire UV spectrum (Fig. 6-10) as a result we were unable to identify it.

PAGE 125

110 Table 6-1. Effect of atmosphere modification and storage period on the color parameters of cooked fresh-cut sweetcorn kernels stored in air or controlled atmosphere. Storage duration (d) Parameter Treatment 0 4 7 10 air 61.46 62.54 a* 64.44 a 62.03 a L* CA 1 63.38 a 62.84 a 64.50 a air -2.25 -1.93 a -1.71 a 2.37 a a* CA -2.13 a -1.95 b -1.71 b air 19.7 20.97 a 23.04 a 24.39 a b* air 96.52 95.26 a 94.24 a 84.45 b CA 20.62a 20.94 b 21.15 b Hue angle air 19.83 21.06 a 23.10 a 24.50 a CA 95.90 a 95.32 a 94.62 a Chroma CA 20.73 a 21.03 b 21.22 b Values in the same column within the same color parameter followed by the 1 CA = controlled atmosphere storage with 2% O2 plus 10% CO2. same letter are not significantly different at p< 0.05 by LSD; n = 4.

PAGE 126

111 cooked fresh-cut sweetcorn kernels stored in air or CA (2% O2 plus 10% CO2) Fig. 6-1. Effect of controlled atmosphere storage on the sugar content (%) of raw and for 10 d at 5 C. Bars indicate the mean + SD; n = 4. 6101214 02404710 8 Storage duration (d) Suga content (%) raw air raw CA cooked air cooked CA r

PAGE 127

112 Fig. 6-2. Typical phenolic acids profiles of fresh-cut sweetcorn kernels stored at 5 C for 10 d (A) Kernels stored in air, raw and cooked, (B) kernels stored in 2% O2 plus 10% CO2, raw and cooked, (C) glucose + amino acid (tryptophan) as a model for HMF. (C) (B) (A) Maillard reaction model Minutes

PAGE 128

113 010020030040050060070004710 Storage durati on (d) raw air raw CA cooked air cooked CATotal souluble phenolics (mg/kg FW) F ig. 6-3. Total soluble phenolics (mg/kg FW) in raw and cooked fresh-cut sweetcorn kernels stored in air or CA (2% O2+10% CO2) for 10 d at 5 C. Bars indicate the mean + SD; n = 4.

PAGE 129

114 012 3g10 45704710Storage duration (d)Lo CFU g-1 6 air storage CA Fig. 6-4. Total aerobic microbial count (log10 CFUg-1) of fresh-cut sweetcorn kernels stored in air or CA (2% O2+10% CO2) for 10 d at 5 C. Bars indicate the mean + SD; n = 4. Fig. 6-5. Raw (A) and cooked (B) fresh-cut sweetcorn kernels at day 0 (just after preparation). A B

PAGE 130

115 Fig. 6-6. Raw (A) and cooked (B) fresh-cut sweetcorn kernels stored in air for 7 d at 5 C. Fig. 6-7. Raw (A) and cooked (B) fresh-cut sweetcorn kernels stored in CA (2% O2+10% CO2) for 7 d at 5 C. A B A B

PAGE 131

116 Fig. 6-8. Raw (A) and cooked (B) fresh-cut sweetcorn kernels stored in air for 10 d at 5 C. Fig. 6-9. Raw (A) and cooked (B) fresh-cut sweetcorn kernels+10% CO2) for 10 d at 5 C. A B stored in CA (2% O2 A B

PAGE 132

117 F ig. 6-10. HPLC chromatogram of the acetone-insoluble fraction of fresh-cut sweetcorn kernel juice (after storing the kernels for 10 d at 5 C in air). Unidentified brown pigments (peaks 1, 2, and 3) increased dramatically upon boiling and are probably responsible for after-cooking browning. Peak 4 was identified as galacturonic acid, which is not affected by boiling. Before boiling Boiling for 60 sec.

PAGE 133

CHAPTER 7 PHENOLICS PROFILE AND ANTIOXIDANT CAPACITY IN FRESH-CUT SWEETCORN DURING STORAGE Introduction The polyphenolic content of fruits and vegetables is an important contributor to olor quality and sensory properties (Macheix et al., 1990). Interest in these compounds has increased in recent years because of their possible health benefits, and these benefits re mainly attributed to their antioxidant characteristics. Epidemiological studies strongly suggest a protective effect of dietary antioxidants against many chronic diseases and suggest that increased consumption of fruits and vegetables may be related to a reduction of the risk of cardiovascular disease and certain types of cancer (Block and Langseth, 1994; Kohlmeier et al., 1995; Steinmetz and Potter, 1996). Also, dietary antioxidants other than vitamins (i.e., polyphenolic compounds) have been shown to be a major dietary factor associated with such protective effects (Hertog et al., 1992; 1994; 1997). al. (2002) found that the contributions of phenolic compounds to antioxidant nectarines, peaches, and plums. Despite these facts, there are no published reports on the polyphenolic composition and antioxidant capacity of sweetcorn. The benefit of using CA/MA in fruit and vegetable storage is well established (Chinnan, 1989; El-Gorani and Sommer, 1981; Kader, 1987; Kader et al., 1989; Shewfelt, 1986; Zagory and Kader, 1988) but there is little available work on the effect of such storage treatments on the antioxidant capacity of fruits and vegetables. Cranberry c a Gil et activity were much greater than those of vitamin C and carotenoids in 25 cultivars of 118

PAGE 134

119 fruit stored in 21% O2 plus 30% CO2 had lower antioxidant activity than air-stored fruit after 2 months of storage at 3 C (Gunes et al., 2002). Storage of blueberries in a high O2 atmosphere for 35 d at 5 C was beneficial in maintaining significantly higher antioxidant capacity compared to air storage, with 80-100% O2 found to be the best treatment (Zheng et al., 2003). The use of CA was found to have a beneficial effect on both sweetcorn cobs or fresh-cut sweetcorn kernels in reducing respiration and hence maintaining higher sugar Riad et al.kernels is tis the major limiting factor for air storage (Riad and Brecht, 2001; Riad et al., 2003). There is no published work on the effect of atmosphere modification on the antioxidant capacity of sweetcorn or fresh-cut sweetcorn kernels. Phytonutrients with antioxidant capacity can be found in free, soluble forms and also in insoluble, bound forms (Adom and Liu, 2002). Most of the insoluble, bound antioxidants are bound to cell wall materials (Bunzel et al., 2001; Sosulski et al., 1982). About 74% and 69% of the total phenolics present in rice and corn, respectively, are in the insoluble, bound forms, with ferulic acid being the major phenolic compound present (Adom and Liu, 2002). A number of methods have been developed to measure the reducing efficiency or antioxidant capacity of dietary antioxidants either as pure compounds or in food extracts. These methods focus on different mechanisms of the antioxidant defense system, i.e., scavenging of oxygen and hydroxyl radicals, reduction of lipid peroxyl radicals, content, the most important characteristic in sweetcorn quality (Riad and Brecht, 2003; 2002), but the most important benefit of using CA for fresh-cut sweetcorn he prevention of after cooking browning, which the

PAGE 135

120 inhibition of lipid peroxidation, or chelation of metal ions. The ORAC (oxygen radical absorbance capacity) assay (Cao et al., 1995) depends on free radical damage to a fluorescent probe to effect a change in ce intensity. The change of fluore of an antioxidant, the inhibition of free radical damage, which is reflected in the protection againair-conditioned car (University ofthen the adjacent rows were cut from the cob near the kernel base and the cut kernels its fluorescen escence intensity is an index of the degree of free radical damage. In the presenc st the change in probe fluorescence, enables quantification of the antioxidant capacity present. The ORAC assay is a common method to measure antioxidant capacity of food products and it was used in this study. The objectives of this study were to determine the polyphenolic composition and antioxidant capacity of fresh-cut sweetcorn kernels in both the free fraction (water soluble fraction) and in the bound fraction (hydrolyzed fraction of the water insoluble compounds) during storage in either air or CA. Materials and Methods Plant Material Fresh harvested sweetcorn (Zea mays L. rugosa var. Prime Time) cobs were harvested from a local farm in Florida on 25 March 2002, precooled, and delivered overnight by refrigerated truck to a local grocery chains refrigerated distribution facility in Jacksonville (Publix distribution center, Jacksonville, Fla.) on 24 March where it was picked up in the morning and transferred within 2 h to the postharvest lab by Florida, Gainesville, Fla.) and the experiment started at the same day. The cobs were de-husked and de-silked, and held at 5 C while the kernels were cut. The kernels were cut from the cobs using a sharp stainless steel knife. The kernels in the first row were popped out first using the edge of the knife and discarded,

PAGE 136

121 were collected. Special care was taken during cutting in order to minimize damage to the kernels. After cutting the kernels, they were dipped in chlorinated (1.34 mM NaOCl), 5 C water at pH 7 for 2 min. for sanitation and kept at 5 C until distributed among the treatments. Controlled At mosphere Treatment 2, which had been previously determined to be the best gas compere stored at 5 C for 10 d with subsamples taken after 4 and 7 d. Threefrozen at -20 C until analysis. Controlled atmosphere was used to determine the effect of atmosphere modification on fresh-cut sweetcorn antioxidant and phenolic content. The CA mixtures were established using a system of pressure regulators, manifolds, and needle valve flowmeters to blend air, N2, and CO2 from pressurized cylinders. The CA was a flowing system with the flow rates set to maintain respiratory CO2 accumulation under 0.5%. The air flow was humidified by bubbling through water before introducing it to the containers. Cut kernels were exposed to two different gas compositions: Air (control) and 2% O2 plus 10% CO osition for maintaining fresh-cut sweetcorn quality and for preventing after cooking browning in the cooked sweetcorn kernels (Riad and Brecht, 2001; Riad et al., 2003). Gas composition was monitored daily using a Servomex O2 and CO2 analyzer (Servomex; Norwood, Mass.). Fresh-Cut Kernel Storage and Sample Preparation The cut kernels w replicate samples per treatment were prepared by putting ~300 g of cut kernels in each of three, 1.2-L plastic containers (Tupperware; Orlando, Fla.). The containers were each fitted with two plastic connections that were attached to plastic tubing and served as the inlet and outlet for the gas flow. After the storage period, the samples were stored

PAGE 137

122 Extraction of Phenolic Compounds Extraction of free phenolic compounds Free phenolic compounds in fresh-cut sweetcorn kernels were extracted by blending 10 g of kernels in 50 mL acetone for 10 min. After centrifugation at 2500 gn for 10 min, the supernatant was removed, and the extraction was repeated on the precipitate. Supernatants were pooled, evaporated at 45 C to about 10 mL, and reconstituted with water to a final volume of 10 mL (Eberhardt et al., 2000). Extraction of bound phenolic compounds The precipitates from free phenolic extraction (i.e., bound phenolics) were digested at room temperature with 10 mL 2M NaOH per sample for 1 h with shaking. The mixture was neutralized with an appropriate amount of 2M HCl. The final solution was extracted several times with acetone. The supernatant was then was evaporated to dryness. Phenolic compounds were reconr with the help of a sonic water al., 1982). Param stituted in 10 mL of wate bath (Sosulski et eters Measurement of polyphenolic compound concentrations Free and bound polyphenolic compounds in raw fresh-cut sweetcorn kernels were separated using a Waters Alliance 2690 HPLC (Waters Corp., Milford, Mass.) equipped with a dual column system using a Supelcosil LC-18 column, 250 x 4.6 mm, and a Waters Spherisorb C-18 ODS (5m) column, 4.6 x 250 mm. Polyphenolic compounds were detected using a Waters 996 photodiode array (PDA) detector. Individual polyphenolic compounds were separated with gradient mobile phases of acidified water (98:2, water: acetic acid) and (68:30:2, water:acetonitrile:acetic acid) (Talcott et al., 2000) at a flow rate of 0.8 mLmin-1 with PDA detection at 280 nm. Ultraviolet spectral

PAGE 138

123 properties (200-400 nm) and retention time of each compound were compared to that of authentic standards obtained (Sigma Chemical Co., St. Louis, Mo.). Total phenolics Total p henolics concentrations were measured in both the free and bound fractions 0-L aliquot of each extract was mixed with 100 L Fonopropane) dihydrochloride, and fluorescence loss was monitored nt microplate reader (485 nm excitaof -Trolox, a water soluble form of vitamin E (Sigma using the Folin-Ciocalteu assay. A 10 lin reagent (Sigma Chemical Co., St. Louis, Mo.) for 3 min to allow reduction of phosphomolybdic acid by phenolic compounds. Then 100 L of 1N Na2CO3 was added as an alkali to form chromophores with a visible blue color. After 7 min, the test solutions were brought up to 1 mL total volume with water and read spectrophotometrically using a Beckman DU 640 Spectrophotometer (Beckman Coulter, Inc., Fullerton, Calif.) at 726 nm after 30 min. Total soluble phenolics were measured as gallic acid equivalents (Howard et al., 1996). Quantification of antioxidant capacity Antioxidant capacity was measured in both the free and bound phenolic fractions using the ORAC assay described by Cao et al. (1995) and modified by Ou et al. (2002) with the use of fluorescein as the fluorescent probe. Peroxyl radicals were generated by 2,2-azobis (2-amidi using a 96-well Molecular Devices fmax fluoresce tion and 538 nm emission; Molecular Devices Corp., Sunnyvale, Calif.) with fluorescent readings taken every 2 min for 70 min at 37 C and the fluorescent decay curves were calculated (Talcott et al., 2003). A 40-fold dilution of each extract (free and bound phenolic extracts) in a pH 7.2 ORAC phosphate buffer (61.6:38.9 v/v, 0.75 M K2HPO4 and 0.75M NaH2PO4 ) was used and compared to a standard curve representing 0, 6.25, 12.5, 25, and 50 M

PAGE 139

124 Chemical Co., St. Louis, Mo.). Measu rements were done by adding 20 L of the diluted rking -phycoerythrin (-phycoerythrin stock solutiay curves were calculated in d in Trolox equivalents. StatisResults and Discussion reased popularity and demand for fresh-cut produce coincides with an esh produce, especially its antiox sample or the Trolox to 160 L of wo on was prepared by dissolving 1 mg in 5.9 mL phosphate buffer and the working -phycoerythrin was prepared by adding 300 L of the stock to 13.4 mL of phosphate buffer) and the initial relative fluorescence were measured, then 20 L of 2,2-azobis (2-amidinopropane) dihydrochloride were added and the relative fluorescence (RF) was measured every 2 min for 70 min. The fluorescent dec relation to Trolox and the data were expresse tical Analysis The experimental design was a completely randomized design with three replicates. The data were analyzed using the Statistical Analysis System (SAS). Analysis of variance (ANOVA) was used to determine the effect of treatments on the dependent variables. Means were separated by the least significant difference (LSD) test at P<0.05. The inc increased consumer interest in the nutritional value of fr idant content. The high antioxidant content of fruits and vegetables adds another health benefit for the consumer since there are many epidemiological studies showing that polyphenolic compounds may be among the most important compounds in nutrition. Most phenolic compounds have a relatively high antioxidant capacity recent work has shown that the contribution of polyphenolics to the total antioxidant capacity of fruits and vegetables is greater than the more well-known antioxidants like vitamin C and carotenoids (Hertog et al., 1992; 1994; 1997).

PAGE 140

125 The pectin galacturonic acid was the first of the acetone insoluble compounds in the sweetcorn extracts to elute (RT ~6.4 min). The CA-stored kerne ls usually had higher galacturonic acid and its concentration increased with increased storage time capacity was much higher in the bound phenolic fraction than in agreement with the work of Adom and Liu (2002 concentrations of (Fig. 7-8), which may have been due to the degradation of the cell walls. Phenolic compounds that were found included p-coumaric acid, p-OH-benzoic acid, and ferulic acid; also, several unidentified phenolic compounds were present in low concentrations (Fig. 7-1 to 7-7). Ferulic acid was the most prominent phenolic compound in the bound fraction (Fig. 7-1, 2, 4, and 6), which is in agreement with work done on field corn, for which ferulic acid was reported to be the major phenolic compound and its concentration in the bound fraction was found to be 1000-fold its concentration in the free fraction (Adom and Liu, 2002) The antioxidant the free fraction (Fig. 7-9), and this is in ), who reported that most of the antioxidant capacity in four grains (rice, corn, wheat, and oat) was in the bound fraction. The fact that most of the antioxidant capacity in sweetcorn kernels are in the bound form dose not reduce its importance since there are many epidemiological studies showing that bound phytochemicals can survive stomach and intestinal digestion to reach the colon where the colonic microflora have the ability to digest them and release the health beneficial compounds (Adam and Liu, 2002). Controlled atmosphere treatment did not have much effect on the sweetcorn kernel antioxidant capacity during the first 7 d of storage (Fig. 7-9), but it significantly reduced antioxidant capacity by 10 d of storage. Similar research has shown that CA prevented an antioxidant capacity increase in cranberry fruit compared to air storage (Gunes et al.,

PAGE 141

126 2002). This reduction in the antioxidant capacity in CA compared with air storage could be due to the effect of CA in delaying deterioration and maintaining cell wall integrity, hence preventing the release of many compounds that have antioxidant activity. Similar results were reported by Dewanto et al. (2002) who found an increase in the antioxidant capacity in heat-processed tomatoes due to the disruption of cell membranes and the release of lycopene from the cell matrix and insoluble fiber during thermal processing. On the other hand, total phenolics content in the fresh-cut sweetcorn kernels followed a pattern mirroring that of the antioxidant capacity. There w as not much change olic content during the first week of storage in either air or CA, excep phenolics in both the free and bound fracti in free or bound phen t for a significant increase in the free phenolic fraction after 10 d for air-stored kernels. This increase in free phenolics might have been due to the release of cell wall bound phenolics as a result of deterioration in the air storage treatment. Also, after 10 d of storage at 5 C, the CA-stored kernels had significantly lower levels of total ons. In conclusion, fresh-cut sweetcorn kernels could serve as a good dietary source for polyphenolics and antioxidants, however, fresh-cut sweetcorn antioxidant capacity declines during storage. Since CA storage is essential to maintain the quality of this highly perishable commodity, it is not recommended to use air storage instead of CA despite the greater loss of antioxidant capacity late in CA storage, because of the many other benefits of atmosphere modification for quality preservation.

PAGE 142

127 Fig. 7-1. HPLC chromatograms of free and bound polyphenolic compounds in sweetcorn was made by comparing retention times and/or spectral properties to authentic 2) unknown, 3) p-coumaric acid, 4) unknown, 5) ferulic acid. extract on day 0 (before the storage treatment). Identification of compounds standards. Tentative peak identifications include 1) galacturonic acid, Minutes

PAGE 143

128 Fig. 7-2. HPLC chromatograms of bound polyphenolic compounds in the sweetco rn extract after 4 d of storage at 5 C in air (control) or CA. Identification of to authentic standards. Tentative peak identifications include 1) galacturonic 6) ferulic acid polymer. compounds was made by comparing retention times and/or spectral properties acid, 2) p-coumaric acid, 3) unknown, 4) ferulic acid, 5) unknown, and Fig. 7-3. HPLC chromatograms of free polyphenolic compounds in the sweetcorn extract after 4 d of storage in air (control) or CA. Identification of compounds was made by comparing retention times and/or spectral properties to authentic standards. Tentative peak identifications include 1) galacturonic acid, 2) p-OH-benzoic acid, 3) tryptophan and 6) tryptophan-containing protein. Minutes Minutes

PAGE 144

129 PLC chromatograms of bound polyphenolic compounds in the sweetcorn extract after 7 d of storage in air (control) or CA. Identification of compounds was made by comparing retention times and/or spectral proper Fig. 7-4. Hties to authentic standards. Tentative peak identifications include 1) galacturonic acid, 2) p-coumaric acid, 3) unknown, 4) ferulic acid, 5) unknown, and 6) ferulic acid polymer. Fig. 7-5. HPLC chromatograms of free polyphenolic compounds in the sweetcorn extract after 7 d of storage in air (control) or CA. Identification of compounds was made by comparing retention times and/or spectral properties to authentic standards. Tentative peak identifications include 1) galacturonic acid, 2) p-OH-benzoic acid and, 3) tryptophan-containing protein. Minutes

PAGE 145

130 Fig. 7-6. HPLC chromatograms of bound polyphenolic compounds in the sweetcorn extract after 10 d of storage in air (control) or CA. Identification of compounds was made by comparing reten tion times and/or spectral properties to authentic standards. Tentative peak identifications include 1) galacturonic acid, 2) p -coumaric acid, 3) unknown, 4) ferulic acid, 5) unknown, and 6) ferulic acid polymer. Fig. 7-7. HPLC chromatograms of free polyphe nolic compounds in the sweetcorn extract after 10 d of storage in air (control) or CA. Iden tification of compounds was made by comparing retention times and/ or spectral properties to authentic standards. Tentative peak identifica tions include 1) galacturonic acid, 2) p -OH-benzoic acid, 3) tryptophan and 6) tryptophanor gallic acid-containing protein. Minutes Minutes

PAGE 146

131 Fig. 7-8. HPLC chromatogram of galacturonic acid in the free phenolic sweetcorn extract during 10 d of storage in CA. Minutes

PAGE 147

132 004710Storage duration (d) 2468101214ORAC (M Trolox/mL) air bound CA bound air free CA freeFig. 7-9. A22) for 10 d at 5 C. Bars indicate the mean +SD. ntioxidant capacity (Trolox equivalents; MmL-1) of free and bound phenolic fractions obtained from fresh-cut sweetcorn kernels as affected by storage in air or CA (2% O plus 10% CO

PAGE 148

133 0100200 300e phe 40050060070004710Storage duration (d)Total solublnolics (mg/kg FW) air bound CA bound air free CA free Fig. 7-10. Total soluble phenolics (mgkg-1 FW) from free and bound fractions obtained from fresh-cut sweetcorn kernels as affected by storage in air or CA (2% O2 plus 10% CO2) for 10 d at 5 C. Bars indicate the mean +SD.

PAGE 149

CHAPTER 8 SUMMARY AND CONCLUSION Sweetcorn Cob Storage Due to the perishable nature of sweetcorn and as a result of its very active metabolism and high respiration rate, sweetcorn is greatly affected by control of the primary and secondary factors used to optimize preservation. The primary factor temperature control was proven again in this work to be the most important factor in sweetcorn storage. Furthermore, this work confirms the previous recommendations on using secoby using controlled atmosphere or modified atmosphere (CA or MA) was also very beneficial in sweetcorn cob storage. The main benefit of CA was to significantly reduce respiration rate over that of air storage and the results suggest that sweetcorn is more sensitive to reduced O2 plus elevated CO2 together than to O2 or CO2 alone since there was greater reduction in the respiration rate when using both of them. As a result of respiration rate reduction in CA treatments, higher sugar levels were maintained, which is the most important quality characteristic for sweetcorn consumers. Also, treatments resulting in greater reduction in respiration had higher sugar levels at the end of the storage duration. Controlled atmosphere treatments also had a positive impact on the visual quality of stored sweetcorn since elevated CO2 significantly reduced loss of greenness and maintained the fresh look of the husks. The CA also improved silk appearance and kernel sweetcorn storage, i.e., storing the cobs as close to 0-1 C as possible. On the other hand, ndary factors to optimize preservation such as atmosphere modification either 134

PAGE 150

135 appearance over the air control, but it had no effect on kernel denting. Storage in 2% O2 plus 15% CO2 resulted in the best score in all visual quality parameters compared with storage in air, 2% O2, or 2% O2 plus 20% CO2. Another benefit of using CA was to maintain significantly higher levels of dimethyl sulfide (DMS), which is the main characteristic volatile component in sweetcorn and is responsible for the characteristic corny smell of cooked sweetcorn. Storing sweetcorn in 2% O2 plus 15% CO2 resulted in the highest DMS content after 14 d of storage at 5 C. In conclusion, CA storage was beneficial in maintaining most sweetcorn quality parameters during 14 d of storage at 5 C. Storage in 2% O2 plus 15% CO2 gave the best result in terms of quality maintenance since it preserved the highest sugar level, reduced tion in sweetcorn visual quality, and maintained the highest DMS content and On the other hand using perforation mediated modified atmosphere packaging (PM-MAP) was another beneficial tool in storing sweetcorn. The desired gas composition was successfully obtained in all treatments at 1 C while at 10 C the obtained levels were less than expected by about 3-5% CO2 and this might have been due to overestimation of respiration rate while designing the tubes for PM-MAP, which led to having steady state CO2 levels around 10, 17, and 22% CO2 instead of 15, 20, and 25% CO2, respectively. Sweetcorn stored at 1 C reached the expected CO2 level in about 5 d with slightly more time required for the higher concentrations. The sweetcorn stored at 10 C was faster in reaching the steady state (about 2 d) and this was expected due to the higher CO2 production rate at the higher temperature. deteriora subsequently better flavor and aroma.

PAGE 151

136 After 10 d of storage in PM-MAP, sugar content was higher in the sweetcorn stored at the lower temperature, but there were no significant differences among the treatments at 1 C. The only difference in sugar concentrations that was observed was in the sweetcorn stored at 10 Cntly lower sugar content than sweetcorn stored in 20 or 222sitions and maintaining them during a 10-d storage periodsevere loss in visual qualit with 15% CO2, which had significa 5% CO2. Also, PM-MAP was beneficial in maintaining the visual quality of all treatments with no sign of microbial growth or deterioration even at the higher temperature (10 C) and the weight loss was very slight in all treatment as a result of the high relative humidity surrounding the cobs. There was no significant production of ethanol or acetaldehyde even with high levels of CO and the low levels of O, which indicate that the sweetcorn cobs did not switch to anaerobic respiration. This work showed that using PM-MAP could be a very useful tool in sweetcorn storage at different storage temperatures. PM-MAP was mostly successful in generating the desired and expected gas compo while maintaining high sweetcorn quality. Fresh-Cut Sweetcorn Kernel Storage This work was carried out in an attempt to introduce fresh-cut sweetcorn kernels as a potential product for the fresh-cut produce industry. Fresh-cut sweetcorn is a very perishable commodity even more perishable than sweetcorn cobs and requires special care during preparation and storage to ensure maintaining quality. Fresh-cut sweetcorn was successfully stored up to 10 d without y even at 5 C. Also, dipping the kernels after cutting in chlorinated water was effective in controlling decay since there was no visual appearance of any microbial growth in these experiments.

PAGE 152

137 Maturity stage of the kernels also affected the quality mainly through its effect on the respiration rate. More mature sweetcorn kernels had a lower respiration rate than the less mature kernels. The respiration rate of kern els was also affected by temperature and gas cokernewere found in fresh-cut sweetcorn including p-couan in the free fraction. mposition. Kernels stored at 5 C had a higher respiration rate than those stored at 1 C, and using CA with 2% O2 plus 10% CO2 significantly reduced the respiration rate. As a result of respiration reduction, kernels stored at 1 C had higher sugar content at the end of the storage period than kernels stored at 5 C. Also, storing sweetcorn ls in 2% O2 plus 10% or 20% CO2 maintained higher sugar content than air storage. Fresh-cut sweetcorn kernels could serve as a good dietary source for polyphenolics and antioxidants, which might have several health benefits for the consumer and could help in reducing the risk of cancer and many chronic diseases. Several phenolic compounds maric acid, p-OH-benzoic acid, and ferulic acid, as well as several unidentified phenolic compounds that were present in low concentrations. The concentration of phenolic compounds was much higher in the bound fraction than the free fraction, but this probably does not impact the nutritional benefits because it has been shown for other food crops that bound phenolics are as nutritionally available as soluble phenolics. The antioxidant capacity followed a similar pattern and was also much higher in the bound phenolic fraction th Controlled atmosphere treatment did not effect the antioxidant capacity during the first 7 d of storage, but it reduced antioxidant capacity by day 10 compared with air storage. Despite these results, CA storage is essential to maintain the quality of this

PAGE 153

138 highlyred with 5 C storage. Also, using a secondary prese C and did not appear in the kerned at 5 C or in air) than in treatments with less or no browningt 1 C or in CA), which suppofore or after cooking, which perishable commodity and it is not recommended to use air storage instead of CA because of the many other benefits of atmosphere modification for quality preservation. This work indicated clearly that temperature is the most important factor in maintaining fresh-cut sweetcorn kernel quality. All quality factors evaluated were better maintained during storage at 1 C compa rvation factor such as CA was beneficial in maintaining the quality of fresh-cut sweetcorn The main problem with fresh-cut sweetcorn was severe browning of kernels after cooking. This browning also was more severe in the kernels from the more mature cobs. Both low temperature and CA were effective in controlling the after cooking browning since it appeared only in cooked kernels stored in air at 5 ls stored in CA (2% O2 plus 10% CO2) at either 1 or 5 C, or in samples stored in air at 1 C. Higher content of free amino acids were found in treatments with more severe browning (more mature kernels and kernels store (less mature kernels and kernels stored a rted the possibility of involvement of Maillard reaction in the after cooking browning. Nevertheless, it was concluded that Maillard reaction is not responsible for the after cooking browning since no 5-hydroxymethylfurfural (the intermediate of the Maillard reaction) was found in any of the sweetcorn samples be excludes Maillard reaction as a cause for the after cooking browning in fresh-cut sweetcorn

PAGE 154

139 On the other hand, there were no differences found in the phenolic profiles of the sweetcorn kernels from the different maturity, temperature, and atmosphere treatments, and there also was no trend or significant difference in the total soluble phenolics contents, which reduces the possibility of the phenolic acids as a cause of the after cooking browning. When the microbial load on the fresh-cut sweetcorn kernels were studied, it was found that the microbial load increase with storage duration and this increase was reduced using CA storage. Since the treatments that had more browning also had a higher microd three yeasts), the after cookiin a soluble form in order to measure it. The pigmeauthentic compounds we had available. bial load, it was thought that the after cooking browning might be caused by microbes, but this was proven to be not totally true. When fresh-cut kernels were inoculated separately with several microbes (five bacteria an ng browning started earlier and was more severe as a result of the increased microbial load, but this effect was not specific to any of the isolated microbes used to inoculate the sweetcorn kernels. These results indicate that after cooking browning is not caused directly by the presence of a specific microorganism, but microbial load could be responsible for after cooking browning in an indirect way, such as by increasing the kernel deterioration rate. Separation of kernel juices from the water insoluble cellular materials allowed formation of the brown pigment to occur nt was found to be associated with the acetone insoluble fraction of the sweetcorn kernel juice. The attempt to identify the brown pigment compound(s) failed since this pigment was very water soluble and it eluted very fast from the HPLC before any of the

PAGE 155

140 In conclusion, fresh-cut sweetcorn kernels can be successfully stored for 10 d in CA with little loss of visual or sensory quality, especially at the lower temperature o f 1 C. e after cooking browning that occurs in stored fresh-cut sweetcorn kernels. In this ss the information we know about the after cooking browning in fresh-cut sweetd, the kernels did brown. This may be the n that the after cooking browning was reduced by CA treatment. This also excludes aerobic microbes as a cause of browning since storing kernels in sealed bags without air for 10 d promoted excessive anaerobic microbial growth. Controlled atmosphere was more critical at the higher temperature (5 C), since after cooking browning was totally controlled by storage in 2% O2 plus 10% CO2. After Cooking Browning, What Is It and What Is It Not? Several preliminary experiments have been conducted in an effort to identify the cause of th ection, a summary of this work and some of the results obtained are presented, followed by some of the available information about after cooking browning. This information is presented in the hope that it may serve as the basis for future work on the same project. After cooking browning, what is it? Here i corn: 1. The browning was never observed to develop on fresh kernels during storage. It occurs only upon exposure to heat, such as during heating in boiling water. 2. After cooking browning occurs only in stored, wounded kernels; it does not occur in freshly cut (i.e., not stored) kernels; neither does it occur in uninjured kernels, whether they are stored attached to the cob or removed from the cob. 3. Development of the disorder is slowed down by low storage temperature. There was no after cooking browning in kernels stored at 1 C for 10 d. It requires storage of cut kernels for at least 4 d at 5 C before the after cooking browning will start to show up. 4. Development of the disorder is oxygen dependant when kernel samples were stored without air for 10 d at 5 C the kernels did not brown, but if the samples were exposed to air after that for just 1 reaso

PAGE 156

141 5. precipitated by acetone. The browning reaction could be enzymatic or the brown crudely concentrate enzymes for further purification. 6. The browning precursors (the pellets that were obtained after precipitation of insoluble in acetone, ethanol, hexane, and ethyl acetate. No other solvent was solubilizes the brown pigment precursors. Also, very small amounts of acetone insoluble solids from stored sweetcorn kernels create a lot of brown soluble pectin, and/or galacturonic acid; their role in the browning was not me of the things that could be excluded as the cause of the after ccell wall material did not brown when heated. 2. It is not native starch (non-gelatinized) or starch-related since most of the native in sweetcorn there are amino acids present, es are present in the precipitate (tested positive in the phenol-sulfuric acid assay although it was not quantified). The brown pigment precursor occurs in the juice of stored sweetcorn kernels and is pigment may be proteinaceous, since acetone precipitates are common ways to sweetcorn juice with acetone) are soluble in water and methanol (not 100%), and identified that pigment. 7. The browning reaction requires high temperature, i.e. around 100 C the browning didnt appear in kernels heated for 10 min in a water bath at 70 C and the severity of the browning increased with increasing the water temperature to boiling. 8. Once the brown pigment was created during boiling it was not soluble and it couldnt be solubilized using any solvent (water, ethanol, acetone, ethyl acetate, etc.) that was tried. 9. The less the water used for boiling, the faster and darker the browning develops. It is a very fast reaction when kernel juice precipitate is boiled, it starts to brown after about 20 s. 10. This reaction is also slowed down by adding a high concentration of reducing agents (5% ascorbic acid or 1-3% SO2) to the boiling water. 11. The extract does contain some carbohydrates (glucose, sucrose, soluble starch, identified). It also has some polyphenolics. After cooking browning, what is it not? The following are so ooking browning: 1. The browning is not associated with cell wall bound compounds since isolated starch was excluded by the centrifugation of the kernel juice. 3. Most probably it is not a Maillard reaction (a non-enzymatic reaction associated with thermal processing of food products) despite the fact that the substrates of the Maillard reaction are presentespecially tryptophan, and also soluble carbohydrat

PAGE 157

142 The reason for ruling out the Maillard reaction is that there was no hydroxymethyl Also, when the acetone insoluble material of sweetcorn juice was re-dissolved in which also rules out a Maillard reaction. 4. Most probably the browning is not phenolic related since all the cell wall furfural (HMF) found, which is the characteristic product of Maillard reactions. a small amount of water, it could turn brown at room temperature without heating, bound phenolics were removed and also most of the soluble phenolics. There is a very low coract, but it is doubtful it has an effect. 5. It is not a microbial reaction, or at least not directly, since the browning was not the more browning appears, which might indicate an indirect effect of microbes. ncentration of unidentified polyphenolics present in the ext caused by a specific microbe, but as a general rule the higher the microbial load,

PAGE 158

LIST OF REFERENCES y, S.D., E.K. Heaton, D.A Golden, and L.R. Beuchat. 1988. Microbiological and sensory quality in unwrapped and wrapped sliced watermelon. J. Food Protection. 51:531-533. AbbeamomAgar, I.T., B. Massantini, B. Hess, and A.A. Kader. 1999. Postharvest CO2 and ethylene Agar, I.T., J. Streif, and F. Bangerth. 1997. Effect of high CO2 and controlled atmosphere Zottola. 1991. Sulfhydryl and ascorbic acid ood Sci. 56:427-430. Amiot, M.J.ar, d storage conditions on phenolic composition and enzymatic m. 43:1132-1137. esRm Ads, M.R., A.D. Hartley, and L.J. Cox. 1989. Factors affecting the efficacy of washing procedures used in the production of prepared salads. Food Microbiol. 6:69-77. Ad, K.K. and R.H. Liu. 2002. Antioxidant activity of grains. J. Agr. Food Chem. 50:6182 6187 production and quality maintenance of fresh-cut kiwifruit slices. J. Food Sci. 64:433-440. (CA) on the ascorbic and dehydroascorbic acid content of some berry fruits. Postharvest Biol. Technol. 11:47-55 Aharoni, A., C. Azica, M. Gil, and E. Falik. 1996. Polyolefin stretch films maintain quality of sweet corn during storage and shelf-life. Postharvest Biol. Technol. 7:171-176. Albrecht, J.A., H.W. Schafer, and E.A. relationship in selected vegetables and fruits. J. F M. Tacchini, S.Y. Aubert, and W. Oleszek. 1995. Influence of cultivmaturity stage, anbrowning of pear fruits. J. Agr. Food Che Anzueto, C.R. and S.S.H. Rizvi. 1985. Individual packaging of apples for shelf life xtension. J. Food Sci. 50:897-905 Appleman, C.O. and J.M. Arthur. 1919. Carbohydrate metabolism in green corn during torage at different temperatures. J. Agr. Res. 17:137-152. Avena-Bustillos, R.J., J.M. Krochta, and M.E. Saltveit. 1997. Water vapor resistance of ed Delicious apples and celery sticks coated with edible caseinate-acetylated onoglyceride films. J. Food Sci. 62:351-354. 143

PAGE 159

144 Babic, c3Babic, S. Aubert. 1993b. Accumulation of chlorogenic acid in shredded carrots during storage in an oriented polypropylene fBabic, affected by controlled atmospheres. Postharvest Biol. Technol. 9:187-193. cBaldwin, E.A. 2002. Fr altveit (eds.). USDA Handbook No. 66. The commercial storage of fruits, vegetables, and florist and nursery stocks. Agr. Handbook 66. U.S. Dept. Agr., Washington, D.C. (May 8, 2004) . Baldwin, E.A., J.K. Burns, W. Kazokas, and J.K. Brecht. 1998. Effect of coating on mango (Mangifera indica L.) flavor. Proc. Fla. State Hort. Soc. 111:247-250. Baldwin, E.A., M.O. Nisperos-Carriedo, and R.A. Baker. 1995a. Use of edible coatings to preserve quality of lightly (and slightly) processed products. CRC Crit. Rev. Food Sci. Nutr. 35:509-524. Baldwin, E.A., M.O. Nisperos-Carriedo, and R.A. Baker. 1995b. Edible coatings for lightly processed fruits and vegetables. HortScience 30:35-38. Baldwin, E.A., M.O. Nisperos-Carriedo, X. Chen, and R.D. Hagenmaier. 1996. Improving storage life of cut apple and potato with edible coating. Postharvest Biol. Technol. 9:151-163. Ballantyne, A., R. Stark, and J.D. Selman. 1988. Modified atmosphere packaging of shredded lettuce. Intl. J. Food Sci. Technol. 23:267-274. Barry-Ryan-C. and D.O. OBeirne. 1999. Ascorbic acid retention in shredded iceberg lettuce as affected by minimal processing. J. Food Sci. 64:498-500. Barth, M.M., E.L. Kerbel, A.K. Perry, and S.J. Schmidt. 1993. Modified atmosphere packaging affects ascorbic acid, enzyme activity and market quality of broccoli. J. Food Sci. 58:140-143. Barth, M.M., A.K. Perry, S.J. Schmidt, and B.P. Klein. 1990. Misting effects on ascorbic acid retention in broccoli during cabinet display. J. Food Sci. 55:1187-1188. I., M.J. Amiot, C. Nguyen-The, and S. Aubert. 1993a. Changes in phenolic ontent in fresh ready-to-use shredded carrots during storage. J. Food Sci. 58:351-56. I., M.J. Amiot, C. Nguyen-The, and ilm. J. Food Sci. 58:840-841. I. and A.E. Watada. 1996. Microbial populations of fresh-cut spinach leaves Bailey, D.M. and R.M. Bailey. 1938. The relation of the pericarp to tenderness in sweet orn. Proc. Amer. Hort. Sci. 36:555-559 uit and vegetable flavor. In: K.C. Gross, C.Y. Wang, and M.A. S

PAGE 160

145 Barth, M.M. and H. Zhuang. 1996. Packaging design affects antioxidant vitamin retention and quality of broccoli florets during postharvest storage. Postharvest Biol. Technol. 9:141-150. Bartz, J.A., M. Mahovic, and D. Concelmo. 2001. Rapid movement of inoculum into wounds on tomato fruit. Phytopathology 91:s6. (Abstr.) Bartz, J.A. and C.I. Wei. 2002. The influence of bacteria, p. 519-541. In: J.A. Bartz and Bayinidirli, A., S. Khalafi, and A. Yeniceri. 1995. Non-enzymatic browning in clarified Beaudry, R.M., A.C. Cameron, A. Shirazi, and D.L. Dostal-Lange. 1992. Modified atmosphere packaging of blueberry fruit: Effect of temperature on the package O Beaudry, R.M., E.R. Uyguanco, and T.M., Lennington. 1993. Relationship between N.Y., USA. C Press, Boca Raton, Fla. nd florist and nursery stocks. Agr. Handbook 66. U.S. Dept. Agr., Beit-H packaging BeuchLopez-Malo (eds.). Minimally processed fruits Beuchssing practices. Emerg. Biacsmins. Biochem. Soc. Trans. 28:839-845. J.K. Brecht (eds.). Postharvest physiology and pathology of vegetables. Marcel Dekker, New York. apple juice in high temperature. J. Food Process. Pres. 19:223-227. 2and CO2. J. Amer. Soc. Hort. Sci. 117:436-441. headspace and tissue ethanol levels of blueberry fruit and carrot roots in sealed HDPE packages, p. 87-94. In: Proc. 6th Intl. CA Research Conf., Vol. 1, Cornell Univ., Ithaca, Beaulieu, J.C. and E.A. Baldwin. 2002. Flavor and aroma of fresh-cut fruits and vegetables, p. 391-425. In: O. Lamikanra (ed.). Fresh-cut fruits and vegetables: Science, technology, and market. CR Beaulieu, J.C. and J.R. Gorny. 2002. Fresh-cut fruits. In: K.C. Gross, C.Y. Wang, and M.A. Saltveit (eds.). USDA Handbook No. 66. The commercial storage of fruits, vegetables, a Washington, D.C. (May 8, 2004) . alachmy, I. and C.H. Mannheim. 1992. Is modified atmosphere beneficial for fresh mushrooms?. Lebensm. Wiss. u. Technol. 34:426-434. at, L.R. 1996. Pathogenic microorganisms associated with fresh produce. J. Food Protect. 59:204-216. Beuchat, L.R. 2000. Use of sanitizers in raw fruit and vegetable processing, p. 63-78. In: S.M. Alzamora, M.S. Tapia, and A. and vegetables. Aspen Publishers, Gaithersburg, Md. at, L.R. and J-H. Ryu. 1997. Produce handling and proce Infect. Dis. 3:459-465. P.A. and H.G. Daood. 2000. Lipoxygenase-catalysed degradation of carotenoids from tomato in the presence of antioxidant vita

PAGE 161

146 Blanchard, M., F. Castaingne, C. Willemot, and J. Makhlouf. 1996. Modified atmosphere preservation of freshly prepared diced yellow onions. Postharvest Biol. Technol. 9:173 -185. Conf., Vol. 1, Cornell Univ., Ithaca, N.Y., USA. Boline storage stability of shredded lettuce. J. Food Sci. 42:1319-1321. Bracks and vegetables. J. Food Qual. 10:195-206. Brackables, p. 269-312. In: R.C. Wiley (ed.). Minimally processed refrigerated fruits and vegetables. Chapman & Hall, New York-London. Brechtables. HortScience 30:18-22. Brechits, vegetables, and florist and nursery stocks. Agr. Handbook 66. U.S. Dept. Agr., Washington, D.C. (May 8, Brecht, J.K., M.E. Saltveit, S.T. Talcott, K.R. Schneider, K. Felkey, and J.A. Bartz. 2004. Brecht. 1988. Quality of su1 and sh2 sweet corn in relation to delay between harvest and cooling and temperature after cooling. HortScience Brecht, J.K., S.A. Sargent, R.C. Hochmuth, and R.S. Tervola. 1990. Postharvest quality of supersweet (sh2) sweet corn cultivars. Proc. Fla. State Hort. Soc. 103:283-288. Blanpied, G.D. and Z. Jozwiak. 1993. A study of some orchard and storage factors that influence the oxygen threshold for ethanol accumulation in stored apples, p. 78-86. In: Proc. 6th Intl. CA Research Block, G. and L. Langseth. 1994. Antioxidant vitamins and disease prevention. Food Technol. 48(7):80-84. Bolin, H.R. and C.C. Huxsoll. 1989. Storage stability of minimally processed fruit. J. Food Biochem. 13:281-292. Bolin, H.R. and C.C. Huxsoll. 1991. Effect of preparation and storage parameters on quality retention of salad-cut lettuce. J. Food Sci. 56:60-62, 67. H.R., A.E. Stafford, A.D. King Jr., and C.C. Huxsoll. 1977. Factors affecting th ett, R.E. 1987. Microbiological consequences of minimally processed fruit ett, R.E. 1994. Microbiological spoilage and pathogens in minimally processed refrigerated fruits and veget t, J.K. 1995. Physiology of lightly processed fruits and vege t, J.K. 1999. Fresh-cut sweetcorn kernels. Citrus and Vegetable Magazine. 64(6):36-37. Brecht, J. K. 2002. Sweetcorn. In: K.C. Gross, C.Y. Wang, and M.A. Saltveit (eds.). USDA Handbook No. 66. The commercial storage of fru 2004) < http : // www. ba.ars.usda.gov/hb66/index.html >. Fresh-cut vegetables and fruits. Hort. Rev. 30:185-251. t, J.K. and S.A. Sargen 21:698. (Abstract)

PAGE 162

147 Brecht, P.E. 1980. Use of controlled atmosphere to retard deterioration of produce. Food Technol. 34(3):45-49. Bunzel, M., J. Ralph, J. M. Martia, R. D. Hatfield, and H. Steinhart. 2001. Diferulates as Buta, J.G., H.E. Moline, D.W. Spaulding, and C.Y. Wang. 1999. Extending storage life CameN.H. Banks, and M.V. Yelanich. 1994. Modified-atmosphere packaging of blueberry fruit: Modeling respiration and package oxygen Cantwr fresh produce, p. 511-518. In: A.A. Kader (ed). Postharvest technology of horticultural crops. Univ. Cao, ood Sci. 55:1033-1038. Charron, C.S. and D.J. Cantliffe. 1995. Volatiles emissions from plants. Hort. Rev. 17:43-72. ChauCherry, J.P.icrobials. Food Chervin, C. 1994. Quality maintenance of ready-to-eat shredded structural components in soluble and insoluble cereal dietary fiber. J. Sci. Food Agr. 81:653-660. Burg, S.P., K.V. Thimann. 1959. The physiology of ethylene formation in apples. Proc. Natl. Acad. Sci. 45:335-344 of fresh-cut apples using natural products and their derivatives. J. Agr. Food Chem. 47:1-6. Buttery, R.G. 1981. Vegetables and fruit flavors, p. 175-216. In: R. Teranishi, R.A. Flath, and H. Sugisawa (eds.). Flavor research, Recent advances. Marcel Dekker, New York. Buttery, R.G., R. Teranishi, and L.C. Ling. 1987. Fresh tomato aroma volatiles A quantitative study. J. Agr. Food Chem. 35:540-544. ron A.C., R.M. Beaudry, partial pressure as function of temperature. J. Amer. Soc. Hort. Sci. 119:534-539. ell, M.I. 2002. Summary table of optimal handling conditions fo Calif., Div. Agr. Natural Resources, Berkeley, Publication 3311. G., C. Verdon, A. Wu, H. Wang, and R. Prior. 1995. Automated oxygen radical absorbance capacity assay using the COBAS FARA II. Clin. Chem. 41:1738-1744. Carlin, F., C. Nguyen, G. Helbert, and Y. Chambroy. 1990. Modified atmosphere packaging of fresh ready to use grated carrots in polymeric films. J. F K.V. 2001. Tube design program for PM-MAP. Unpublished computer program. 1999. Improving the safety of fresh produce with antim Technol. 53(11):54, 56, 59. and P. Boisseau. carrots by gamma irradiation. J. Food Sci. 59:359-361.

PAGE 163

148 Chinnan, M.S. 1989. Modeling gaseous environment and physicochemical changes of fresh fruit and vegetables in modified atmosphere storage, p. 189-201. In: J.J. Jen (ed.). Qualit y factors of fruit and vegetables. Amer. Chem. Soc., Washington, D.C. Cisneros-Zevallos, L.A., M.E. Saltveit, and J.M. Krochta. 1995. Mechanism of surface 68-75. In: Proc. 7th Intl. CA Research Conf., Vol. 5, Univ. of Calif., Davis, Calif., USA. Clevif carotenoids from vegetables. HortScience 35:585-588. Cobb endosperm. Plant Physiol. 67:107-109. Court, and P.R. Mosely. 1988. Classification of vegetable corns. HortScience 23:449-450. CreecCulpepper, C.W. and C.A. Magoon. 1927. A study of the factors determining quality in Daniels, F.A., R. Krishnamurthi, and S.H. Rizvi. 1985. A review of effects of carbon Davidson, M. P. 2001. Chemical preservatives and natural antimicrobial compounds, p. Deak. Extending the shelf life of fresh sweet corn by shrink-wrapping, refrigeration, and irradiation. J. Food Sci. Delaquis, P.J., S. Stewart, P.M.A. Toivonen, and A.L. Moyls. 1999. Effect of warm, Dewanto, V., X. Wu, K.K. Adom, and R.H. Liu. 2002. Thermal processing enhances the Chipley, J.R. 1993. Sodium benzoates and benzoic acid, p. 11-48. In: P.M. Davidson and A.L. Branen (eds.). Antimicrobials in foods. 2nd ed. Marcel Dekker, New York. white discoloration of peeled (minimally processed) carrots during storage. J. Food Sci. 60:320-323, 333. Clarke, R. and C.P. De Moor. 1997. The future of film technology: A tunable packaging system for fresh produce, p. dence, B., I. Paetau, and J.C. Smith. 2000. Bioavailability o D.C. and J.A. Hannah. 1981. The metabolism of sugars in maize er J.W., A.M. Rhodes, D.L. Garwood h, R.G. 1968. Carbohydrate synthesis in maize. Adv. Agron. 20:275-322. sweet corn. J. Agr. Res. 34:413-433. dioxide on microbial growth and food quality. J. Food Protect. 48:532-544. 593-628. In: M.P. Doyle, L.R. Beuchat, and T.J. Montville (eds.). Food microbiology: Fundamentals and frontiers. ASM Press, Washington, D.C. T., E.K. Heaton, Y.C. Hung, and L.R. Beuchat. 1987 52:1625-1631. chlorinated water on the microbial flora of shredded iceberg lettuce. Food Res. Intl. 32:7-14. nutritional value of tomatoes by increasing antioxidant activity. J. Agr. Food Chem. 50:3010-3014.

PAGE 164

149 Di Pentima, J.H., J.J. Rios, A. Clemente, and J.N. Olias. 1995. Biogenesis of off-odor in broccoli under low-oxy gen atmosphere. J. Agr. Food Chem. 43:1310-1313. e chemical and biochemical changes, p. 111-126. In: S.M. Alzamora, M.S. Tapia, and A. Lopez-Malo (eds.). Minimally processed fruits Dubois, M., K.A. Gilles, J.K. Hamilton, P.A. Rebers, and F. Smith. 1956. Colorimetric method for determination of sugars and related substances. Anal Chim. 28:350-356. Eberh 405:903-904. ev. 3:412-461. Emond, J.P. and K.V. Chau. 1990b. Use of perforation in modified atmosphere Emonntration profile in modified atmosphere bulk packaging. ASAE paper No. 92-6021. Evensfresh and stored sweet corn. J. Amer. Soc. Hort. Sci. 111:734-738. Examphere packaging of fruits and vegetables: Gas transfer properties and effect of temperature fluctuation. Acta Hort. 343:175-180 Fain, A.R. 1996. A review of the microbiological safety of fresh salads. Dairy Food Environ. Sanitation 16:146-149. Farkas, J., T. Saray, C. Mohacsi-Farkas, C. Horti, and E. Andrassy. 1997. Effects of low-9:111-119. Minn. Doehlert, D.C., T.M. Kuo, J.A. Juvik, E.P. Beers, and S.H. Duke. 1993. Characteristics of carbohydrate metabolism in sweet corn (sugary-1) endosperm. J. Amer. Hort. Sci. 108:661-666. Dorantes-Alvarez, L. and A. Chiralt. 2000. Color of minimally processed fruits and vegetables as affected by som and vegetables. Aspen Publishers, Gaithersburg, Md. ardt, M.V., C.Y. Lee, and R.H. Liu. 2000. Antioxidant activity of fresh apples. Nature El-Goorani, M.A. and N.F. Sommer. 1981. Effects of modified atmosphere on postharvest pathogens of fruit and vegetables. Hort. R Emond, J.P. and K.V. Chau. 1990a. Effect of package physical properties on gas concentration profiles in MA packaging. ASAE paper No. 90-6511. packaging. ASAE paper No. 90-6512. d, J.P., K.V. Chau, and J.K. Brecht. 1992. Modeling of gas conce en, K.B. and C.D. Boyer. 1986. Carbohydrate composition and sensory quality of a, A., J. Arul, R. Lencki, and Z. Li. 1993. Suitability of various plastic films for modified atmos dose gamma irradiation on shelf life and microbiological safety of precut/prepared vegetables. Adv. Food Sci. 1 Farr, D.F., G.F. Bills, G.P. Chamuris, and A.Y. Rossman. 1989. Fungi on plants and plant products in the United States. APS Press, St. Paul,

PAGE 165

150 Flora, L.F. and R.C. Wiley. 1974a. Sweet corn aroma, chemical components, and relative importance in the overall flavor response. J. Food Sci. 39:770-773. Flora, L.F. and R.C. Wiley. 1974b. Influence of cultivar, process, maturity and planting Foley, D.M., A. Dufour, L. Rodriguez, F. Caporaso, and A. Prakash. 2002. Reduction of Fonseca, S.C., F.A.R. Oliveira, K.V. Chau, and J.K. Brecht. 1997. Modeling the effects ing, p. 77-82. In: Proc. 7th Intl. CA Research Conf., Vol. 5, Univ. of California, Davis, Calif., USA. Fonseca, S.C., F.A.R. Oliveira, I.B.M. Lino, J.K. Brecht, and K.V. Chau. 2000. Modeling O and CO exchange for development of perforation-mediated modified Forney, C.F. and W.J. Lipton. 1990. Influence of controlled atmospheres and packaging injury of Fornepounds produced by Francfety of Garcigetables: Garwnsformations and processed quality of high sugar maize genotypes. J. Amer. Soc. Hort. Sci. 101:400-404. Geesore packaging to extend the shelf life of tomatoes. J. Food Technol. Gersh. date on the dimethyl sulfide and hydrogen sulfide in sweet corn. J. Agr. Food Chem. 22:816-819. Folchi, A., G.C. Pratella, S.P. Tian, and P. Bertolini. 1995. Effect of low oxygen stress in apricot at different temperatures. Intl. J. Food Sci. 3:245-253. Escherichia coli 0157:H7 in shredded iceberg lettuce by chlorination and gamma irradiation. Radiation Physics Chem. 63:391-396. of perforation dimensions and bed porosity on gas exchange in the perforation-mediated modified atmosphere packag 22atmosphere packaging. J. Food Eng. 43:9-15. on chilling sensitivity, p. 257-267. In: Wang, C.Y. (ed.). Chilling horticultural crops. CRC Press, Boca Raton, Fla. y, C.F., J.P. Mattheis, and R.K. Austin. 1991. Volatile com broccoli under anaerobic conditions. J. Agr. Food Chem. 39:2257-2259. is, G.A., C. Thomas, and D. O'Beirne. 1999. The microbiological sa minimally processed vegetables. Intl. J. Food Sci. Technol. 34:1-22. a, E. and D.M. Barrett. 2002. Preservative treatments for fresh-cut fruits and vegetables, p. 267303. In: O. Lamikanra (ed.). Fresh-cut fruits and ve Science, technology, and market. CRC Press, Boca Raton, Fla. ood, D.L., F.J. McArdle, S.F. Vanderslice and J.C. Shannon. 1976. Postharvest carbohydrate tra n, J.D., K.M. Browne, K. Maddison, J. Shepherd, and F. Guaraldi. 1985. Modified atmosphe 20:339-349. off, S.N. 1993. Vitamin C (ascorbic acid): New roles, new requirements? Nutr. Rev. 51:313-326

PAGE 166

151 Gil, M.I., M. Castaner, F. Ferreres, F. Artes, and F.A. Toms-Barbern. 1998a. Modified-atmosphere packaging of minimally processed Lollo Rosso (Lactuca sa tiva) Phenolic metabolites and quality changes. Z. Lebensm. Unters. Forsch. 206:350-Gil, M, and F.A. Toms-Barbern. 1998b. Effect of modified atmosphere packaging on the flavonoids and vitamin C content of minimally processed Swiss Gil, M and vitamin C) of fresh-cut spinach. J. Agr. Food Chem. 47:2213-2217. Gil, Mother polyphenols in response to carbon dioxide treatments. J. Agr. Food Chem. Gil, M.I., F.A. Tomas-Barberan, B. Hess-Pierce, and A.A. Kader. 2002. Antioxidant 6-4982. d grape juice. Lebensm. Wiss. u. Technol. 31:196-200. Gornyuality changes in c. 6th Intl. CA Research Conf., Vol. 1, Cornell Grunaine in corn. J. Plant Nutrition 14:653-662. 67. 354. .I., F. Ferreres chard (Beta vulgaris subspecies cycla). J. Agr. Food Chem. 46:2007-2012. .I., F. Ferreres, and F.A. Toms-Barbern. 1999. Effect of postharvest storage and processing on the antioxidant constituents (flavonoids .I., D.M. Holcroft, and A.A. Kader. 1997. Changes in strawberry anthocyanins and 45:1662-1667. capacities, phenolic compounds, carotenoids, and vitamin C contents of nectarine, peach, and plum cultivars from California. J. Agr. Food Chem. 50:497 Goddard, M.S., and R.H. Matthews. 1979. Contribution of fruits and vegetables to human nutrition. HortScience 14:245-247. Gogus, F., H. Bozkurt, and S. Eren. 1998. Kinetics of Maillard reaction between the major sugars and amino acids of boile J.R., B. Hess-Pierce, R.A. Cifuentes, and A.A. Kader. 2002. Q fresh-cut pear slices as affected by controlled atmospheres and chemical preservatives. Postharvest Biol. Technol. 24:271-278. Gorny, J.R., B. Hess-Pierce, and A.A. Kader. 1999. Quality changes in fresh-cut peach and nectarine slices as affected by cultivar, storage atmosphere and chemical treatments. J. Food Sci. 64:429-432. Gran, C.D. and R.M. Beaudry. 1993. Modified atmosphere packaging determination of lower oxygen limits for apple fruit using respiratory quotient and ethanol accumulation, p. 54-62. In: Pro Univ., Ithaca, N.Y., USA. u, J. A. and J.M. Swiader. 1991. Chromatographic quantitation of free amino acids: S-methylmethionine, methionine, and lys Gunes, G., J.H. Hotchkiss, and C.B. Watkins. 2001. Effects of gamma irradiation on the texture of minimally processed apple slices. J. Food. Sci. 66:63

PAGE 167

152 Gunes, G., R.H. Liu, and C.B. Watkins. 2002. Controlled-atmosphere effects on postharvest quality and antioxidant activity of cranberry fruits. J. A gr. Food Chem. 50:5932-5938. Gunetion of apple slices. J. Sci. Food Agr. 80:1169-1175. 2868. Hansen, M., R.G. Buttery, D.J. Cantwell, and L.C. Ling. 1992. Broccoli storage under Hardenburg, R.E., A.E. Watada, and C.Y. Wang. 1986. The commercial storage of fruit, Heard, G.M. 1999. Microbial safety of ready-to-eat salads and minimally processed Heard G.M. 2002. Microbiology of fresh-cut produce, p. 187-248. In: O. Lamikanra Hertog, M.G.L., E.J.M. Feskens, P.C.L. Hollman, M.B. Katan, and D. Kromhout. 1994. Hertog, M.G.L., P.C.L. Hollman, and M.B. Katan. 1992. Content of potentially Hertog, M.G.L., P.M. Sweetnam, A.M. Fehily, P.C. Elwood, and D. Kromhout. 1997. y. Amer. J. Clin. Nutr. 65:1489-1494. s, G., C.B. Watkins, and J.H. Hotchkiss. 2000. Effects of irradiation on respiration and ethylene produc Hagenmaier, R.D. and R.A. Baker. 1997. Low-dose irradiation of cut iceberg lettuce in modified atmosphere packaging. J. Agr. Food Chem. 45:2864Hagenmaier, R.D. and R.A. Baker. 1998. Microbial population of shredded carrot in modified atmosphere packaging as related to irradiation treatment. J. Food Sci. 63:162-164. Hanotel, L., A. Fleuriet, and P. Boisseau. 1995. Biochemical changes involved in browning of gamma-irradiated cut Witloof chicory. Postharvest Biol. Technol. 5:199-210. low oxygen atmosphere: Identification of higher boiling volatiles. J. Agr. Food Chem. 40:850-852. vegetables, and florist and nursery stock. Agriculture Handbook Number 66, U.S. Dept. Agr., Washington, D.C. vegetables and fruits. Food Austral. 51:414-420. (ed.). Fresh-cut fruits and vegetables: Science, technology, and market. CRC Press, Boca Raton, Fla. Heaton, J.W., R.Y. Yada, and A.G. Marangoni. 1996. Discoloration of coleslaw is caused by chlorophyll degradation. J. Agr. Food Chem. 44:395-398. Dietary flavonoids and cancer risk in the Zutphen elderly study. Nutr. Cancer 22:175184. anticarcinogenic flavonoids of 28 vegetables and 9 fruits commonly consumed in the Netherlands. J. Agr. Food Chem. 40:2379-2383. Antioxidant flavonols and ischemic heart disease in a Welsh population of men: The caerphilly stud

PAGE 168

153 Hintlian, C.B. and J.H. Hotchkiss. 1986. The safety of modified atmosphere packaging: A review. Food Technol. 40(12):70-76. Holcroft, D.M., M.I. Gil, and A.A. Kader. 1998. Effect of carbon dioxide on anthocyanins, phenylalanine ammonia lyase and glucosyltransferase in the arils of Holcrlor and anthocyanin synthesis of stored strawberry fruit. HortScience 34:1244-1248. HosoHortScience 20:290-291. Howard, L.R. and T. Dewi. 1995. Sensory, microbiological and chemical quality of mini-Howa the composition and sensory quality of mini-peeled carrots. J. Food Sci. 61:643-645, Howard, L.R. and L.E. Griffin. 1993. Lignin formation and surface discoloration of Howard, L.R. and C. Hernandez-Brenes. 1998. Antioxidant content and market quality of jalapeno pepper rings as affected by minimal processing and modified atmosphere Hu, FHurstrocessed fruits and vegetables. HortScience Izumi, H. and A.E. Watada. 1994. Calcium treatments affect storage quality of shredded Izumi stored pomegranates. J. Amer. Soc. Hort. Sci. 123:136-140. oft, D.M., and A.A. Kader. 1999. Carbon dioxide-induced changes in co ki, T., H. Hiura, and M. Hamada. 1985. Breaking bud dormancy in corms, tubers, and trees with sulfur-containing compounds. Howard, L.R., D. D. Braswell, and J. Aselage. 1996. Chemical composition and color of strained carrots as affected by processing. J. Food Sci. 61:372-330 peeled carrots as affected by edible coating treatment. J. Food Sci. 60:142-144 rd, L.R., and T. Dewi. 1996. Minimal processing and edible coating effects on 651. minimally processed carrot sticks. J. Food Sci. 58:1065-1067, 1072. packaging. J. Food Qual. 21:317-327. .B., E.B. Rimm, M.J. Stampfer, A. Aschero, D. Spiegelman, and C. Willett. 2000. Prospective study of major dietary patterns and risk of coronary heart disease in men. Amer. J. Clin. Nutr. 72:912-921. W.C. 1995. Sanitation of lightly p30:22-24. International Fresh-cut Produce Association (IF PA). 2000. Fresh-cut facts. (May 8, 2004) < http : // www. fresh-cuts.org/fcf.html >. carrots. J. Food Sci. 59:106-109. H. and A.E. Watada. 1995. Calcium treatment to maintain quality of zucchini squash slices. J. Food Sci. 60:789-793.

PAGE 169

154 Jiang, Y.M. and J.R. Fu. 1999. Postharvest browning of litchi fruit by water loss and its prevention by controlled atmosphere storage at high relative humidity. Lebensm Wiss. U. Techno l. 32:278-283. espiration response to reduced oxygen, enhanced carbon dioxide, and temperature. J. Amer. Kader, A.A. 1980. Prevention of ripening in fruits by use of controlled atmospheres. Kader, A.A. 1985. Ethylene-induced senescence and physiological disorders in harvested horticultural crops. HortScience 20:54-57. Kadend vegetables. Food Technol. 40(6):117-121. ral crops. Univ. Calif., Div. Agr. Natural Resources, Berrkeley, Publication 3311. Kade(ed.). Fresh-cut fruits and vegetables: Science, technology, and market. CRC Press, Boca Raton, Fla. Kadere packaging of fruits and vegetables. CRC Crit. Rev. Food Sci. Nutr. 28:1-30. Kays,SA. Joles, D.W., A.C. Cameron, A. Shirazi, P.D. Petracek, and R.M. Beaudry. 1994. Modified atmosphere packaging of Heritage red raspberry fruit: R Soc. Hort. Sci. 119:540-545. Food Technol. 34(3):51-54. r, A.A. 1986a. Potential application of ionizing radiation in postharvest handling of fresh fruits a Kader, A.A. 1986b. Biochemical and physiological basis for effects of controlled and modified atmospheres on fruits and vegetables. Food Technol. 40(5):99-100 102-104. Kader, A.A. 1987. Respiration and gas exchange of vegetables, p. 30-31. In: J., Weichmann (ed.). Postharvest physiology of vegetables. Marcel Dekker, New York. Kader, A.A. 1992. Postharvest biology and technology: An overview, p.15-20. In: A.A. Kader (ed.). Postharvest technology of horticultu r, A.A. 2002. Quality parameters of fresh-cut fruit and vegetable products, p. 11-20. In: O. Lamikanra r, A.A., D. Zagory, and E.L. Kerbel. 1989. Modified atmosphe S.J. 1999. Preharvest factors affecting appearance. Postharv. Biol. Technol. 15:233-247. Ke, D., M. Mateos, and A.A. Kader. 1993. Regulation of fermentative metabolism in fruits and vegetables by controlled atmospheres, p. 63-77. In: Proc. 6th Intl. CA Research Conf., Vol. 1, Cornell Univ., Ithaca, N.Y., U Kim, D.M., N.L. Smith, and C.Y. Lee. 1993. Apple cultivar variations in response to heat treatment and minimal processing. J. Food Sci. 58:1111-1114, 1124.

PAGE 170

155 Kim, D.M., N.L. Smith, and C.Y. Lee. 1994. Effect of heat treatment on firmness of apples and apple slices. J. Food Process. Pres. 18:1-8. Klein, B.P. 1987. Nutritional consequences of minimal processing of fruits and Kohlmeier, L., N. Simonsen, and K. Mohus. 1995. Dietary modifiers of carcinogenesis. Korsten, L. and F.C. Wehner. 2002. Fungi, p. 485-518. In: J.A. Bartz and J.K. Brecht -life foods. J. Food Proc. Pres. 13:1-69. Lange7-490. of maize. Genetics 38:485-499. rvest Biol. Technol. 20:201-220. nal. Biochem. 14:71-77. tharvest Biol. Technol. 14:51-60. King, A.D. and H.R. Bolin. 1989. Physiological and microbiological storage stability of minimally processed fruits and vegetables. Food Technol. 43(2):132-135; 139. vegetables. J. Food Qual. 10:179-193. Environ. Health Perspect., 103 (Suppl 8):177184. (eds.), Postharvest physiology and pathology of vegetables. Marcel Dekker, New York. Labuza, T.P., and W.M. Breene. 1989. Application of active packaging for improvement of shelf life and nutritional quality of fresh and extended shelf D.L. 2000. New film technologies for horticultural products. HortTechnology. 10:48 Langerak, D.I. 1975. The influence of irradiation and packaging on the keeping quality of prepared cut endive, chicory and onions. Acta Alimentaria 4:123-138 Langiou, P., E. Samoli, A. Lagiou, J. Peterson, A. Tzonou, J. Dwyer and D. Trichopoulos. 2004. Flavonoids, vitamin C and adenocarcinoma of the stomach. Cancer Causes Control 15:67-72. Larsen, M. and C.B. Watkins. 1995. Firmness and concentrations of acetaldehyde, ethyl acetate and ethanol in strawberries stored in controlled and modified atmospheres. Postharvest Biol. Technol. 5:39-50. Laughnan, J.R. 1953. The effect of the sh2 factor on carbohydrate reserves in the mature endosperm Lee, S. K. and A.A. Kader. 2000. Preharvest and postharvest factors influencing vitamin C content of horticultural crops. Postha Lee, Y.P. and T. Takahashi. 1966. An improved colorimetric determination of amino acids with the use of ninhydrin. A Li, P. and M.M. Barth. 1998. Impact of edible coatings on nutritional and physiological changes in lightly processed carrots. Pos

PAGE 171

156 Lidster, P.D., M.A. Tung, M.R. Garland, and, S.W. Porritt. 1979. Texture modification of processed apple slices by a postharvest heat treatment. J. Food Sci. 44:998-1000, 1007. Lin, C.H. and C.I. Wei. 1997. Transfer of Salmonella montevideo onto the interior surfaces of tomatoes by cutting. J. Food Protect. 60:858-863. Guzman, I. and D.M. Barrett Luna-. 2000. Comparison of calcium chloride and calcium lactate effectiveness in maintaining shelf stability and quality of fresh-cut Luna-Guzman, I., M. Cantwell, and D.M. Barrett. 1999. Fresh-cut cantaloupe: Effects of wines. Crit. Rev. Food Sci. Nutr. 30:441-486. Mainrmness and quality characteristics of whole and sliced strawberries after freezing and thermal processing. J. Food Sci. 51:391-394. Manns for modified atmosphere storage of fresh produce, p. 178-186. Proc. 5th Intl. CA Research Conf., Vol. 2, Washington State Univ., Pullman, Wash., USA. Manz in different atmospheres. J. Sci. Food Agr. 67:521-529 Marrolism. J. Agr. Food Chem. 39:1602-1605. Maulent, C.A. Sims, E.A. Baldwin, M.O. Balaban, and D.J. Huber. 2000. Tomato flavor and aroma quality as affected by storage temperature. J. Food Sci. McCaf fruits and vegetables subject to minimal processes, p. 313-326. In: R.C. Wiley (ed.). Minimally Miller, K.S. and J.M. Krochta. 1997. Oxygen and aroma barrier properties of edible films: A review. Trends Food Sci. Technol. 8:228-237. cantaloupes. Postharvest Biol. Technol. 19:61-72. CaCl2 dips and heat treatments on firmness and metabolic activity. Postharvest Biol. Technol. 17:201-213. Macheix, J.J., J.C. Sapis, and A. Fleuriet. 1991. Phenolic compounds and polyphenoloxidase in relation to browning in grapes and G.L., J.R. Morris and, E.J. Wehunt. 1986. Effect of preprocessing treatments on the fi apperuma. J.D., D. Zagory, R.P. Singh, and A.A. Kader. 1989. Design of polymeric package ano, M., B. Citterio, M. Maifreni, M. Paganssi and, G. Comi. 1995. Microbial and Sensory quality of vegetables for soup packaged w, D.A. 1998. Where are we with vitamin E? J. Thrombosis Thrombolysis 5:209-214. Mattheis, J.P., D.A. Buchanan, and J.K. Fellman. 1991. Change in apple fruit volatiles after storage in atmospheres inducing anaerobic metabo F., S.A. Sarg 65:1228-1237. rthy, M.A. and R.H. Matthews. 1994. Nutritional quality o processed refrigerated fruits and vegetables. Chapman & Hall, New York-London.

PAGE 172

157 Molins, R.A., Y. Motarjemi, and F.K. Kaferstein. 2001. Irradiation: A critical control point in ensuring the microbiological safety of raw f oods. Food Control 12:347-356. Morafied atmosphere packaging of sweet corn on cob. J. Food Proc. Pres. 18:279-293. MorriL. Main, and E.J. Wehunt. 1985. Effect of cultivar, postharvest storage, preprocessing dip treatments and style of pack on the Nakhasi, S., D. Schlimme, and T. Solomos. 1991. Storage potential of tomato harvested Nguyen-The, C., and F. Carlin. 1994. The microbiology of minimally-processed fresh fruit and vegetables. CRC Crit. Rev. Food Sci. Nutr. 34:371-401. Nicoln: A. Scalbert (ed.), Polyphenolic phenomena, INRA Editions, Paris. Nisperos, M.O. and E.A. Baldwin. 1996. Edible coatings for whole and minimally processed fruits and vegetables. Food Austral. 48:27-31. Nune Controlling temperature and water loss to maintain ascorbic acid levels in strawberries during Obenland, D.M., R.E. Rij, and A.G. Aung. 1995. Heat-induced alteration of methanethiol emission from anaerobic broccoli. J. Hort. Sci. 70:657-663. Olsenultivars. Sci. Hort. 44:179-189 d Sci. 58:623-626. en radical absorbance capacity (ORAC) and ferric reducing antioxidant power (FRAP) Paradis, C., F. Castaigne, T. Desrosiers, J. Fortin, N. Rodrigue, and C. Willemot. 1996. Park, W.P. and D.S. Lee. 1995. Effect of chlorine treatment on cut watercress and onion. J. Food Qual. 18:415-424. les-Castro, J., M.A. Rao, J.H. Hotchkiss, and D.L. Downing. 1994. Modi s, J.R., W.A. Sistrunk, C.A. Sims, G. processing quality of strawberries. J. Amer. Soc. Hort. Sci. 110:172-177. at breaker stage using modified atmosphere packaging. J. Food Sci. 56:55-59 as, J., V. Cheynier, A. Fleuriet, and M.A. Rouet-Mayer, 1993. Polyphenols and enzymatic browning, p. 165-175. I s, M.C.M., J.K. Brecht, A.M.M.B. Morais, and S.A. Sargent. 1998. postharvest handling. J. Food Sci. 63:1033-1036. J.K., J.E. Giles, and R.A. Jordan. 1990. Post-harvest carbohydrate changes and sensory quality of three sweet corn c Omary, M., R. Testin, S. Barefoot, and J. Rushing. 1993. Packaging effects on growth of Listeria innocua in shredded cabbage. J. Foo Ou, B., D. Huang, M. Hampsch-Woodill, J.A. Flanagan, and E.K. Deemer. 2002. Analysis of antioxidant activities of common vegetables employing oxyg assays: A comparative study. J. Agr. Food Chem. 50:3122-3128. Sensory, nutrient and chlorophyll changes in broccoli florets during controlled atmosphere storage. J. Food Qual. 19:303-316.

PAGE 173

158 Pesis, E., D. Aharoni, Z. Aharoni, R. Ben-Arie, N. Aharoni, and Y. Fuchs. 2000. Modified atmosphere and modified humidity packaging alleviates chilling injury sympto ms in mango fruit. Postharvest Biol. Technol. 10:195-200. 65:549-553. aling. J. Agr. Food Chem. 40:569-572. Riad, G.S. and J.K. Brecht. 2001. Fresh-cut sweetcorn kernels. Proc. Fla. State Hort. Soc. Riad,ced O2 with or without elevated CO2 and effects of controlled atmosphere storage on quality. Proc. Fla. Riad,packaging of sweetcorn. Proc. Fla. State Hort. Soc. 115:71-75. Riad,ted by controlled atmosphere storage. Acta Hort. 628:387-394. Rice-Evans, C.A., N.J. Miller, and G. Paganga. 1997. Antioxidant properties of phenolic compounds. Trends Plant Sci. 4:152-159. Richmeinhold, New York. Risse, L.A. and R.E. McDonald. 1990. Quality of supersweet corn film-overwrapped in Robb Poovaiah, B.W. 1986. Role of calcium in prolonging storage life of fruits and vegetables. Food Technol. 40(5):86-89. Prakash, A., A.R. Guner, F. Caporaso, and D.M. Foley. 2000. Effects of low-dose gamma irradiation on the shelf life and quality characteristics of cut Romaine lettuce packaged under modified atmosphere. J. Food Sci. Ramamurthy, M.S., B. Maiti, P. Thomas, and P.M. Nair. 1992. High-performance liquid chromatography determination of phenolic acids in potato tubers (Solanum tuberosum) during wound he Reyes, F.G.R., G.W. Varseveld, and M.C. Kuhn. 1982. Sugar composition and flavour quality of high sugar (shrunken) and normal sweet corn. J. Food Sci. 47:753-755. 114:160-163 G.S. and J.K. Brecht. 2003. Sweetcorn tolerance to redu State Hort. Soc. 116:390-393. G.S., J.K. Brecht, and K.V. Chau. 2002. Perforation-mediated modified atmosphere G.S., J.K. Brecht, and S.T. Talcott. 2003. Browning of fresh-cut sweetcorn kernels after cooking is preven ond, D.V. 1973. Sulfur compounds, p. 41-55. In: L.P. Miller (ed.). Phytochemistry. Van Nostrand R Rin, Y.L., J.M. Desmarchelier, P. Williams, and R. Delves. 2001. Natural levels of dimethyl sulfide in rough rice and its products. J. Agr. Food Chem. 49:705-709 trays. HortScience 25:322-324. s, P.G., J.A. Bartz, G. McFie, and N.C. Hodge. 1996. Causes of decay of fresh-cut celery. J. Food Sci. 61:444-448.

PAGE 174

159 Rodov, V., A. Copel, N. Aharoni, Y. Aharoni, A. Wiseblum, B. Horev, and Y. Vinokur. 2000. Nested modified atmosphere packages maintain quality of trimmed sweet corn during cold storage and the shelf life period. Postharvest Biol. Technol. 18:259-266. Rosence of sliced pear and strawberry fruits. J. Food Sci. 54:656-659. Rump maturity attained and changes in quality during storage. J. Sci. Food. Agr. 23:193-197. SadovHigh levels of microbial contamination of vegetables irrigated with wastewater by the Saltveit, M.E. 1989. A summary of requirements and recommendations for the controlled Saltveit, M.E. 1997. Physical and physiological changes in minimally processed fruits Press, London. Saltveit, M.E. 2000. Wound induced changes in phenolic metabolism and tissue Saltveit, M.E. 2002. Fresh-cut vegetables, p. 691-712. In: J.A. Bartz and J.K. Brecht Sargeell. 1994. Edible films reduce surface drying of peeled carrots. Proc. Fla. Sta. Hort. Soc. 107:245-Schou storage on visual quality aspects, sugar, and ethanol content of sweet corn, p. 78-86. Proc. 6th Intl. CA Research J.C. and A.A. Kader. 1989. Postharvest physiology and quality maintenan f, G., J. Mawson and H. Hansen. 1972. Gas chromatographic analysis of the soluble substances of sweet corn kernels as a method indicating the degree of ski, A.Y., B. Fattal, D. Goldberg, E. Katzenelson, and H.I. Shuval. 1978. drip method. Appl. Environ. Microbiol. 36:824-830. and modified atmosphere storage of harvested vegetables, p. 78-86. Proc. 5th Intl. CA Research Conf., Vol. 2, Washington State Univ., Pullman, Wash., USA. and vegetables, p. 205-220. In: F.A. Toms-Barbern and R.J. Robins (eds.). Phytochemistry of fruit and vegetables. Oxford University Saltveit, M.E. 1999. Effect of ethylene on quality of fresh fruits and vegetables. Postharvest Biol. Technol. 15:279-292. browning are altered by heat shock. Postharvest Biol. Technol. 21:61-69. (eds.), Postharvest physiology and pathology of vegetables. Marcel Dekker, New York. Salunkhe, D.K. and B.B. Desai. 1984. Postharvest biotechnology of vegetables. CRC Press, Boca Raton, Fla.. Sams, C.E. 1999. Preharvest factors affecting postharvest texture. Postharvest Biol. Technol. 15:249-254. nt, S.A., J.K. Brecht, J.J. Zoellner, E.A. Baldwin, and C.A. Campb 247. ten, S.P. 1993. Effect of temperature and CA Conf., Vol. 2, Cornell Univ., Ithaca, N.Y., USA.

PAGE 175

160 Shewfelt, R.L. 1986. Postharvest treatment for extending the shelf life of fruits and vegetables. Food Technol. 40(5):70-80, 89. Showalter, R.K. and L.W. Miller. 1962. Consumer preference for high-sugar sweet corn Silva, F.M., K.V. Chau, J.K. Brecht, and S.A. Sargent. 1999a. Tubes for modified Silva, F.M., K.V. Chau, J.K. Brecht, and S.A. Sargent. 1999b. Modified atmosphere Simpson, K.L., T.-C. Lee, D.B. Rodriguez, and C.O. Chichester. 1976. Metabolism in nen (eds.). Antimicrobials in foods. 2nd ed. Marcel Dekker, New York. Sosulon of phenolic acids in cereal and potato flours. J. Agr. Food Chem. 30:337340. Spaldeet corn stored in controlled atmospheres or under low pressure. J. Amer. Soc. Hort. Sci. 103:592-SteinmK.A., and J.D. Potter. 1996. Vegetables, fruit, and cancer prevention: A review. J. Amer. Diet. Assoc. 96:10271039. Talco and sensory quality of processed carrot puree as influenced by stress-induced phenolic compounds. J. Agr. Food Chem. Talcott, S.T., L. Howard, and C.H. Brenes. 2000. Antioxidant changes and sensory bility of fortified yellow passion fruit (Passiflora Tapie.D. Tew. 2004. The role of carotenoids in the prevention of human pathologies. Biomed. Pharmacother. 58:100-110. varieties. Proc. Florida State Hort. Soc. 75:278-280. atmosphere packaging of fresh fruits and vegetables: Effective permeability measurements. Applied Eng. Agric. 15:313-318. packaging for mixed loads of horticultural commodities exposed to two postharvest temperatures. Postharvest Biol. Technol. 17:1-9. senescent and stored tissues, p. 780-842. In: T.W. Goodwin (ed.). Chemistry and biochemistry of plant pigments 2nd ed. Academic Press, London. Sofas, J.N. and F.F. Busta. 1993. Sorbic acid and sorbates, p. 49-94. In: P.M. Davidson and A.L. Bra ski, F., K. Krygier, and L. Hogge. 1982. Free, esterified, and insoluble-bound phenolic acids. 3 Compositi ing, D.H., P.L. Davis, and W.F. Reeder. 1978. Quality of sw 595. etz, tt, S.T. and L.R. Howard. 1999. Chemical 47:1362-1366. properties of carrot puree processed with and without periderm tissue. J. Agr. Food Chem. 48:1315-1321. Talcott, S.T., S.S. Percival, J. Pittet-Moore, and C. Celoria. 2003. Phytochemical composition and antioxidant sta edulis). J. Agr. Food Chem. 48:1315-1321. ro, H., D.M. Townsend, and K

PAGE 176

161 Tatsumi, Y., A.E. Watada, and W.P. Wergin. 1991. Scanning electron microscopy of carrot stick surface determines cause of white translucent appearance. J. Food Sci. 56:1357-1359. Toivonen, P.M.A. 1997. Non-ethylene, non-respiratory volatiles in harvested fruits and Vamos-Vigyazo, L. 1981. Polyphenol oxidase and peroxidase in fruits and vegetables. Wangent or low O2 storage. J. Amer. Soc. Hort. Sci. 108:125-129. Wang444. g and its prevention. ACS Symp.Ser.600. Amer. Chem. Soc., Washington, D.C. Wiley, R.C. 1985. Sweet corn aroma: Studies of its chemical components and influence on flavor, p. 346-366. In: H.E. Pattee (ed.). Evaluation of quality of fruits and Williams, M.P. and P.E. Nelson. 1973. Effects of hybrids and processing on the dimethyl WilliaM.P. and P.E. Nelson. 1974. Kinetics of the thermal degradation of methylmethionine sulfonium ions in citrate buffers and in sweet corn and tomato Wily,n, p. 121-155. In: N.A.M. Eskin (ed.). Quality and preservation of vegetables, CRC Press, Boca Raton, Fla. vegetables: Their occurrence, biological activity and control. Postharvest Biol. Technol. 12:109-125. Tudela, J.A., E. Cantos, J.C. Espin, F.A. Toms-Barbern, and M.I. Gil. 2002a. Induction of antioxidant flavonol biosynthesis in fresh-cut potatoes. Effect of domestic cooking. J. Agr. Food Chem. 50:5925-5931. Tudela, J.A., J.C. Espin, and M.I. Gil. 2002b. Vitamin C retention in fresh-cut potatoes. Postharvest Biol. Technol. 26:75. CRC Crit. Rev. Food Sci. Nutr. 15:49-127. C.Y. 1983. Postharvest responses of Chinese cabbage to high CO2 treatm C.Y. and L. Qi. 1997. Modified atmosphere packaging alleviates chilling injury in cucumbers. Postharvest Biol. Technol. 10:195-200. Wann, E.V., G.B. Brown, and W.A. Hills. 1971. Genetic modification of sweet corn quality. J. Amer. Soc. Hort. Sci. 96:441Weichmann, J. 1986. The effect of controlled-atmosphere storage on the sensory and nutritional quality of fruits and vegetables. Hort. Rev. 8:101-127. Whitaker, J.R. and Lee, C.Y. 1995. Recent advances in chemistry of enzymatic browning: an overview, p. 2-7. In: C.Y. Lee and J.R. Whitaker, (eds.). Enzymatic brownin vegetables. AVI, Westport, Conn. sulfide potential of sweet corn. J. Food Sci. 38:1136-1138. ms, serum. J. Food Sci. 39:457-460. R.C., F.D. Schales, K.A. Corey. 1989. Sweet cor

PAGE 177

162 Wong, D.A., J.A. Juvik, D.C. Breeden, and J.M. Swiader. 1994. Shrunken2 sweet corn yield and the chemical components of qual ity. J. Amer. Soc. Hort. Sci. 119:747-755. Wrighd-atmosphere storage on the quality and carotenoid content of sliced persimmons and peaches. Postharvest Biol. Wrighnd controlled-atmosphere storage on the ascorbate content and quality of strawberries and persimmons. Postharvest Yahiassive and semi-active atmosphere to prolong the postharvest life of avocado fruit. Lebensm. Wiss. u. Technol. Zagorf fresh produce. Food Technol. 42(9):70-77. Zhengeres on blueberry phenolics, anthocyanins, and antioxidant capacity. J. Agr. Food Chem. 51:7162-7169. Ziegl t, K.P. and A.A. Kader. 1997a. Effect of controlle Technol. 10:89-97. t, K.P. and A.A. Kader. 1997b. Effect of slicing a Biol. Technol. 10:39-48. E.M. and G. Gonzalez-Aguilar. 1998. Use of pa 31:602-606. y, D. and A.A. Kader. 1988. Modified atmosphere packaging o Y., C.Y. Wang, S.Y. Wang, and W. Zheng. 2003. Effect of high-oxygen atmosph er, RG. 1991. Vegetable, fruits and carotenoids and the risk of cancer. Amer. J. Clin. Nutr. 53:251-259. Zink, D.L. 1997. The impact of consumer demands and trends on food processing. Emerging Infect. Dis. 3:467-469.

PAGE 178

BIOGRAPHICAL SKETCH Riad,untant; and he has only one brother, Gamal has been working as an assistant researcher in the National Research Center, Giza,ry and secondary education in Cairo, was a PhD program in the samnited Florid Gamal Riad was born on October 20, 1970, in Cairo, Egypt. He is son of Samir a retired teacher; and Hanaa Attia, an acco Ahmed. Egypt since 1993. He received his prima Egypt; and Sebha, Libya. He graduated from Elkoba High School, Cairo, Egypt in 1986. He attended the College of Agriculture at Ain Shams University, Cairo, Egypt where he warded a BS degree in Horticultural Science in 1991. In 1991, he entered a graduate program (in the Horticultural Sciences Department) at Ain Shams University, where he was awarded an MS degree in 1996 and enrolled in a e university from 1997 to 1999 until he traveled to the U States. He got an assistantship toward his PhD degree in 2000 at the University of a. Upon completing his PhD degree, he will return to the National Research Center as a researcher. 163


Permanent Link: http://ufdc.ufl.edu/UFE0004269/00001

Material Information

Title: Atmosphere Modification to Control Quality Deterioration during Storage of Fresh Sweetcorn Cobs and Fresh-Cut Kernels
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0004269:00001

Permanent Link: http://ufdc.ufl.edu/UFE0004269/00001

Material Information

Title: Atmosphere Modification to Control Quality Deterioration during Storage of Fresh Sweetcorn Cobs and Fresh-Cut Kernels
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0004269:00001


This item has the following downloads:


Full Text











ATMOSPHERE MODIFICATION TO CONTROL QUALITY DETERIORATION
DURING STORAGE OF FRESH SWEETCORN COBS AND FRESH-CUT KERNELS













By

GAMAL RIAD


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA


2004

































Copyright 2004

by

Gamal Riad

































This dissertation is dedicated to those in the world who care: my Mother, Hanaa Attia;
my father, Samir Riad; my brother, Ahmad Samir; and all of my friends.















ACKNOWLEDGMENTS

First and foremost, I would like to express my deepest grateful and thanks to Allah,

the God Almighty, for his continuous blessings. Then my deepest gratitude is due to my

mother, father, and brother for their continuous unconditional love, support,

encouragement, and guidance throughout my life, without which I wouldn't have been

able to finish this work.

Endless thanks and gratitude are due to my supervisory committee chair Dr. Jeffrey

K. Brecht for contributing his time and effort in providing constructive criticism, advice

and moral support throughout each step in my studies at the University of Florida.

I am personally indebted to all my supervisory committee members, Dr. Khe V.

Chau, Dr. Donald J. Huber, Dr. Steven A. Sargent, Dr. Charles A. Sims, and Dr. Stephen

T. Talcott for their continuous help, valuable discussions, and support; all of them

showed me the true meaning of the word advisor. I would also like to thank Dr. Stephen

Talcott for his assistance and help with the phenolic measurements, and helpful

discussions.

I am grateful to my former committee in Egypt, Dr. Taha el-Shourbagy and Dr.

Mordy Atta-Aly for their support and help before my travel to the United States.

Special appreciation is due to Kim Cordasco and Abbie Fox for their help

throughout this work, Special thanks go to Gary Harvey, produce manager of Publix

supermarket warehouse (Jacksonville, Fla.) for his help in providing a continuous supply









of sweetcorn during these experiments. Also I would like to thank Thermo King

Corporation (Minneapolis, Minn.) for financial support through my assistantship.

I would like to thank all my friends and colleagues in the Horticultural Sciences

Department at the University of Florida; the National Research Center (Giza, Egypt); and

Ain Shams University (Cairo, Egypt) for their help, encouragement, and support.

Finally, I would like to thank my country, Egypt, for the financial support in the

first 2 years of my study.
















TABLE OF CONTENTS

page

A C K N O W L E D G M E N T S ................................................................................................. iv

LIST O F TA B LE S ...........................................................................................

LIST OF FIGU RE S ......... ...... .............................. .. ...... ........... xi

ABSTRACT .......................................... .......... xiv

CHAPTER

1 IN TRODU CTION .................. ....................................... .. .. ... ........ ..

2 LITERA TURE REVIEW .......................................................... ..............4

Sw eetcorn .................. .............................................................. .. 4
Factors Optimizing Postharvest Preservation.............................................................6
Factors Optimizing Postharvest Preservation in Sweetcorn.......................................8
Controlled Atmosphere (CA) and Modified Atmosphere (MA) Technology ............10
Beneficial Effects of CA and M A .................................................................... 10
Detrimental Effects of CA and MA.............................................. ................. 11
Modified Atmosphere Packaging (MAP).......................................................11
A ctive and P massive M A ................................................................................. 15
CA and MA and Volatile Production .............. ............................................15
CA/M A and Sw eetcorn Storage ........................................ ........................... 18
Fresh-Cut Fruits and V vegetables ........................ .. ................ ............... .... 22
Limitations of Fresh-Cut Fruit and Vegetable Production..............................23
W ater lo ss ................ ........ .................................................. 2 3
Texture change and loss of tissue firmness............... ................... 25
C olor change ................................................................... 27
M icrobial grow th ..................................... ...... ........ .... ........ .... 31
N u trien t lo ss ................................................. ................ 3 6
L oss of flavor and arom a......................................... .......... ............... 41
Fresh-Cut Sw eetcorn K ernels .............. ........................................... ............... 43









3 SWEETCORN TOLERANCE TO REDUCED 02 WITH OR WITHOUT
ELEVATED CO2 AND EFFECTS OF CONTROLLED ATMOSPHERE
STORA GE ON QU ALITY ............................................... ............................. 45

In tro d u ctio n .......................................................................................4 5
M materials and M methods ....................................................................... ..................47
P lant M material ...............................................................4 7
Controlled Atm osphere Storage ........................................ ....... ............... 48
P a ra m e te rs ..................................................................................................... 4 8
R espiration rate ........................ .. .. .............. ............ .... .. ........... 48
S u g ar c o n ten t .......................................................................................... 4 9
Dimethyl sulfide (DMS) content......................................................50
V isual appearance evaluation.................................... ....................... 51
Statistical A analysis ........................................ .................... .... .. ..51
Results and Discussion .................. ............................. .. .... ................. 51

4 PERFORATION MEDIATED MODIFIED ATMOSPHERE PACKAGING (PM-
M AP) OF SW EETCORN .......................................................... ............... 60

In tro d u ctio n .......................................................................................6 0
M materials and M methods ....................................................................... ..................65
P lan t M material ......................... .... .......... .......... ........ ............... 6 5
Perforation Mediated Modified Atmosphere Packaging Treatment ...................66
Sw eetcorn Storage ............................................... .. .... .. .. ............ 66
P aram ete rs ...................................................... ................ 6 7
Gas com position .................. ......................... .. ........ ................. 67
R respiratory quotient ............................................................................... 67
W eight loss .................................................................................. 67
Su g ar content ................... ............ ...... .......................................... 6 8
Ethanol and acetaldehyde content.............................. ..... ............. 68
Statistical A analysis ........................................ .......... ......... ........ 69
R results and D discussion ....................... .................. ................... .. ......69

5 SELECTION OF MATURITY STAGE, STORAGE TEMPERATURE, AND
ATMOSPHERE FOR OPTIMUM POSTHARVEST QUALITY OF FRESH-CUT
SW EE TC O R N K E R N E L S .............................................................. .....................78

In tro d u ctio n ...................................... ................................................ 7 8
M materials and M methods ....................................................................... ..................80
P lant M material: ....................................................................................80
Controlled Atmosphere Treatment................. ............................................ 81
Fresh-Cut Kernel Storage and Cooked Sample Preparation ...............................81
P a ra m e te rs ..................................................................................................... 8 2
R espiration rate ........................ .. .. .............. ............ .... .. ........... 82
Sugar content ................................................................................. 82
F ree am ino acids............. ................................................ .... . ........... 83
Statistical A analysis ........................................ .......... ......... .... .. .. 84









R esu lts an d D iscu ssion ............................................................... .... .................... 84

6 REDUCED OXYGEN AND ELEVATED CARBON DIOXIDE TO PREVENT
BROWNING OF FRESH-CUT SWEETCORN KERNELS AFTER COOKING.... 95

In tro d u ctio n .......................................................................................9 5
M materials and M methods .................................... ........................ ...............97
Experiment I: CA Effect on Fresh-Cut Sweetcorn Kernels .............................97
Plant m material ......................................... .............. ............ 97
Preparation of the fresh-cut sweetcorn .....................................................98
Storage and treatm ents ........................................ ........................... 98
Sugar content .............................................................................. 99
T otal Soluble phenolics ........................... .......... ...... ............... ......99
Phenolic acids and 5-hydroxymethylfurfural (HMF)..............................100
M icrobial analy sis. ......................................................... ....................10 1
A after cooking brow ning. ........................................ ........ ............... 101
Statistical analysis .................................................... ...... .... ............ 101
Experiment II: Microbial Load Effect on the After Cooking Browning........... 102
Plant m material ........................... ......... ...... ....... ............ 102
Preparation of the fresh-cut sw eetcorn...................................................... 102
Isolation of the m icrobes ................................................................ ...... 102
Preparation of the inoculum ....................... ........... ...............102
Inoculation process ................. .. ...... .......................... 103
Controlled atmosphere treatment, storage, and sampling ........................103
After cooking browning measurement................................................... 103
Experiment III: Attempts to Identify the Browning Reaction Source.............104
R results and D discussion .................. ........................... .. .... .. .. .. ........ .... 105
Experiment I ................................... ......... .................. 105
E xperim ent II .......................................... ............ ..... ........ .... 108
Experim ent III ................................................ .............. .. 108

7 PHENOLICS PROFILE AND ANTIOXIDANT CAPACITY IN FRESH-CUT
SWEETCORN DURING STORAGE ............................................. ..............118

Introdu action ................................................................................................ ..... 118
M materials and M methods ........................................... ....................................... 120
Plant M material .............................................. ...................... 120
Controlled Atmosphere Treatment: .................................. .............. 121
Fresh-Cut Kernel Storage and Sample Preparation: .....................................121
Extraction of Phenolic Compounds .............. ............................................122
Extraction of free phenolic compounds ....................................................122
Extraction of bound phenolic compounds................................................ 122
P aram eters ................... .................... ....... .......................... 122
Measurement of polyphenolic compound concentrations.......................... 122
Total soluble phenolics.............................................. 123
Quantification of antioxidant capacity .....................................................123
Statistical A naly sis ............................................ .. .. ...... .... ...........124


viii









R results and D discussion ............................................................................ ........ 124

8 SUMMARY AND CONCLUSION .....................................................................134

Sw eetcorn C ob Storage....................................................................................... 134
Fresh-Cut Sweetcorn Kernel Storage ......................................................... 136
After Cooking Browning, What Is It and What Is It Not?........... ...............140
After cooking browning, what is it? ...................................... ............... 140
After cooking browning, what is it not?...... .... ............................ .. ......... 141

L IST O F R E FE R E N C E S ........................................................................ ................... 143

BIOGRAPHICAL SKETCH ............................................................. ............... 163
















LIST OF TABLES


Table p

3-1. Description of the visual quality ratings used for visual quality evaluation of
sweetcorn after storage ........................... ..... ..... ....... .............. 55

4-1. Diameters and lengths of brass tubes used for creating PM-MAP with desired CO2
levels at different tem peratures. ........................................ ......................... 73

6-1. Effect of atmosphere modification and storage period on the color parameters of
cooked fresh-cut sweetcorn kernels stored in air or controlled atmosphere. .........110
















LIST OF FIGURES


Figure page

3-1. Effect of controlled atmosphere storage on respiration rate of sweetcorn stored in air
or 2% 02 plus 0, 15, or 25% CO2 at 5 C. ..................................... ...............56

3-2. Effect of controlled atmosphere storage on the sugar content in sweetcom kernels
from cobs stored in different gas compositions.. ....................................... ........ 57

3-3. Effect of controlled atmosphere storage on the different attributes of sweetcorn
visual quality after storage in different gas compositions at 5 C..........................58

3-4. Effect of controlled atmosphere storage on dimethyl sulfide content in sweetcorn
kernels from cobs stored at 5 OC in different gas compositions at 5 C..................59

4-1. Gas composition in perforation-mediated MAP in sweetcorn packages stored for 10
d a t 1 C ......................................................................... 7 4

4-2. Gas composition in perforation-mediated MAP in sweetcorn packages stored for 10
d a t 1 0 C ........................................................................ 7 5

4-3. Effect of different levels of CO2 generated through PM-MAP on the respiratory
quotient of sweetcom stored for 10 d at 1 OC or 10 C .......................................76

4-4. Effect of temperature and different CO2 levels built up through PM-MAP on total
sugars in sweetcom stored for 10 d at 1 or 10 C. ................................................77

5-1. The differences between the freshly harvested more mature and less mature
sw eetcorn cobs. ....................................................................... 80

5-2. Effect of maturity stage, storage temperature and controlled atmosphere on
respiration rate of fresh-cut sweetcorn........................ ... .................................... 87

5-3. Effect of maturity stage, storage temperature and controlled atmosphere on on sugar
content in fresh-cut sw eetcom ......................................... ............................. 88

5-4. Effect of maturity stage, storage temperature and controlled atmosphere on on free
am ino acids in fresh-cut sweetcorn. ............................... ................................. 89

5-5. Fresh-cut sweetcorn kernels at day 0 (just after preparation)................. ........ 90

5-6. Cooked fresh-cut sweetcorn kernels at day 0 (just after preparation).. ......................90









5-7. Fresh-cut sweetcorn kernels after storage for 10 d at 1 OC in air. ............................91

5-8. Cooked fresh-cut sweetcorn kernels after storage for 10 d at 1 OC in air ..............91

5-9. Fresh-cut sweetcorn kernels after storage for 10 d at 5 OC in air. ...........................92

5-10. Cooked fresh-cut sweetcorn kernels showing after cooking browning after storage
for 10 d at 5 C in air.......... ... ...................... ............ .......... ............92

5-11. Fresh-cut sweetcorn kernels after storage for 10 d at 1 OC in 2% 02+ 10% CO2....93

5-12. Cooked fresh-cut sweetcorn kernels after storage for 10 d at 1 OC in 2% 02 + 10%
CO 2.......... ....... ..... ................................... ......... 93

5-13. Fresh-cut sweetcorn kernels after storage for 10 d at 5 OC in 2% 02+10% CO2.....94

6-1. Effect of controlled atmosphere storage on the sugar content of raw and cooked
fresh-cut sweetcorn kernels stored in air or CA at 5 C ...................................... 111

6-2. Typical phenolic acids profiles of fresh-cut sweetcorn kernels stored at 5 OC for 10 d
in airo r C A ......................... .. ........... ...... ........... ........................... 1 12

6-3. Total soluble phenolics in raw and cooked fresh-cut sweetcorn kernels stored in air
or C A for 10 d at 5 C ............... ........................................ ............ .... 113

6-4. Total aerobic microbial count of fresh-cut sweetcorn kernels stored in air or CA for
10 d at 5 oC ................. .......................................................... 114

6-5. Raw and cooked fresh-cut sweetcorn kernels at day 0 ....................................114

6-6. Raw and cooked fresh-cut sweetcorn kernels stored in air for 7 d at 5 C...............115

6-7. Raw and cooked fresh-cut sweetcorn kernels stored in CA for 7 d at 5 C ...........115

6-8. Raw (A) and cooked (B) fresh-cut sweetcorn kernels stored in air for 10 d at 5 OC. 116

6-9. Raw (A) and cooked (B) fresh-cut sweetcorn kernels stored in CA (2% 02+10%
CO 2) for 10 d at 5 C .................. .......................... .. ...... .. ........ .. .. 116

6-10. HPLC chromatogram of the acetone-insoluble fraction of fresh-cut sweetcorn
kernel juice after storing the kernels for 10 d at 5 OC in air. ................................117

7-1. HPLC chromatograms of free and bound polyphenolic compounds in sweetcorn
extract on day 0 (before the storage treatment).....................................................127

7-3. HPLC chromatograms of free polyphenolic compounds in the sweetcorn extract after
4 d of storage in air (control) or CA .................................................................... 128









7-4. HPLC chromatograms of bound polyphenolic compounds in the sweetcorn extract
after 7 d of storage in air (control) or CA.. ............................................................129

7-5. HPLC chromatograms of free polyphenolic compounds in the sweetcorn extract after
7 d of storage in air (control) or CA............................................................ 129

7-6. HPLC chromatograms of bound polyphenolic compounds in the sweetcom extract
after 10 d of storage in air (control) or CA.. .........................................................130

7-7. HPLC chromatograms of free polyphenolic compounds in the sweetcorn extract after
10 d of storage in air (control) or CA ................................... .......... ............... 130

7-8. HPLC chromatogram of galacturonic acid in the free phenolic sweetcorn extract
during 10 d of storage in CA .................................................... .................. 131

7-9. Antioxidant capacity of free and bound phenolic fractions obtained from fresh-cut
sweetcorn kernels as affected by storage in air or CA .................................. 132

7-10. Total soluble phenolics (mg-kg-1 FW) of free and bound fractions obtained from
fresh-cut sweetcorn kernels as affected by storage in air or CA............................133















Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

ATMOSPHERE MODIFICATION TO CONTROL QUALITY DETERIORATION
DURING STORAGE OF FRESH SWEETCORN COBS AND FRESH-CUT KERNELS


By

Gamal Riad

August 2004

Chair: J. K. Brecht
Department: Horticultural Sciences

Controlled atmosphere (CA) storage and modified atmosphere packaging (MAP)

are beneficial tools for extending the postharvest life of fresh fruits and vegetables, but

specific tolerance levels to gas composition must be determined in order to apply these

techniques. Perforation-mediated modified atmosphere packaging (PM-MAP) for

sweetcorn utilizing impermeable containers with a diffusion window was designed to

establish 15, 20 or 25% CO2 atmospheres at 1 and 10 C. The desired CO2 concentrations

were obtained at 1 OC, but were about 3-5% lower than expected at 10 OC. It took about 5

d to reach the equilibrium atmospheres at 1 OC, and about 2 d at 10 OC. Sweetcorn cobs in

CA tolerated 2% 02 and up to 25% CO2 alone for 2 weeks at 5 OC, but elevated

respiration suggested that they may be damaged by the two gas levels in combination,

although no significant ethanol or acetaldehyde production was detected in any CA or

PM-MAP treatment.









The best atmosphere composition tested for maintaining sweetcorn quality was 2%

02 plus 15% CO2. The CA reduced sweetcorn respiration, maintained higher sugar

concentrations, reduced loss of husk greenness, and improved silk and kernel appearance.

This CA also maintained the highest concentration of dimethyl sulfide (DMS), the main

characteristic aroma component in sweetcom.

The potential for storing and handling fresh-cut sweetcorn kernels was also

examined. Fresh-cut sweetcorn kernels are extremely perishable. Successful handling

requires low storage temperature and optimum maturity stage. Quality was maintained

for 10 d in air at 1 OC or in CA (2% 02 plus 10% CO2) at 5 OC, but brown kernel

discoloration after cooking limited shelf life in air at 5 OC especially in the more mature

kernels. The CA reduced fresh-cut sweetcorn respiration, inhibited sugar loss, and, most

importantly, prevented after cooking browning. After cooking browning was not due to

typical Maillard reaction (5-hydroxymethylfurfural was not present) nor due to changes

in soluble phenolics. Higher aerobic microbe counts were associated with increased after

cooking browning but not with a specific species. A water soluble brown pigment

precursor was isolated from kernel juice but was not identified.














CHAPTER 1
INTRODUCTION

Sweetcom (Zea mays L. rugosa) is widely used as a fresh or processed vegetable. It

is sweeter than wild type (field) corn because it has a recessive mutant gene sugary-1

(sul) that restricts the conversion of sugar into starch (Creech, 1968; Laughnan, 1953;

Wann et al., 1971). However, this conversion still occurs, and continues after harvest and

during storage, resulting in rapid quality loss (Doehlert et al., 1993). Since consumer

surveys have shown that most consumers prefer sweetcom with higher natural sweetness

(Showalter and Miller, 1962), one of the areas of genetic improvement of sweetcorn has

involved the selection of mutant strains that produce high sugar levels in the seed

endosperm (Courter et al., 1988; Garwood et al., 1976; Laughnan, 1953; Showalter and

Miller, 1962; Wann et al., 1971). The most successful of these mutants is shrunken-2

(sh2), which blocks sucrose conversion to starch, and is found, alone or in combination

with sul, in almost all modem commercial sweetcorn cultivars.

Controlled atmosphere (CA) and modified atmosphere (MA), i.e., elevating carbon

dioxide (C02) levels and reducing oxygen (02) levels around stored vegetables after

harvest, can be useful supplements to maintenance of optimum temperature and relative

humidity in maintaining postharvest quality of fresh fruits and vegetables. The main

effects of CA/MA are reduced respiration and ethylene production; and, consequently,

delayed ripening or senescence, reduced weight loss, and prolonged shelf life (Kader et

al., 1989; Weichmann, 1986). The elevated CO2 used in CA/MA also competitively









inhibits ethylene action (Burg and Thimann, 1959) and inhibits postharvest diseases

(El-Gorani and Sommer, 1981; Daniels et al., 1985).

Fresh sweetcorn is a perishable food product prone to rapid postharvest

deterioration caused by kernel desiccation, loss of sweetness, husk discoloration, and

development of decay. Keeping cobs in CA/MA with high CO2 and/or low 02 levels

inhibits respiration; and, consequently, it reduces sugar loss and other metabolic

reactions, and slows the growth of pathogens; CA/MA also decreases husk yellowing by

inhibiting chlorophyll degradation.

Nowadays there is increased acceptance and demand for fresh-cut fruits and

vegetables (sometimes called minimally processed or ready-to-eat produce) for many

reasons such as their convenience, perceived high nutritional value, and freshness. The

flourishing of the fresh-cut industry in the last decade encourages the development of

new fresh-cut products and there is now greater feasibility of sweetcorn kernels being

developed as a fresh-cut product, but work is needed to determine the limiting factors in

storing and handling such a value-added product.

Study Objectives. The objectives of this work were as follows:

* Determine the specific tolerance levels of sweetcorn to low 02 and/or to high CO2
as an essential requirement to successfully apply controlled and/or modified
atmosphere techniques in sweetcorn storage.

* Determine the interactive effects of 02/CO2 combinations and temperature on
induction of anaerobic respiration in sweetcorn (using perforation-mediated MAP).

* Determine the effects of low 02 and high CO2 on levels of aroma volatiles and
other quality factors in sweetcorn during storage.

* Introduce fresh-cut sweetcorn kernels and determine its feasibility and value as a
fresh-cut product.









The last objective required investigating the following:

* The effect of sweetcorn maturity, storage temperature, and 02/CO2 levels on
fresh-cut sweetcorn quality in order to determine the best postharvest treatments to
reduce the occurrence of after-cooking browning of fresh-cut kernels.

* The cause of the browning of cooked fresh-cut kernels after storage, and testing the
possibility of the Maillard reaction as its cause.














CHAPTER 2
LITERATURE REVIEW

Sweetcorn

The quality of fresh sweetcorn [Zea mays L. rugosa (the old name was Zea mays L.

saccharata)] depends to a large extent on the kernel texture and flavor, which are directly

related to the sugar and polysaccharide content of the endosperm (Culpepper and

Magoon, 1927; Flora and Wiley, 19974a). Sweetness in sweetcorn is closely related to

kernel sucrose content (Reyes et al., 1982), which is the primary sugar in developing

kernels (Cobb and Hannah, 1981). Texture and eating quality of sweetcorn consists of

several factors, including pericarp tenderness (Bailey and Bailey, 1938), levels of soluble

sugars and water-soluble polysaccharides (WSP) in the endosperm (Culpepper and

Magoon, 1927; Evensen and Boyer, 1986), and moisture content (Wann et al., 1971). On

the other hand, flavor and aroma, which are not as easily defined as sweetness and

texture, are most often associated with kernel content of many volatiles, but mainly

dimethyl sulfide (DMS) (Flora and Wiley, 1974b; Wiley, 1985; Williams and Nelson,

1973; 1974). Therefore, a major goal has been to develop sweeter sweetcorn and to find

postharvest treatments to delay the depletion of this high level of sugars.

The basic genetic difference between standard sweetcorn cultivars and starchy field

corn is the presence of a recessive allele at the sugary-] (su-1) locus on chromosome 4 in

sweetcorn. This recessive gene conditions an 8- to 10-fold increase in WSP, extreme

starch reduction (about 50% or less than normal corn) and 2-fold increase in sugar over

the normal corn (Creech, 1968; Laughnan, 1953; Wann et al., 1971). Doehlert et al.









(1993) stated that the differences between normal kernels and su-1 kernels are that the

su-1 kernels accumulate less weight, retain kernel moisture longer, have a thinner

pericarp, and contain altered storage protein.

Many other mutants have been introduced that improve the sugar content in

sweetcorn, with one of the most important being the shrunken-2 (sh2) gene on

chromosome 3, which blocks the conversion of sucrose to WSP and starch, resulting in

accumulation of sugar (Laughnan, 1953). Cultivars containing this sh2 gene alone or in

combination with the sul gene supersweetet' sweetcorn) usually contain more than twice

the sugar content of the standard sul sweetcorn (Courter et al., 1988; Showalter and

Miller, 1962; Wann et al., 1971) and can retain higher sugar and moisture content for a

longer time (Garwood et al., 1976); and consequently are preferred by consumers in taste

tests (Evensen and Boyer, 1986; Showalter and Miller, 1962).

There are many other genetic mutations in sweetcorn that have been introduced

with limited success commercially, such as the endosperm mutants brittle-] (btl) and

brittle-2 (bt2), which produce a high sugar content with a relatively low starch content in

kernels, and which are adapted mainly for tropical climates. Another mutation is the

sugary enhancer (se or se/sul) gene, which acts as an independent genetic modifier of the

sul gene. The se gene produces nearly double the sugar compared with sul; and unlike

the watery endosperm of sh2 kernels, se varieties have a creamy endosperm due to

production of WSP. The main disadvantage of the se varieties is that the sugar is rapidly

converted to starch after harvest, unlike the sh2 varieties (Wily et al., 1989).

Sweetcorn has a high respiration rate, which results in a high rate of heat evolution,

and it loses sugars very rapidly after harvest (Brecht and Sargent, 1988; Evensen and









Boyer, 1986; Wann et al., 1971). That is why fast cooling after harvest and keeping the

cob temperature as close as possible to 0 OC is the most important step in maintaining

sweetcorn quality (Brecht, 2002). The sugar content, which so largely determines quality

in sweetcorn, declines rapidly at room temperature and decreases less rapidly if the

sweetcorn is kept at about 0 C. Early work by Appleman and Arthur (1919) showed that

sugar loss is about four times as rapid at 10 OC as at 0 OC. At 30 OC, 60% of the sugars in

sul sweetcorn may be converted to starch in a single day as compared with only 6% at

0 C (Brecht, 2002). While in sh2 varieties the sugar loss is actually at the same rate, the

higher initial sugar in these cultivars helps in keeping it sweet tasting for a longer period

(Brecht at al., 1990). Similar results were obtained by Olsen et al. (1990) who found that

the sugar depletion rate was higher in sh2 than sul, but the sh2 still contained

significantly more sugar after storage. These two types of sweetcorn lose sweetness and

aroma during storage, but the main difference between them during storage is that sul

(and se) kernels tend to become starchy while sh2 varieties tend to taste more watery and

bland (Brecht, 2002).

Factors Optimizing Postharvest Preservation

Harvested fruits and vegetables are highly perishable products. Over-ripening and

senescence, mechanical injuries, trimming, water loss, and biological factors such as

diseases and pests are the principal causes of postharvest losses. Postharvest preservation

of these commodities is thus comprised of efficient techniques to reduce the tremendous

amount of fresh produce losses, maintain produce quality, and extend shelf life

throughout the postharvest chain, consequently increasing their commercial value.









Postharvest deterioration can be controlled by primary and secondary factors. The

primary factors to optimize preservation of a horticultural commodity (Kader et al., 1989)

are as follows:

* Selecting varieties of the crop that have desirable postharvest and storage
characteristics,
* Application of the ideal preharvest treatments,
* Harvesting at the optimum maturity stage,
* Minimizing mechanical injuries during harvesting and postharvest handling,
* Using proper sanitation procedures to reduce microbial infection,
* Providing the optimum temperature and relative humidity throughout the
postharvest chain


Temperature control (precooling and cold storage) has been identified as the most

crucial factor for extending the shelf life of produce since biological reactions generally

increase 2- to 3-fold for every 10 C rise in temperature.

Secondary factors to optimize preservation of a horticultural commodity include

modification of 02 and CO2 concentrations in the atmosphere surrounding the commodity

to levels different than normal air (Kader et al., 1989; Saltveit, 1989). Atmosphere

modification is achieved either by reducing the concentration of 02 (which is required for

respiration and for ethylene synthesis); or by increasing CO2 concentration (which

inhibits respiration, ethylene action, and microbial growth) (Chinnan, 1989; Daniels et

al., 1985; El-Gorani and Sommer, 1981; Kader, 1987; Kader et al., 1989; Labuza and

Breene, 1989; Shewfelt, 1986). This is referred to as controlled atmosphere (CA) or

modified atmosphere (MA) storage.

Although the secondary factors are not as significant as the primary factors, their

additive effect is important to preserve the overall postharvest quality of many

commodities. It was also demonstrated that using secondary factors could solve some









important postharvest problems in specific commodities. For example, the use of CA/MA

technology on chilling sensitive produce may overcome the impact of low temperature

injury (Forney and Lipton, 1990; Pesis et al., 2000; Wang and Qi, 1997)

Factors Optimizing Postharvest Preservation in Sweetcorn

Sweetcorn has one of the highest metabolic and respiration rates among vegetable

crops, which makes it a very perishable product that requires special attention to the

postharvest practices used in order to prolong its shelf life. The main sources of

postharvest loss in sweetcorn are sugar loss, husk yellowing and drying, and kernel

denting. Denting is caused by water loss, primarily from the husks, which in turn draw

moisture from the cob and kernels, causing the latter to collapse, causing the dented

appearance. It was estimated that a loss of only 2% moisture may result in objectionable

kernel denting (Hardenburg et al., 1986). Mechanical injuries, which occur especially

when sweetcorn is harvested mechanically or during trimming, can be a serious cause of

postharvest losses by promoting water loss and decay. Sweetcorn is also affected by

biological factors such as diseases and pests that increase postharvest losses.

As discussed above, postharvest deterioration can be controlled by primary and

secondary factors. The primary factors to optimize preservation of sweetcorn are

* Selection of varieties of the crop that have desirable postharvest and storage
characteristics. For example, cultivation ofsh2 varieties ensures high sugar content;
and hence increases the tolerance for diverse postharvest conditions and extends the
postharvest life of sweetcorn (Spalding et al., 1978).

* Application of the ideal preharvest treatments ensures high postharvest quality and
long shelf life. It has been proven that preharvest factors have a great impact on
postharvest quality (Kays, 1999) For example, use of the optimum nitrogen and
sulfur fertilization rates increased the flavor quality in harvested sweetcorn due to
increased dimethyl sulfide in the kernels (Wong et al., 1995).









* Harvesting at the optimum maturity stage. The early work by Rumpf et al. (1972)
demonstrated that the highest levels of sugars were found when the cobs were
harvested at the optimum degree of maturity.

* Minimizing mechanical injuries during harvesting and postharvest handling.

* Using proper sanitation procedures throughout the postharvest chain, to ensure
reduced microbial infection, and thus reduce postharvest losses.

* Providing the optimum temperature and relative humidity throughout the
postharvest chain.

Similar to all perishable horticultural crops, temperature control (precooling and

cold storage) is the most crucial factor in extending the shelf life of sweetcorn. It is well

documented that fast precooling and storing at a low temperature (0-1 C) and high

relative humidity (>90%) is the key factor in ensuring the highest postharvest quality in

sweetcorn (Brecht, 2002; Brecht and Sargent, 1988; Evensen and Boyer, 1986). This is

due to the reduction in metabolic rates at lower temperatures, which reduces the

respiration rate and consequently the sugar consumption (high sugar content being the

main quality factor in sweetcorn). Moreover, the low metabolic rate reduces the

conversion of sugars to starch, which helps in keeping the high sugar content, which is

very helpful in the genotypes that covert sugar to starch. Also, low temperature reduces

water loss and subsequently reduces denting and/or husk drying of the sweetcorn cobs.

On the other hand, low temperature also helps in reducing microbial growth and hence

reduces pathological postharvest losses.

The secondary factors to optimize preservation of sweetcorn as mentioned above

also include using MA or CA, by reducing 02 levels and/or increasing CO2 levels (see

discussion below). The MA and CA help in reducing sweetcorn metabolic rates,

including respiration (Riad et al., 2002; Riad and Brecht, 2003) and carbohydrate

metabolism (Risse and McDonald, 1990). They also reduce water loss from sweetcorn









husks (Deak et al., 1987) because of the restricted gas exchange that is integral to MA

and CA technology. Reduced 02 and more importantly elevated CO2 levels in MA and

CA also reduce microbial growth on sweetcorn husks and kernels (Aharoni et al., 1996;

Schouten, 1993). All these benefits help in reducing sweetcorn postharvest losses.

Controlled Atmosphere (CA) and Modified Atmosphere (MA) Technology

The technique of modification of the atmosphere surrounding perishable products

is referred to as CA or MA. In CA, the atmosphere is created artificially and the gas

composition is continuously monitored and adjusted to maintain the optimum gas

concentration. On the other hand, in MA, the gaseous environment is modified naturally

by the interplay among the physiology of the commodities and the physical environment,

thus the control of the atmosphere in MA is less precise than CA. In MA, the respiration

rate (02 consumption and CO2 evolution) of the commodity being stored is in equilibrium

with the 02 and CO2 concentrations in the surrounding environment. Several articles have

been published on the benefits of CA/MA technology on the extension of perishable

product shelf life (Anzueto and Rizvi, 1985; Nakashi et al., 1991; Zagory and Kader,

1988).

Beneficial Effects of CA and MA

Using CA and MA have a wide range of benefits (Kader, 1980; Kader et al., 1989)

such as the following:

* The CA and MA conditions reduce the respiration rate (as long as the levels of 02
and CO2 are within those levels the commodity can tolerate and don't induce
anaerobic respiration), which results in delayed ripening and senescence and better
maintenance of the quality of the commodity.

* The CA and MA conditions reduce ethylene production and reduce sensitivity to
ethylene (action) and this has many beneficial effects such as delaying fruit
ripening and tissue senescence, delay of chlorophyll degradation, and maintenance
of textural quality (decrease in lignification, etc.).









* The CA and MA conditions allow handling of chilling sensitive fruits (such as
tomato, banana, and mangoes) at temperatures lower than the chilling threshold
temperatures in normal air storage.

* Using CA and MA reduces the incidence and severity of some physiological
disorders such as the disorders induced by ethylene or by chilling injury.

* Since delaying senescence, including fruit ripening, reduces the susceptibility to
pathogens, CA/MA has a beneficial effect in decreasing postharvest diseases
(Daniels et al., 1985; El-Goorani and Sommer, 1981). Levels of 02 below 1% and
levels of CO2 above 10% can also have a significant direct effect on fungal growth.
Carbon dioxide levels above 10-15% (in commodities that tolerate such levels) can
be used to provide a fungistatic effect.

Detrimental Effects of CA and MA

Most CA/MA disadvantages are related to severe reductions of 02 and/or increases

in CO2 that force the product into anaerobic respiration (Kader et al., 1989; Brecht,

1980). Anaerobic respiration causes many disorders such as

* Increased susceptibility to decay and shortening of the storage life.

* Irregular ripening.

* Accumulation of ethanol, acetaldehyde, and other compounds that produce off-
flavors and other metabolic dysfunctions.

* Physiological disorders (such as brown stain in lettuce, internal browning and
surface pitting in pome fruits).

* Activation of the growth of some anaerobic pathogens that are considered to be
major health hazards.

Modified Atmosphere Packaging (MAP)

Modified atmosphere packaging (MAP) is an atmosphere control technique that

relies on the natural process of respiration of the product and the gas permeability of the

package holding the product. Due to respiration, there is a buildup of CO2 and a depletion

of 02, and the package material helps to maintain the modified gas levels until the

package reaches steady state because of restricted gas permeability. In the steady state









condition, the 02 flow entering the package equals the 02 consumed by respiration, and

the CO2 flow leaving the package equals the CO2 produced by respiration. Because of the

limitation of CA storage to relatively large-scale systems, the MAP technique was

developed to provide the optimal atmosphere; not for the entire storage facility, but for

just the product, thus maintaining the desired atmosphere during almost all of the

postharvest chain even during the retail display. The MAP can vary from a whole

shipping container to a small retail package. As well as the benefits of modifying the 02

and CO2 levels, MAP has the additional benefits of water loss prevention, product

protection, and brand identification. To achieve the desired atmosphere more rapidly,

modification of the package atmosphere can be accelerated by using absorbents, or by

using active modification instead of passive modification (i.e., initially replacing the

package atmosphere with the desired one by gas flushing) (Kader et al., 1989)

The development of MAP has faced several problems (Kader, 1987; Kader et al.,

1989) such as

* Lack of commodity respiration data under several temperatures and gas
compositions.

* Lack of permeability data for packaging materials at different temperatures and
relative humidities.

* Lack of consistency in respiration data gathered for the same commodity.

In medium and high-respiring commodities (like sweetcorn), using commonly

available films such as low-density polyethylene (LDPE), polyvinyl chloride (PVC), and

polypropylene is not ideal due to their low gas transmission rates, which may lead to

respiration switching toward anaerobic respiration (Morales-Castro et al., 1994). Fonseca

at al. (2000) summarized some of the limitations of using the flexible polymeric films

that result from their structure and permeation characteristics such as









* Films are not strong enough for large packages.

* Film permeability characteristics change unpredictably when films are stretched or
punctured.

* Some films are relatively good barriers to water vapor, causing condensation inside
packages when temperature fluctuations occur and consequently increasing
susceptibility to microbial growth.

* Film permeability may be affected by water condensation.

* The uniformity of permeation characteristics of films is not yet satisfactory.

* Film permeability is too low for high-respiring products.

* Products that require high CO2/02 concentrations may be exposed to anaerobiosis
because of the high ratio of the CO2 versus 02 permeability coefficients.

Perforation-mediated MAP is a potential technique for postharvest preservation of

fresh horticultural commodities. In this technique, instead of using the common

polymeric films, a package is used in which the regulation of gas exchange is achieved by

single or multiple perforations or tubes that perforate an impermeable package (Emond

and Chau, 1990a,b; Emond et al., 1992).

Using the perforation-mediated package has many potential advantages (Fonseca et

al., 1997) such as

* The high values of mass transfer coefficients, implying that a reduced size and
number of perforations for gas exchange are required, thus high-respiring produce
can be packed in this system.

* A MAP using perforations can be adapted easily to any impermeable container,
including large bulk packages. Polymeric films are not strong enough for packs
much larger than those used for retail, but perforations can be applied to retail
packages as well as shipping containers, because rigid materials can be used. Rigid
packages also can prevent mechanical damage of the product.

* A flexible system is obtained due to the ability to change the gas transfer
coefficients by selecting the adequate size and shape of the perforation.

* Commodities requiring high CO2 concentrations and relatively high 02
concentrations can be packed with this system.









But on the other hand there are some limitations to this technique (Fonseca et al.,

1997) such as

* Although it may be applied to products that could not be packed in conventional
MAP, the range of products is not very wide, because the CO2/02 transfer
coefficient ratio averages 0.8, the ratio of the diffusion values of CO2 and 02 in the
air. This may eventually be overcome by the use of perforation packed with
materials with different affinities for CO2 and 02.

* Water loss in the product may become a problem, but packed perforations may
solve this problem.

* Non-uniformity of concentrations inside the package due to gas stratification may
also be a problem in large containers.

* More fundamental research and experimental validation is needed before its
eventual commercial use.

One commercially available film (Intellipac from Landec Corporation, Menlo Park,

Calif.) is claimed by the manufacturer to be able to automatically adjust its permeability

in response to temperature changes by a phase change in the film polymer structure

(Clarke and De Moor, 1997). Permeability characteristics of this package are provided by

using a highly permeable membrane over an aperture in the wall of the package. The

membrane is made by coating a microporous substrate with a side chain crystallizable

(SCC) polymer. In cold temperature, the SCC polymer exists in a crystalline solid phase;

but when the temperature increases above the pre-selected switch temperature, the

polymer changes to a more permeable liquid phase. Because this transformation involves

a physical effect and not a chemical change, the metamorphosis is reversible. By

changing the properties of the polymer and coating thickness, it is possible to obtain the

permeability selectivity and the temperature switch required. Lange (2000) found that

Intellipac-stored strawberries had ethanol and ethyl acetate levels similar to those of fresh









fruit, while samples stored in regular perforated film had levels of these fermentative

products 7-fold higher than those of fresh samples.

Active and Passive Modified Atmosphere

A MAP system maintains an adequate atmosphere within the package under steady

state conditions through interaction of the respiration rate of the commodity and the

package size and gas permeation of the package material (Kader et al., 1989). When

atmospheres are modified passively by commodity respiration, it may require hours to

days until the gas concentrations reach the steady state in the package, the required time

being mainly a function of the package void volume (Ballantyne et al., 1988; Geeson et

al., 1985). Using active or semi-active atmosphere modification i.e., removal or addition

of a determined gas volume from the package, allows the desired atmosphere to be more

quickly achieved, and thus further prolongs the storage life of the produce (Yahia and

Gonzalez-Aguilar, 1998). This may be very important in sweetcorn, since addition of

even a single day to its short storage life would be a significant benefit.

Controlled and Modified Atmosphere and Volatile Production

High CO2 and/or low 02 can induce anaerobic metabolism, resulting in

accumulation of ethanol and acetaldehyde (Kader, 1989; Ke et al., 1993). Methanethiol

production is also induced under low 02 atmospheres in Brassica crops (Forney at al.,

1991); and production of other volatiles has also been shown to be enhanced under low

02 and/or high CO2 atmospheres (Hansen et al., 1992; Larsen and Watkins, 1995;

Mattheis et al., 1991)

The production of some volatiles may be modified if both 02 and CO2 are modified

simultaneously. Obenland et al. (1995) found that low 02 levels enhanced methanethiol

production in broccoli. However, the application of high CO2 levels in parallel with low









02 inhibited methanethiol production. Hence, the levels of both gases may be important

in determining the production of volatiles.

The critical 02 concentration that results in induction of anaerobic respiration is

dependent on the character of the product in question (Gran and Beaudry, 1993). Also,

temperature can influence the threshold of anaerobic induction. The 02 threshold for

anaerobic induction increases with increasing temperature (Beaudry et al., 1992; Gran

and Beaudry, 1993; Joles et al., 1994). For example, Cameron et al. (1994) found that

packages designed for blueberries at 0 OC reached the threshold for anaerobic induction at

5 C. Also Beaudry et al. (1992) reported that highbush blueberry fruit can tolerate

-1.8% 02 when stored at 0 OC whereas at 25 C they require -4% 02 to avoid elevating

the respiratory quotient (RQ). They concluded that the risk of anaerobiosis within LDPE

packages was increased by high temperature and the critical 02 level that induced

anaerobic respiration was increased with increasing temperature. There are also several

reports indicating that varieties of the same commodity have different potentials for

accumulation of acetaldehyde and ethanol (Blanpied and Jozwiak, 1993; Folchi et al.,

1995; Gran and Beaudry, 1993)

Sulfur-containing volatiles are produced via free, sulfur-containing amino acids,

peptides, thioglucosides, and thiophenes in plant tissue (Buttery, 1981; Richmond, 1973).

The accumulation of many of these compounds is associated with off-odors and off-

flavors (Di Pentima et al., 1995; Forney et al., 1991; Hansen et al., 1992). There are some

reports indicating that hydrogen sulfide and allyl sulfide cause respiration enhancement

over short periods of time (Hosoki et al., 1985). This could have a significant effect on









MAP, since films used in these packages are selected on the basis of respiratory activity

and film permeability (Toivonen, 1997).

In natural ecosystems, plant tissue evolution of volatiles is influenced to a large

degree by evapotranspiration (Charron and Cantliffe, 1995). The evolution of volatiles

under low transpiration is much lower, and therefore the volatiles can accumulate in the

tissues. A potential problem in CA/MA and especially MAP is that restricted ventilation

may lead to accumulation of volatile compounds within the plant tissues that may be

damaging or at least objectionable, from a quality standpoint. Among the compounds that

may accumulate in CA and MA and cause off-odors during storage are those usually

associated with anaerobic respiration such as ethanol and acetaldehyde, as well as

acetone, dimethyl sulfide, hydrogen sulfide, methanethiol, and ethanethiol.

By enhancing evapotranspiration in CA/MA storage, the accumulation of volatiles

in the tissue can be reduced (Beaudry et al., 1993; Blanpied and Jozwiak, 1993).

Toivonen (1997) reported that use of desiccants or a combination of desiccants and a

volatile adsorbent in MAP significantly improved raspberry shelf life and quality at 10 C

since it lowered the concentrations of ethanol and acetaldehyde. This treatment lowered

the concentrations of ethanol and acetaldehyde in the fruit without much effect on their

concentration in the headspace of the package, and increased acceptability from 42% to

75% with a slight increase in weight loss. These results agree with the previous

suggestion that water movement from the product is important in lowering volatile levels

in the tissue.

Most of the odors present in fresh sweetcorn may be considered to contribute to its

characteristic flavor; especially DMS, which is responsible for the characteristic aroma of









cooked sweetcorn (Wiley, 1985; Williams and Nelson, 1973; 1974; Flora and Wiley,

1974b). It was noticed that the levels of these compounds are increased by canning or

freezing (Flora and Wiley, 1974a). There is very little information about the effect of

CA/MA storage and MAP on the levels of these compounds, or about the possible

changes in their levels, or the levels of precursor compounds, during storage.

CA/MA and Sweetcorn Storage

Early work by Spalding et al. (1978) found that sweetcorn appearance and flavor

were not significantly improved when stored for 3 weeks at 1.7 OC under CA or low

pressure. In those experiments, sweetcorn cobs were stored either in air, low-pressure

atmosphere with the equivalent of 2% 02, or in CA containing 2 or 21% 02 plus either 0,

15, or 25% CO2. The increase in ethanol level was much higher with the increase of CO2

level over 15% than with the decrease of 02 level from 21% to 2%. Kernels from

sweetcorn cobs stored in 2% 02 plus 25% CO2 were not significantly higher in sugars

than air-stored sweetcorn and contained the highest amount of ethanol; but were the best

treatment in terms of flavor rating, better than the air storage treatment, while storing in

2% 02 plus 15% CO2 was similar to air storage in sugar content and flavor rating. Also,

in this experiment, the highest off-flavor levels detected were obtained when storing

sweetcorn in 21% 02 plus either 15 or 25% CO2; but there was no significant difference

between air storage and storing in 2% 02 plus either 15, or 25% CO2. Despite these

results, it was concluded that sweetcorn appearance and flavor maintenance were not

significantly improved by CA storage conditions; and that breeding of cultivars that

better retain quality in combination with prompt precooling offers more potential for

success than CA storage.









On the other hand, there are many reports stating the beneficial effects of using

MAP for sweetcorn. Deak et al. (1987) demonstrated that using MAP (shrink wrap film

with moisture permeability of 0.1 g.100.cm-224 h-1 and 02 permeability that varied from

0.4 to 40 mL-100 cm-224 h-1) eliminated water loss and maintained beneficial CO2 and

02 levels within the package. These effects, together with low storage temperature (5 C),

markedly reduced postharvest deterioration and hence resulted in an at least three-fold

extension of shelf life (29 versus 8 d). But MAP treatment increased microbial growth

due to the water-saturated atmosphere. Similar results were obtained by Risse and

McDonald (1990) when sweetcorn was stored at 1, 4, or 10 OC for 26 d unwrapped or

wrapped in stretch or shrink wrap. It was concluded that film wrapping maintained

freshness and reduced moisture loss better than the lack of wrapping. Also these authors

recommended stretch wrap over shrink wrap since stretch wrap generated higher CO2

levels (4-9% versus 1-3%) and lower 02 levels (14-19% versus 18-21%), which resulted

in higher total soluble solids retention. In that experiment also, an increase in microbial

growth was experienced in the MAP treatments, especially in the presence of damaged

husks or kernels, presumably due to the higher relative humidity within the MAP.

However, reduced 02 and elevated CO2 have also been reported to reduce decay

and maintain sweetcorn husk chlorophyll levels (Aharoni et al., 1996; Schouten, 1993).

Aharoni et al. (1996) demonstrated that sweetcorn wrapped with PVC film benefited

from the reduction of water loss, but the limiting factor affecting the shelf life of fresh

sweetcorn in this case was the increase in pathogens on the trimmed ends of the cobs.

Polyolefin stretch film has lower permeability rates for 02 and CO2 than PVC film, and

consequently generated higher CO2 levels (-10% versus -3%) and lower 02 levels (-7%









versus -15%) in the packages. This resulted in a significant reduction in the decay

incidence and water loss and significantly better maintenance of the general appearance

quality (Aharoni et al., 1996). These levels of 02 and CO2 did not trigger anaerobic

respiration, and ethanol levels in the packages were very low until the packages were

transferred to 20 OC to simulate retail conditions. Upon transfer to 20 OC, there was a

sudden increase in CO2 (20-25%) and decrease in 02 levels (2-4%), which increased the

ethanol concentration significantly in the sweetcorn in those packages. Nevertheless,

these high levels of ethanol had little effect on the general quality of the sweetcorn, since

off-odor occurred only in two types of packages, out of the six different combinations

used in the experiment.

Aharoni et al. (1996) suggested that the increase in microbial growth in MAP

reported by Deak et al. (1987) and Risse and McDonald (1990) was probably due to

relatively low CO2 levels within their packages, a consequence of the high ratio of the

CO2 versus 02 permeability coefficients of plastic films as mentioned previously.

Schouten (1993) demonstrated that CA storage with higher CO2 (2% 02 plus 10% CO2)

retained higher sugar content than air storage and CA was more beneficial when used at a

higher temperature (5-6 C compared with 1-2 OC). The CA storage significantly reduced

respiration and pathological breakdown, and also resulted in an increase in ethanol

content; but the higher content of sugars had a more positive influence on the taste than

the negative influence of the ethanol. On the other hand, Rumpf et al. (1972) clearly

proved that the loss in sucrose content, the decisive factor in determining the taste quality

of sweetcorn, may be delayed by both low temperature (0 C versus 5 or 10 C) and low

02 content, since 1% 02 was the best treatment in retaining the sucrose levels after 11 d









of storage at 5 C followed by 2% 02 then 4% 02 and finally air storage. All the CA

treatments in this experiment had 0% CO2.

The main difficulty in using MAP for sweetcorn is that the film permeability is

usually designed to maintain a desirable atmosphere during storage, but a rapid depletion

of 02 may occur in the retail display where the temperature is higher, resulting in a shift

toward anaerobic metabolism and causing rapid deterioration and quality loss. Recently

Silva et al. (1999b) introduced the idea of using MAP designed for the retail display

temperature and selecting the surrounding atmosphere inside a CA container during

transport to overcome the negative effect of changes in surrounding temperatures during

the postharvest chain. A similar idea was used by Rodov et al. (2000) who used a nested

sweetcorn package i.e., a film wrap for retail packages (PVC film-wrapped trays

containing two cobs of sweetcorn) suitable for the display temperature (20 C) along with

a master carton liner that provided the desirable atmosphere during shipping and storage

temperature (2 C). The low 02 levels (10-15%) and the high CO2 levels (5-10%)

obtained in both cases were beneficial in inhibiting mold growth but weren't severe

enough to increase the ethanol levels.

Due to the lack of work on CA/MA effects on sweetcorn, there are no specific

limits to cite for 02 and/or CO2 levels or the threshold of sweetcorn sensitivity to reduced

02 or elevated CO2 for more than a few specific temperatures and storage times, however

some work has shown that sweetcorn can tolerate up to 25% CO2 and down to 0.5% 02

without damage for 2 weeks at 1 OC, scoring the best in flavor ratings (Riad and Brecht,

2003; Spalding et al., 1978). The usual recommended CA combination for sweetcorn is a

more conservative 2% 02 plus 15% C02, which is assumed to show benefits and not









cause damage over the likely commercial range of temperatures and storage times

(Brecht, 2002; Saltveit, 1989).

Fresh-Cut Fruits and Vegetables

Fresh-cut fruits and vegetables (sometimes called minimally processed or lightly

processed fruits and vegetables) represent a relatively new and rapidly developing

segment of the fresh produce industry. Fresh-cut processing involves preparing fresh

produce to be ready to eat or cook by the final consumer. According to the International

Fresh-Cut Produce Association (IFPA), fresh-cut produce is defined as any fresh fruit or

vegetable or any combination thereof that has been physically altered from its original

form, but remains in a fresh state. Regardless of commodity, it has been trimmed, peeled,

washed and cut into 100% usable product that is subsequently bagged or prepackaged to

offer consumers high nutrition, convenience, and value while still maintaining freshness

(IFPA, 2002). Sales of fresh-cuts have grown from about $5 billion in 1994 to $10-12

billion in 2002, which is about 10% of total produce sales (includes retail and foodservice

sales). Packaged salads alone topped the $1.6 billion mark in retail sales in 1999, marking

a 15.9% increase from the previous year (IFPA, 2002).

Fresh-cut fruit and vegetable products are different from traditional, intact

vegetables and fruits in terms of their physiology and their handling requirements.

Fresh-cut produce is essentially purposely-wounded plant tissue that must subsequently

be maintained in a viable, fresh state for extended periods of time. Fresh-cut vegetables

deteriorate faster than intact produce as a direct result of the wounding associated with

processing, which leads to a number of physical and physiological changes affecting the

viability and quality of the produce (Brecht, 1995; Saltveit, 1997).









Limitations of Fresh-Cut Fruit and Vegetable Production

Water loss

Plant tissues are mainly composed of water and any small changes in water content

may have a large impact on produce quality and could cause a variety of negative

characteristics such as limpness, flaccidity, shriveling, wrinkling, and/or tissue

desiccation. Sams (1999) demonstrated that losing a small amount of water content like

3% or 5% of the water content in spinach or apple, respectively, render these

commodities unmarketable.

On the other hand, crispness, an important characteristic of fresh produce that is

related to water turgor pressure in the tissue, could be easily lost due to water loss. The

loss in crispiness results in softening and flaccidity. Leafy vegetables are particularly

susceptible to desiccation because of their large surface-to-volume ratios; moreover loose

leaves, such as spinach, are more prone to desiccation than a compact head, such as a

whole lettuce head (Salunkhe and Desai, 1984). Therefore, as a consequence of water

loss, appearance changes such as wilting and reduced crispness may occur.

In case of fresh-cut products, the possibilities to undergo severe water loss are

much higher than in the case of intact produce since during fresh-cut preparation the

produce goes through peeling and cutting or shredding, slicing, etc. Losing the skin has a

critical effect on many fresh-cut products because the skin is a protective waxy coating,

highly resistant to water loss, and thus peeled, fresh-cut fruits and vegetables are more

perishable and more susceptible to turgor loss and desiccation. On the other hand, the

mechanical injury brought on by cutting, shredding and/or slicing, directly exposes the

internal tissues to the atmosphere, promoting the evaporation of intercellular water and









hence starting desiccation of the tissue. Furthermore, fresh-cut preparation increases the

relative product surface area per unit mass or volume, which also increases the water loss.

Another factor affecting the water loss affects products that require rinsing after

cutting, since this is frequently followed by centrifugation. If accelerated centrifugation

speed or long centrifugation times are applied, increased desiccation can result, as

reported for cut lettuce (Bolin and Huxsoll, 1989).

The control of water loss in fresh-cut fruits and vegetables is possible through the

use of appropriate handling techniques, including temperature and relative humidity

control, which can greatly help minimize the rate of water loss. Reduction of water loss

can be achieved basically by decreasing the capacity of the surrounding air to hold water,

which can be achieved by lowering the temperature and/or increasing the relative

humidity. On the other hand, applying edible coatings to the fresh-cut commodity can

greatly reduce the water loss since such coatings provide an additional barrier to moisture

movement (Baldwin et al., 1995a). When a thin layer of protective material is applied to

the surface of the fruit or vegetable as a replacement for the natural protective tissue

(epidermis, peel), it can have several possible effects (Baldwin et al., 1996; Baldwin et

al., 1995a,b; Li and Barth, 1998; Nisperos and Baldwin, 1996). The coating may act as a

barrier to water loss or, alternatively, hygroscopic coating materials may serve to

maintain a moist surface appearance of the fresh-cut product. The coating may also work

as a semipermeable barrier that helps in reducing gas diffusion and hence increase

internal CO2 levels and reduce 02 levels, which results in a MA-like effect. This can help

reduce respiration, as well as associated senescence processes such as color and texture

changes, and may help to retain aroma volatiles. Coatings containing antimicrobial









compounds or with low pH may be used to reduce microbial growth. For example, the

primary parameter affecting fresh-cut celery quality is water loss because small

reductions in water content (2.5-5%) may lead to flaccidity, shriveling, wrinkling, and

pithiness. A significant increase in moisture retention by celery sticks was obtained with

the application of a caseinate acetylated monoglyceride coating (Avena-Bustillos et al.,

1997).

Texture change and loss of tissue firmness

During fruit ripening, one of the most notable changes is softening, which is related

to biochemical alterations at the cell wall, middle lamella, and membrane levels, although

significant roles in the softening process have been attributed to the pectic enzymes,

polygalacturonase, and pectin methylesterase (PME). Fresh-cut fruits and vegetables may

be considered to undergo a ripening-like process due to wound-induced ethylene

production and the subsequent increase in respiration, which results in loss of tissue

firmness.

Textural changes in fresh-cut vegetables and fruits are minimized at low

temperatures. Thus, temperature management is the first step to maintain the initial, fresh

textural quality of these products. Several treatments have been proven to be effective in

reducing fresh-cut firmness loss. A common treatment used to improve tissue firmness is

to spray or dip fruit or vegetable pieces in aqueous calcium salts, as described for

shredded carrots, zucchini slices, and fresh-cut pears, strawberries, kiwifruit, nectarines,

peaches, and melons (Agar et al., 1999; Gorny et al., 1999; 2002; Izumi and Watada,

1994; 1995; Luna-Guzman et al., 1999; Luna-Guzman and Barrett, 2000; Main et al.,

1986; Morris et al., 1985; Rosen and Kader, 1989). This is a result of the pivotal role

played by calcium in maintaining the textural quality of produce due to its effect of









rigidifying cell wall structure by cross-linking ester groups and also preserving the

structural and functional integrity of membrane systems (Poovaiah, 1986).

The use of CA or MA and MAP also retards senescence, lowers respiration rates,

and consequently slows the rate of tissue softening (Kader, 1992). Rosen and Kader

(1989) demonstrated that texture loss in strawberry fruit slices was reduced significantly

by using CA. Strawberry slices kept under CA for a week had comparable firmness to

whole and to freshly sliced fruit. Also, the best treatment to retain firmness of peach

halves was a combination of a dip in 2% calcium chloride and 1% zinc chloride, followed

by packaging with an 02 scavenger and storage at 0 to 2 oC (Bolin and Huxsoll, 1989).

Heat treatment also appears to have the potential to beneficially affect product

texture. Heat treatment of whole apple fruit resulted in firmer fresh-cut products when

compared with non-heated fruit (Kim et al., 1993). However, while heat treatment of

whole apples improved apple slice firmness, the storage temperature of the whole fruit

after heating also had a significant effect on product firmness. Mild heat treatment (45 C

for 1.75 h) of whole fruit prior to cutting retained greater fresh-cut apple firmness during

storage for 21 d at 2 OC (Kim et al., 1994).

Heat treatment could be applied prior to fresh-cut preparation, for example, apples

that were kept at 38 C for 6 d immediately after harvest, then cold stored for 6 months

prior to slicing and dipping in calcium solution, produced slices that were firmer than

slices from control fruit (Lidster et al., 1979). Also heat treatment could be used during

the calcium dipping treatment, as Luna-Guzman et al. (1999) demonstrated for fresh-cut

muskmelons cylinders dipped in 2.5% calcium chloride solution for 1 minute at different

temperatures (60, 40, or 20 C). The texture was firmer in samples treated at higher dip









temperatures, perhaps due to the activation of PME, which is known to be activated in the

55-70 C range, resulting in reduced pectin methyl esterification. The de-esterified pectin

chains may crosslink with either endogenous calcium or added (exogenous) calcium to

form a tighter, firmer structure.

However, there is a problem noticed when calcium chloride is used as a firming

agent, which is that it may result in undesirable bitter flavor of the product. Fresh-cut

cantaloupe cylinders dipped in calcium lactate solutions resulted in a textural

improvement similar to calcium chloride-treated fruit cylinders. Sensory evaluation

indicated that results were better since less bitterness and a more detectable melon flavor

were perceived (Gorny et al., 2002; Luna-Guzman and Barrett, 2000).

Color change

Most fresh-cut fruits and vegetables experience some kind of color change after

preparation. There are many reasons and degrees of this color change. The most common

color change is the browning discoloration found in many fresh-cut commodities due to

oxidation of colorless phenolic compounds to produce brown phenolic pigments.

Browning. Brown discoloration is one of the most limiting factors in the shelf-life

of fresh-cut products. The main reason for brown discoloration is enzymatic browning.

During the preparation stages, produce is subjected to operations where cells are broken,

causing enzymes to be liberated from tissues and put in contact with their substrates.

Enzymatic browning is the discoloration that results from the action of a group of

enzymes called polyphenol oxidases (PPO), which have been reported to occur in all

plants, and exist in particularly high amounts in mushroom, banana, apple, pear, potato,

avocado, and peach. Enzymatic browning must be distinguished from non-enzymatic

browning, which results upon heating or storage after processing of foods; types of non-









enzymatic browning include the Maillard reaction, caramelization, and ascorbic acid

oxidation.

Enzymatic browning is a complex process, which can be subdivided into two parts.

The first reaction is mediated by PPO, resulting in the formation of o-quinones (slightly

colored), which, through non-enzymatic reactions, lead to the formation of complex

brown pigments. The o-quinones are highly reactive and can rapidly undergo oxidation

and polymerization. The o-quinones react with other quinone molecules, other phenolic

compounds, aromatic amines, thiol compounds, ascorbic acid, and the amino groups of

proteins, peptides and amino acids (Nicolas et al., 1993; Whitaker and Lee, 1995).

Usually, brown pigments are formed, but in addition, reddish-brown, blue-gray,

and even black discolorations can be produced on some injured plant tissues. Color

variation among products of enzymatic oxidation is related to the phenolic compounds

involved in the reactions (Amiot et al., 1997), and both color intensity and hue of

pigments formed vary widely (Nicolas et al., 1993). Consequences of enzymatic

browning are not restricted to discoloration, since undesirable-tasting chemicals can also

be produced and loss of nutrient quality may result (Vamos-Vigyazo, 1981).

Heat treatments can be used to control fresh-cut produce browning. Brief exposures

to temperatures in the range of 40 to 60 OC can redirect plant tissue metabolism toward

production of heat shock proteins, which can, in some cases, prevent undesirable

metabolic processes from occurring (Saltveit, 2002). For example, the synthesis of

wound-induced enzymes such as the phenolic biosynthesis enzyme polyphenol oxidase

(PAL) can be prevented by giving lettuce tissue a brief heat shock (e.g., immersion in

45 C water for 90 s) after processing (Saltveit, 2000). While this technique is very









effective at preventing browning in plant tissue with constitutively low levels of phenolic

compounds (e.g., celery and lettuce), it is ineffective in tissue with constitutively high

levels of phenolic compounds (e.g., artichokes and potatoes).

Also there are different chemical treatments that help in controlling fresh-cut

product browning such as using acidulants, reducing agents, or chelating agents.

Polyphenol oxidase, which catalyzes the formation of o-quinones from o-diphenols,

beginning the sequence of reactions leading to formation of brown phenolic pigments in

vegetable and fruit tissue, has a pH optimum of 6.0 to 6.5 and shows little activity below

pH 4.5. Therefore, using citric, ascorbic, or erythorbic acids as acidulants helps by

reducing the pH and hence inhibiting PPO activity. The use of reducing agents such as

ascorbic acid or its isomer erythorbic acid helps in inhibiting the active oxidation

reactions and helps prevent brown pigment formation. Also, the use of chelating agents

such as ethylenediamine tetraacetic acid (EDTA) can reduce browning by chelating the

copper required for the PPO active site and thus inhibiting its activity (Brecht et al.,

2004).

Fresh-cut onions and sweetcorn kernels sometimes develop a brown or black

discoloration of unknown cause after cooking ('after-cooking browning'). This

discoloration, which limits the shelf life of these products, increases with longer storage

time of the fresh tissue at higher temperature, and is reduced by lowered 02 and/or

elevated CO2 (Blanchard et al., 1996; Langerak, 1975; Riad and Brecht, 2001; Riad et al.,

2003). Wounding before storage is apparently a prerequisite for sweetcorn after-cooking

browning because it does not occur if sweetcorn kernels cut from stored cobs are cooked

or when intact kernels are separated from the cob before storing (Brecht, 1999).









White blush. Another common color change is 'white blush' or 'white bloom' of

carrots, in which the bright orange color of fresh carrots disappears in stored fresh-cut

products, particularly when abrasion peeling is used. Fresh-cut carrots with white blush

develop a white layer of material on the peeled surface, giving a poor appearance to the

product. Upon peeling, the protective superficial layer (epidermis) of carrots is removed,

generally by abrasion, leaving cell debris and an irregular surface, which while moist

presents the natural orange color of carrots. The sequence of disruption of surface tissues

followed by dehydration and white blush formation was confirmed by scanning electron

microscopy, when comparing carrots peeled with a knife and a razor sharp blade.

Knife-peeled carrot surfaces appeared severely damaged, compressed, and separated

from underlying tissue, therefore prone to dehydration. Razor-peeled carrot surfaces were

cleaner and apparently only a thin layer of cells had been removed, resulting in a product

that upon drying did not acquire the whitish appearance (Tatsumi et al., 1991). It has been

suggested that phenolic metabolism may be activated by peeling, inducing increases in

lignin production, which result in the color change (Cisneros-Zevallos et al., 1995;

Howard and Griffin, 1993). Consumers perceive white blush carrots as aged or not fresh,

and using edible coatings was very successful in treating the white blush in carrots

(Sargent et al., 1994). Sensory results showed preference for carrots coated with edible,

cellulose-based coating, due to a fresh appearance (Howard and Dewi, 1995; 1996).

Yellowing. Yellowing of plant tissue due to chlorophyll degradation and exposure

of the preexisting yellow carotenoid pigments is a normal process in ripening or

senescence of many fruits and vegetables and this change can be accelerated by ethylene.

In fresh-cut products, the stress of the wounding during preparation results in increased









ethylene production, and hence increases the yellowing discoloration in green tissues.

Yellow discoloration was observed during storage of leafy and other green fresh-cut

products. Shredded Iceberg lettuce darkens (develops brown discoloration) during

storage, particularly at high temperatures. Simultaneously, loss of green pigmentation is

observed (Bolin et al., 1977; Bolin and Huxsoll, 1991). In a study of cabbage processed

into coleslaw, the color changed from green to a lighter white color during the cold

storage period as a result of chlorophyll degradation and because cabbage lacks yellow

pigments (Heaton et al., 1996).

The use of MAP (10% 02 plus 8% CO2) was beneficial in maintaining green color

in broccoli and reduced the yellowing of the florets at 10 oC compared to an unpackaged

control, which lost about 10% of its initial chlorophyll within 3 d of storage at 10 C

(Barth et al., 1993).

Microbial growth

Prior to harvest, plants are covered with a protective layer (the skin or epidermis),

which protects the plant cells from microbial attack. Due to tissue damage and the fact

that these tissues lose their protective skin during fresh-cut preparation, fresh-cut

products are more prone to increased microbial growth. This increase is also promoted as

a result of the release of cells fluids that microorganisms can use as nutrients (King and

Bolin, 1989). Cell sap released from cut cells floods adjacent intercellular spaces and,

when it comes in contact with bacteria, a suspension is formed that allows bacterial cells

to move into the intercellular space. Bacterial cells that contact the cell sap become

suspended, can move in the flooded intercellular spaces, and become protected from

surface treatments. Bartz et al. (2001) demonstrated that within 5 seconds of application

to a cut surface, cells of Erwinia carotovora became located in sites within tomato fruit









tissue that could not be successfully disinfected with 1.34 mM chlorine at pH 6.0.

Furthermore, the passage of a knife through plant tissues during preparation can drag

bacteria into contact with damaged cells (Lin and Wei, 1997).

The growth of bacteria, fungi, and/or yeasts may directly limit the life of fresh-cut

vegetables and fruits by causing changes in the appearance and texture of the products, or

through production of off-flavors and slimes that make them inedible. Many bacterial,

fungal, and yeast species have been found to limit shelf-life of fresh-cut fruits and

vegetables (Bartz and Wei, 2002; Farr et al., 1989; Heard, 1999; King and Bolin, 1989;

Korsten and Wehner, 2002; Robbs et al., 1996). In general, bacteria and certain fungi

cause decays in vegetables, including 'fruit vegetables', whereas fungi cause most of the

decays in fruits. This is because fruit tissues are low pH (3.5-4.5), which favors mold and

yeast growth, while vegetable tissues are nearer neutral pH (6.0-6.5), which favors

bacteria growth. The major reasons for the separation between classes of microbes and

plant hosts are not only the low pH of true fruits, but also the quantity and nature of the

acidulants responsible for the low pH (Brecht et al., 2004).

Microbial contamination and growth on or in fresh-cut vegetables and fruits is a

major concern for the industry (Beuchat, 1996; Fain, 1996; Francis et al., 1999; Hurst,

1995; Nguyen-the and Carlin, 1994; Zink, 1997) especially the possibility that certain

human pathogens also can grow or survive on fresh-cut vegetables and fruits. These

microbes ordinarily have no direct effect on the life of the product, but their presence

turns the fresh-cut product into an unacceptable product regardless of other quality

factors (Brecht et al., 2004). The major consideration in fresh-cut safety is inoculation of

the nutrient-rich flesh of vegetables and fruits with human pathogens during preparation.









The presence of human pathogens is of particular concern with fresh-cut products

because they are almost always consumed raw (i.e., without a heat treatment). In

addition, it has been suggested that the elimination of spoilage microbes without

elimination of human pathogens may extend shelf-life of a fresh-cut product to the point

that safety is compromised because the human pathogens may be more likely to

proliferate (Brackett, 1994; Hintlian and Hotchkiss, 1986).

Contamination of fruits and vegetables with human pathogens can occur during

growth in the fields, during harvesting and postharvest handling, in the course of

processing, and during transport (Beuchat, 1996). Human pathogens that have been found

on produce include bacteria, viruses, and parasites. These pathogens make contact with

the produce by cross contamination or by being naturally present in the environment.

That environment can include fields, air, and dust within a processing or packing facility.

In the field, produce is subjected to irrigation, fertilization, and animal contact. Irrigation

water is often not potable water and may contain pathogens (Sadovski et al,. 1978).

Beuchat and Ryu (1997) pointed out that soil contact can lead to accidental

contamination by immature compost or environmentally present pathogens. They also

stated that animal removal and control should be monitored frequently and, if possible,

all animals should be eliminated from entering the premises of vegetable and fruit

production and processing facilities.

There are many methods to control microbial growth in fresh-cut fruits and

vegetables such as sanitation, temperature control, modifying storage atmosphere

(CA/MA), and irradiation. Irradiation has the potential to eliminate vegetative forms of

bacterial pathogens as well as parasites and extend shelf life (Chervin and Boisseau,









1994; Farkas et al., 1997; Foley et al., 2002; Gunes et al., 2000; Hagenmaier and Baker,

1997; 1998; Molins et al., 2001; Prakash et al., 2000). However, irradiation doses

required to eliminate some microorganisms may cause vitamin C losses, negative textural

changes (Gunes et al., 2001), and enzymatic browning (Hanotel et al., 1995) in some

vegetable and fruit tissues. Irradiation levels of 1.5-20 kGy are necessary to destroy

yeasts and molds, which may exist as spores, and these levels are damaging to plant

tissues (Brackett, 1987; Kader, 1986a).

Sanitation of the produce, the washing tanks, and the fresh-cut preparation facility

is critical in controlling microbial growth in fresh-cut produce. Use of chemical sanitizers

has been successful in preventing contamination of food products by maintaining low

levels of microorganisms in the processing environment. Rigorous sanitation of

preparation areas reduces the level of microbial contamination, while chemical treatments

and low temperatures restrict microbial growth during storage and marketing. Sanitation

is usually done using cold (0-1 C) chlorinated water (0.67-2.7 mM free chlorine at

pH 7 or less). Application of chlorine is not very effective at reducing microbial levels on

contaminated tissues, but rather primarily acts to reduce microbial loads in the water and

prevent cross-contamination (Hurst, 1995). The chlorine rinse also removes cellular

contents at cut surfaces that may support microbial growth as well as promote browning,

and may also directly inhibit some browning reactions (Brecht et al., 2004). Heated water

may also be useful alone or as a supplement to sanitizer treatment in reducing microbial

populations on fresh-cut products. Delaquis et al. (1999) demonstrated a 3-log reduction

in microbial (mainly pseudomonad) levels on fresh-cut lettuce washed in chlorinated









(1.34 mM NaOC1) water at 47 C for 3 minutes compared with a 1-log reduction using

4 C chlorinated water.

Nowadays, there are many other alternatives sanitizers to chlorine that have been

used or proposed for use in fresh-cut plants include chlorine dioxide (CO02), bromine and

iodine compounds, hydrogen peroxide (H202), peroxyacetic acid, and ozone (Beuchat,

2000).

On the other hand, several chemicals can be used by the fresh-cut produce industry

to control microbial growth during storage including acidulants such as sorbic acid and

benzoic acid, which lower pH to levels unfavorable to microbes and thus inhibit their

growth during storage (Chipley, 1993; Sofas and Busta, 1993). Although the actual

mechanism of action of sorbic acid against bacteria is not known, there are some theories

as to the action which include the possibility that sorbate inhibits amino acid uptake

resulting in either destruction or disruption of the membrane There is also the theory the

sorbate effects enzyme activity by the accumulation of beta unsaturated fatty acids

preventing the function of dehydrogenase inhibiting metabolism and growth and the last

possibility states that sorbate potentially inhibits respiration by competitive action with

acetate in acetyl coenzyme A formation. (Davidson 2001).

Beside their other benefits, edible coatings help retain acidulants and antimicrobials

on cut surfaces (Baldwin et al., 1995a,b). Incorporating the antimicrobials potassium

sorbate and sodium benzoate into edible coatings on fresh-cut apple and potato improved

their effectiveness compared with aqueous dips (Baldwin et al., 1996).

Also, one of the important factors in controlling microbial growth on fresh-cut

products is management of temperature and relative humidity (Babic and Watada, 1996;









Omary et al., 1993; Riad and Brecht, 2003). Maintenance of low temperature throughout

the postharvest chain plays a pivotal role in controlling microbial growth either by

retarding the microbe's activity or by enhancing the produce quality by delaying ripening

and senescence (Heard, 1999) and delaying tissue senescence.

On the other hand, CA/MA storage helps in controlling microbial growth directly

and indirectly. The direct effect came from the effect of CA/MA on the microorganisms

and the indirect effect is a result of the effect of CA/MA on the plant physiology. It is

known that CA/MA reduces the respiration rate and delays ripening and senescence and,

since delaying senescence (including fruit ripening), reduces susceptibility to pathogens,

CA/MA has a beneficial effect in decreasing postharvest diseases (Daniels et al., 1985;

El-Goorani and Sommer, 1981). Levels of 02 below 1% and levels of CO2 above 10%

can have a significant inhibitory effect on fungal growth. Carbon dioxide levels of

10-15% (in commodities that tolerate such levels) can be used to provide a fungistatic

effect (Kader, 1986b; Kader et al., 1989). It is well known that sweetcorn can tolerate

very high CO2 levels, as high as 25%, without any symptoms of injury (Riad et al., 2003;

Spalding et al., 1978)

Nutrient loss

Nowadays there is more awareness of the importance of fruits and vegetables as a

great source of antioxidant compounds such as polyphenolics, vitamin C, vitamin E,

P-carotene, and other carotenoids. It has been suggested that these phytonutrients have

long-term health benefits and may reduce the risk of diseases such as cancer and heart

disease. Most of these antioxidant compounds are known to inhibit cellular and DNA

damage caused by reactive oxygen species and free radicals that may lead to degenerative









diseases (Hu et al., 2000; Lagiou et al., 2004; Marrow, 1998; Tapiero et al., 2004).

Vegetables and fruits are the primary source of these antioxidant compounds in our daily

diet and there is a lot of epidemiological work that shows strong correlations between the

delay or suppression of certain diseases and consumption rates of fruits and vegetables

(Block and Langseth, 1994; Gershoff, 1993; Steinmetz and Potter, 1996; Ziegler, 1991).

Retaining maximum bioactivity of these phytonutrients in fresh-cut fruits and vegetables

is a very important goal and this could be achieved through better understanding of the

effect of fresh-cut processing, packaging, and storage on bioactivity of these compounds.

Many investigations with fresh-cut fruits and vegetables have demonstrated that

concentrations of vitamins and other phytochemical compounds are reduced following

fresh-cut operations as a result of wounding and are affected by conditions of handling,

packaging, and storage (Kader, 2002; Klein, 1987). But, on the other hand, stress

associated with processing may also initiate biosynthesis of many compounds that affect

antioxidant content and product quality. The synthesis of wound ethylene after fresh-cut

operations can stimulate a variety of physiological responses including loss of vitamin C

and chlorophyll and induction of polyphenolic metabolism (Kader, 1985; Saltveit, 1999;

Tudela et al., 2002a,b).

Carotenoids. Carotenoids are important compounds in vegetables and fruits for

their excellent antioxidant properties and the diversity of color they provide. Human daily

consumption of carotenoids is mainly from vegetables and fruits (Goddard and Matthews,

1979). Carotenoids have diverse roles in the biological functioning of both plants and

humans. They possess provitamin A and antioxidant activity, modulate detoxifying

enzymes, regulate gene expression, aid in cellular communication, and augment immune









functions (Clevidence et al., 2000). Due to their role as antioxidant compounds, as well as

the color characteristics they impart to vegetables and fruits, exploration of techniques to

retain carotenoids is vital for nutritional and sensory quality characteristics. Disruption of

plant tissues by mechanical means or during senescence can also lead to rapid destruction

of carotenoids through the action of oxidase enzymes, and may be prevented by the use

of reducing agents or MA (Biacs and Daood, 2000; Simpson et al., 1976).

Vitamin C. Ascorbic acid (vitamin C) is a water-soluble antioxidant long

associated with inhibition of oxidative reactions and is a key marker compound for

determining the extent of oxidation in fresh-cut vegetables and fruits (Barth et al., 1993).

Ascorbic acid is easily destroyed during fresh-cut operations and levels are affected by

cutting technique (Barry-Ryan and O'Beirne, 1999), gas composition (Gil et al., 1998a;

1999), package design (Barth and Zhuang, 1996), water loss and storage

time/temperature (Nunes et al., 1998; Lee and Kader, 2000), light intensity, heat, oxidase

enzymes, and pro-oxidant metals (Albrecht et al., 1991; Lee and Kader, 2000).

Polyphenolics. Polyphenolics are a major category of antioxidant compounds

present in vegetables and fruits that encompass thousands of individual compounds in

various commodities and concentrations. Recent findings have increased the interest in

polyphenolic compounds present in fresh and fresh-cut vegetables and fruits due to their

elevated antioxidant capacity.

Polyphenolics, along with carotenoids and ascorbic acid, constitute a significant

portion of the overall antioxidant capacity of vegetables and fruits; therefore maintaining

their level in fresh-cut produce is critical for optimal human health. In previous reviews,

the nutritional content was believed to decrease in fresh-cut as compared with intact









vegetables and fruits, especially levels of vitamin C (Klein, 1987; McCarthy and

Matthews, 1994). Following tissue wounding and exposure to light and air, antioxidant

phytochemicals may be lost to enzymatic and oxidative action at the site of cellular

disruption, in secondary or coupled oxidation reactions with lipids, in reactions with

wound ethylene, from exposure to chlorinated sanitizers, or from mild desiccation (Barth

et al., 1990; Nunes et al,. 1998; Park and Lee, 1995; Wright and Kader, 1997a,b).

Therefore, developing postharvest treatments to alleviate phytonutrient loss following

fresh-cut operations is vital to insure that maximum levels of phytonutrients reaching the

consumer.

Several treatments are used in the fresh-cut industry to achieve the goal of

maintaining maximum nutritional value in fresh-cut products. For example, proper

temperature control is among the most critical factors influencing nutrient retention in

fresh-cut vegetables and fruits as enzymatic and oxidative reactions occur more rapidly at

elevated storage temperatures. Temperature control serves to reduce microbial

populations and slow chemical reactions that affect sensory characteristics and

phytochemical concentrations. Also, different physical and chemical treatments have

been investigated in fresh-cut vegetables and fruits as means to maintain fresh-like

characteristics and nutritional quality. Mild heat treatments or surface acidification have

been used as means to inhibit oxidative enzymes and serve to protect some nutrient

compounds, as long as a high water-activity is maintained (Dorantes-Alvarez and Chiralt,

2000). Maintaining a high RH during storage was shown to be effective in retaining

antioxidant compounds (Jiang and Fu, 1999). Ascorbic acid is commonly applied to cut

surfaces through edible coatings or dips to prevent browning on cut surfaces: acting both









as an acidulant and a reducing agent, ascorbic acid can reduce quinones back to colorless

phenolic compounds. Combinations of reducing agents and acids were effective in

prevention of surface browning and retention of sugars and organic acids in fresh-cut

apples and their effect may be enhanced when combined with additional preservation

techniques such as MAP or proper temperature control (Buta et al., 1999).

On the other hand, the use of MAP has proven to be an effective means to reduce

enzymatic and autooxidative reactions affecting phytonutrients in fresh-cut vegetables

and fruits by reducing concentrations of 02 and increasing CO2. Modified atmospheres

were used to maintain higher levels of provitamin A and vitamin C in fresh-cut broccoli

(Barth et al., 1993; Barth and Zhuang, 1996; Paradis et al., 1996) and jalapeno peppers

(Howard and Hernandez-Brenes, 1998), but had little effect on provitamin A

concentrations in peach and persimmon slices (Wright and Kader, 1997a), and was

ineffective in ascorbic acid retention in sliced strawberry or persimmon (Wright and

Kader, 1997b). Extreme CO2 concentrations (>20%) may actually cause greater

degradation or suppressed synthesis of vitamin C (Agar et al., 1997; Tudela et al., 2002b;

Wang, 1983) and anthocyanins (Gil et al., 1997; Holcroft et al., 1998; Holcroft and

Kader, 1999; Tudela et al., 2002a), while certain CO2 levels may induce biosynthesis of

provitamin A carotenoids (Weichmann, 1986). Modified atmospheres did not affect

flavonoid content of Swiss chard, but significantly reduced levels of ascorbic acid (Gil et

al., 1998a), while flushing packages of fresh-cut lettuce with 100% N2 retained higher

ascorbic acid concentrations than passive MAP and air controls (Barry-Ryan and

O'Beirne, 1999). In fresh-cut spinach, flavonoid content remained constant during

storage in air or MAP, but spinach in MAP contained higher dehydroascorbic acid









concentrations that resulted in lower antioxidant activity compared with air-stored

spinach (Gil et al., 1999). However decreases in flavonoids were observed in 'Lollo

Rosso' lettuce stored in MAP (Gil et al., 1998b), further indicating a commodity-specific

role of gas composition on overall phytonutrient retention.

Furthermore, the application of food-grade compounds into wash water or on the

surface of fresh-cut vegetables and fruits as coatings has an advantage of immediate

benefits at the active site of phytonutrient deterioration. The benefits of these edible

coatings include decreased respiration rate, browning inhibition, and retention of various

quality factors by creating a barrier to 02, which influences enzymatic and nonenzymatic

oxidation rates (Li and Barth, 1998). Edible coatings are a common method to extend the

fresh-like appearance and quality characteristics of many vegetables and fruits (Baldwin

et al., 1995a,b; 1996). Fresh-cut carrots with cellulose-based edible coatings retained

greater provitamin A levels during storage in one study (Li and Barth, 1998), but another

coating had no effect (Howard and Dewi, 1996).

Loss of flavor and aroma

Flavor quality of fresh-cut vegetables and fruits is critical to their acceptance and

appreciation by consumers. Sensory attributes such as sweetness and characteristic aroma

may be the most important indicators of shelf life from the consumer's point of view. The

challenge in fresh-cut vegetable and fruit handling is to maintain the taste and aroma

attributes of the original whole product. Due to the short shelf life of fresh-cut fruits and

vegetables, starting with produce at its optimum maturity or ripeness stage and using the

highest quality standards is extremely important in maintaining flavor shelf life (Beaulieu

and Gorny, 2002).









Taste and aroma together make up flavor, which contributes to the recognizable

nature of a food. Taste refers to detection of nonvolatile compounds on the tongue while

aroma is related to volatile compounds detected in the nose. These two aspects of flavor

are inextricably linked as it has been shown that perception of aroma can be influenced

by levels of taste components, and vice versa (Beaulieu and Baldwin, 2002).

The taste component in fruits and vegetables mainly depends on the sugar and

organic acid levels and the relation between them. As a result of increased respiration due

to wounding during preparation of fresh-cut produce, there is a depletion of sugar levels

in the commodity and therefore organoleptic quality is reduced, especially in products

like melons whose quality depends on high levels of sugars. This problem tends to be

worse in fruits such as tomato or melon that have very limited capacity to replenish

soluble sugars lost to accelerated respiration during storage or ripening, in comparison

with fruits such as banana and apple that have a reserve of starch and can convert it to

sugars during ripening. Organic acids are one of the major respiratory substrate and the

increase in tissue pH in fresh-cut apples has been attributed to utilization of organic acids

in respiration (Kim et al., 1993). Depletion of acids also can have negative organoleptic

effects in fruits like apple, peach, and mango for which the balance of sweetness (sugars)

and tartness (acids) is an important flavor attribute.

The second component of flavor, aroma, is related to synthesis of volatiles during

the growth and development of fruits or vegetables, but the most dramatic production

coincides with fruit ripening. These volatiles include alcohols, aldehydes, esters, ketones,

lactones, and other compounds (Baldwin, 2002). Little is known about the effects of

different storage temperatures and atmospheres on normal aroma volatile production in









fresh-cut fruits and vegetables other than the inhibition that occurs as a consequence of

chilling injury (Buttery et al., 1987; Maul et al., 2000). Brecht et al. (2004) stated in their

review that fresh-cut fruit taste becomes bland during the course of extended storage due

to the loss and/or the reduced production of aroma volatiles. Recently there is an interest

in designing edible coatings to retain volatile flavor compounds within the tissue as a

method to overcome this problem (Baldwin et al., 1998; Miller and Krochta, 1997).

Fresh-Cut Sweetcorn Kernels

There is not much information about sweetcorn as a fresh-cut commodity. A few

European retailers prepare fresh-cut sweetcorn kernels onsite and don't store them due to

the excessive perishability of the product. It is well documented that sweetcorn is a very

perishable product as a result of its very high respiration rate, which results in a quick

loss of the sweetness (the most important characteristic of sweetcorn) unless it was

rapidly cooled and stored at a low temperature (as close to 0 oC as possible).

Supplementation of proper temperature management by gas modification (CA or MA)

greatly helps in maintaining high quality through reducing the high respiration rate

(Brecht, 2002; Riad and Brecht, 2001; Riad et al., 2003). As a result of fresh-cut

processing (cob trimming, de-husking, and kernel separation), the respiration rate

increases and also there may be increased microbial load due to the open wounds in the

kernels. This makes fresh-cut sweetcorn kernels a very delicate product, yet the early

work by Brecht (1999) showed that there is great potential for sweetcorn kernels as a

fresh-cut product. The main problem in fresh-cut sweetcorn kernels storage is the

formation of brown pigment in kernels when they are cooked. This after-cooking

browning is more pronounced as kernel maturity advances, increases with longer storage

period and higher storage temperature, and is also higher in cut kernels than whole









kernels (Brecht, 1999). Riad et al. (2003) found that the browning starts to appear after

7 d of storage at 5 C and was prevented during 2 weeks storage at 0 OC. Also, storing the

cut kernels in CA (2% 02 plus 10% CO2) prevented this after cooking browning even

when the kernels were stored at 5 OC.

It has been suggested (Brecht, 1999, Riad and Brecht, 2001) that sweetcorn after-

cooking browning may be caused by the Maillard reaction, a non-enzymatic reaction

usually associated with food processing due to a reaction between free sugars and basic

amino acids at elevated temperatures, forming a brown pigment. Sweetcorn has a high

background of soluble sugars (Courter et al., 1988) and amino acids (Grunau and

Swiader, 1991). More work is needed to determine the exact cause for sweetcorn after-

cooking browning.














CHAPTER 3
SWEETCORN TOLERANCE TO REDUCED 02 WITH OR WITHOUT ELEVATED
CO2 AND EFFECTS OF CONTROLLED ATMOSPHERE STORAGE ON QUALITY

Introduction

Storing fruits or vegetables in controlled (CA) or modified atmosphere (MA)

enriched with high CO2 and/or utilizing low 02 levels could be a very beneficial tool in

maintaining product quality and extending shelf life. Atmosphere modification is

achieved either by reducing the concentration of 02, which is required for respiration, or

by increasing the CO2 concentration, which inhibits respiration and ethylene action.

Reducing respiration consequently retards sugar loss and the growth of pathogens; and it

also decreases yellowing through decreasing chlorophyll degradation. Several articles

have been published on the benefits of CA/MA technology on the extension of perishable

product shelf life (Chinnan, 1989; El-Goorani and Sommer, 1981; Kader, 1987; Kader

et al., 1989; Shewfelt, 1986; Zagory and Kader, 1988).

On the other hand, increasing CO2 above, or decreasing 02 below the commodity

tolerance level may result in product deterioration due to anaerobic respiration and off-

flavor and odor development. These tolerance levels may be affected by the physiological

condition of the commodity, the storage temperature, and the storage duration. One of the

challenges in CA storage is to determine the tolerance levels that result in the maximum

benefits with the minimum possibility of injury to the stored commodity.

Fresh sweetcorn (Zea mays L. rugosa) is a very perishable food product and is

prone to rapid postharvest deterioration caused by kernel desiccation, loss of sweetness,









husk discoloration, and loss of taste and aroma. Most of these problems are mainly due to

its high respiration rate, which suggested that using CA/MA could be beneficial in

sweetcorn storage.

The early work by Spalding et al. (1978) found that sweetcorn appearance and

flavor were not significantly improved using CA storage and they concluded that

breeding cultivars that better retain quality, in combination with prompt pre-cooling,

offers more potential for success than MA or CA. On the other hand, there are many

other reports stating the beneficial effects of using gas modification either by CA or MA

for sweetcorn. Deak et al. (1987) demonstrated that using modified atmosphere

packaging (MAP) eliminated water loss and maintained beneficial CO2 and 02 levels

surrounding the sweetcorn. These effects together with lower storage temperature

markedly reduced postharvest deterioration and extended the shelf life. But MAP

treatment increased microbial growth due to the water-saturated atmosphere. Similar

results were obtained by Risse and McDonald (1990). However, reduced 02 and elevated

CO2 have also been reported to reduce decay and maintain husk chlorophyll levels

(Aharoni et al., 1996; Schouten, 1993). Aharoni et al. (1996) suggested that the increase

in microbial growth reported by Deak et al. (1987) and Risse and McDonald (1990) was

probably due to relatively low CO2 levels during storage. The main benefit of CA/MA is

that the loss in sucrose content, the decisive factor in determining the taste quality of

sweetcorn, was delayed by high CO2 and/or low 02 content (Riad et al., 2002, Risse and

McDonald, 1990; Rumpf et al., 1972; Schouten, 1993).

Generally, the quality of fresh sweetcorn depends to a large extent on the kernel

sweetness, texture, and flavor. Sweetness in sweetcorn is closely related to kernel sucrose









content (Reyes et al., 1982). Texture and eating quality of sweetcorn consists of several

factors, including pericarp tenderness, levels of water-soluble polysaccharides (WSP) in

the endosperm, and moisture content (Bailey and Bailey, 1938; Culpepper and Magoon,

1927; Wann et al., 1971). On the other hand, flavor and aroma, which are not as easily

defined as sweetness and texture, are most often associated with kernel content of many

volatiles that are present in fresh sweetcorn and considered to contribute to its

characteristic flavor, especially dimethyl sulfide (DMS), which is responsible for the

characteristic aroma of cooked sweetcorn (Flora and Wiley, 1974a,b; Wiley, 1985;

Williams and Nelson, 1973; 1974). Despite its importance in the overall quality of

sweetcorn, there is no or very little information on the effect of CA on the flavor and

aroma and specifically on the levels of DMS during storage.

The objective of this work was 1) to give a better idea about the tolerance level of

sweetcorn to reduced levels of 02 with and without elevated levels of C02; 2) to

determine the best gas composition that results in the best quality after storage; and 3) to

determine the effects of CA storage on the different quality attributes of sweetcorn.

Materials and Methods

Plant Material

Fresh harvested sweetcorn (Zea mays L. rugosa var. Prime Time) cobs were

harvested at a local farm in Florida (Hugh Branch Inc., Pahokee) on 26 November

1999 precooled, and delivered overnight by refrigerated truck to a local grocery chain's

distribution facility in Jacksonville (Publix Distribution Center, Jacksonville, Fla.) on

27 November, where it was picked up, transferred to the postharvest lab (University of

Florida, Gainesville, Fla.), and the experiment started on the same day. As soon as the

sweetcorn arrived at the lab, the cobs were sorted, trimmed, and the outermost leaves









were removed. A window of about 3-4 cm width was opened in the husk to show

3-4 rows of kernels. The sweetcorn cobs were kept at 5 OC for about 3 h until they were

distributed among the treatments at the start of the CA application.

Controlled Atmosphere Storage

A flow-through CA system was used. Four sweetcorn cobs were placed in each of

three, 3.8-L glass jars per treatment in a 5 C storage room, then the jars were closed and

the CA gas mixture was introduced to the jars. Six gas compositions were used in this

study, namely air, air plus 15 or 25% CO2 (which resulted in 02 concentrations of

17.7 and 15.6%, respectively), 2% 02, and 2% 02 plus 0, 15, or 25% CO2. The balance in

all CA treatments was made up with N2. The CA gas mixtures were established using a

system of pressure regulators, manifolds, and needle valve flowmeters to blend air, N2,

and CO2 from pressurized cylinders. The CA was a flowing system with flow rates set to

maintain respiratory CO2 accumulation under 0.5%. The gas flow was humidified by

bubbling the gas mixture through water before introducing it to the containers. Gas

composition in CA was monitored daily for 14 d using a Servomex 02 and CO2 analyzer

(Servomex, Norwood, Mass.). The 02 analyzer measures the paramagnetic susceptibility

of the sample gas by means of a magneto-dynamic type measuring cell, while CO2 is

measured by infra-red absorption using a single beam. The analyzer was calibrated using

N2 and certified 02 and CO2 standards.

Parameters

Respiration rate

Respiration rate was measured daily using a static method in which the storage

containers were closed and incubated for 2 h. Carbon dioxide concentration was

measured before and after incubation, and respiration rate was calculated.









Carbon dioxide levels were measured using a Gow Mac, series 580 gas

chromatograph (Gow Mac Instruments Co., Bridgewater, N.J.) equipped with a thermal

conductivity detector (TCD) and a Hewlett Packard Model 3390A integrator (Hewlett

and Packard Co. Avondale, Pa.). Column, detector, and injector temperatures were

40, 28, and 28 C, respectively. Detector current was 90 mA during the analysis. Carrier

gas was He with a 30 mLmin-1 flow rate at 276 KPa (40 psi). A 1-mL sample was

withdrawn from the storage container headspace using a 1.0 mL plastic syringe and 0.5

mL was injected in the gas chromatograph. Calibration was done prior to sample analysis

using a certified standard mixture.

Sugar content

Total soluble sugars were measured in raw and cooked kernels using the

phenol-sulfuric method described by Dubois et al. (1956). In this experiment, 15 g of

sweetcorn kernels were blended in a commercial blender with 85 mL of 95% ethanol for

2-3 min then were heated for 20 min in an 85 C water bath. Then the samples were

stored overnight at -20 OC to precipitate ethanol-insoluble materials. The samples were

then filtered through Whatman #2 filter paper in a Buchner funnel attached to a side-arm

Erlenmeyer flask. A vacuum pump was connected to the side arm to expedite the

extraction. The collected filtrates were volumetrically adjusted to 200 mL using 95%

ethanol and 20 mL portions of the extracts were stored in scintillation vials at -20 OC until

the measurements were performed.









To prepare samples for measurement of total soluble sugars, 0.5 mL of 5% phenol

(w/w) was added to 0.5 mL of diluted (1:250) ethanol extract in test tubes, the mixture

was vortexed, then 2.5 mL concentrated sulfuric acid was added and the mixture vortexed

again. The test tubes were let to stand for 10 min then were placed in a 25 C water bath

for another 10 min to stop the reaction. The total soluble sugars were measured by

reading the absorbance at 490 nm using a Shimadzu UV-1201 spectrophotometer

(Shimadzu Scientific Instruments, Inc.; Tokyo, Japan). The absorbance values were

compared to a standard curve of glucose.

Dimethyl sulfide (DMS) content

Dimethyl sulfide was measured chromatographically using a modified method from

Ren et al. (2001) in which a water bath was used instead of a microwave oven. A 10-g

sample of sweetcorn kernels was placed in a 40-mL glass vial fitted with a septum under

the screw cap (Fisher Scientific, Pittsburgh, Pa., Cat. No. 05-727-6) then the vial was

placed in a water bath (70 C) for 2 h. A 1-mL sample of the vial head space was

withdrawn with a 1.0-mL plastic syringe and injected on a HP 5890 Series II gas

chromatograph (Hewlett Packard Co. Avondale, Pa.) equipped with a flame ionization

detector and a Hewlett Packard Model 3390A integrator. A 3 mm by 2 m stainless steel

column with 20% OV-101 on 80/100 CWHP packing (Alltech, Deerfield, Ill.) was used

and the carrier gas was N2 (30 mL-min1). Oven, injector, and detector temperatures were

70, 150, and 200 C, respectively. Calibration was done prior to sample analysis using

standards that were prepared from the authentic compound (Sigma-Aldrich Co. St. Louis,

Mo.; Cat. No. 47,157-7)









Visual appearance evaluation

Husk, silk, and kernel appearance, and kernel denting were measured using a scale

from 1 to 9, where a score of 9 represents excellent quality and a score of 1 represents the

lowest quality level as described in Table 3-1.

Statistical Analysis

The experimental design was a completely randomized design with three replicates

consisting of four sweetcorn cobs in each of three jars per treatment. The data were

analyzed using the Statistical Analysis System (SAS). Analysis of variance (ANOVA)

was used to determine the effect of treatments on the dependent variables. Means were

separated by the least significant difference (LSD) test at P<0.05.

Results and Discussion

Controlled atmosphere storage is known for its beneficial effects on fresh fruits and

vegetables. Sweetcorn was expected to benefit from this technique during storage since

CA can reduce the high respiration rate of sweetcorn and consequently reduce sugar,

water, and flavor losses. In this work, sweetcorn was stored in different gas compositions

in a flow-through CA system in which fresh sweetcorn cobs were stored in air or 2% 02

plus 0, 15, or 25% CO2 at 5 C to determine the best atmosphere composition for

maintaining quality and in order to determine the tolerance levels of sweetcorn to low 02

and/or high CO2.

Respiration rate (Fig. 3-1) was significantly affected by CA treatment. Storing

sweetcorn in 2% 02 plus either 15 or 25% CO2 significantly increased the respiration rate

compared with air storage, which might be a result of the sweetcorn metabolism

switching to anaerobic respiration. There was no significant effect of 2% 02 plus 0% CO2

on respiration, and storage in either 15 or 25% CO2 in air (17.7 or 15.6% 02,









respectively) significantly reduced respiration compared with the air control. These

results suggest that sweetcorn is more sensitive to reduced 02 plus elevated CO2 together

than to either alone. Riad et al. (2002) reported that no ethanol or acetaldehyde was

detected in the atmosphere surrounding sweetcorn stored in perforation-mediated

modified atmosphere packaging (PM-MAP) even with steady state CO2 levels as high as

25% plus 02 levels as low as 1%, but de-husked sweetcorn was used in those

experiments, which might have affected the tolerance levels to high CO2 and low 02.

Also, these results are not with agreement with Morales-Castro et al. (1994), who

reported that elevated CO2 had a negligible effect on respiration rate in the range from

0 to 15% while reduced 02 had a large influence on respiration rate in the range from 21

to 0%.

While all CA treatments had significantly higher levels of sugars than the air

control (Fig. 3-2), which is the main quality parameter for sweetcorn (Evensen and

Boyer, 1986; Showalter and Miller, 1962), the highest sugar content was found in the two

CA treatments that resulted in reduction of respiration rate (Fig. 3-1). These results are in

agreement with the results obtained by Riad et al. (2002) who reported that higher sugar

content was maintained in PM-MAP treatments with CO2 concentrations up to 25%.

Also, in the early work by Rumpf et al. (1972), higher sugar levels were obtained with all

CA treatments over air storage, even with 02 concentration as low as 1%. Schouten

(1993) observed that even when CA treatment resulted in a significantly higher ethanol

production, which might indicate fermentative metabolism, there was no effect on the

sugar content. On the other hand, these results are not in agreement with the results









obtained by Spalding et al (1978) who found that CA treatment resulted in no or negative

impact on sugar content.

Controlled atmosphere treatments had a positive impact on the visual quality of

stored sweetcorn (Fig. 3-3). Elevated CO2 significantly reduced loss of greenness and

maintained the fresh appearance of the husks. The CA also improved silk appearance

(reduced discoloration) over the air control, but it had no effect on kernel denting, which

might be due to the minimal water loss that occurred in all treatments since all the gas

mixtures were humidified before being introduced to the storage containers. Kernel

appearance was also improved in all CA treatments, especially air plus 15% CO2 and 2%

02 plus 15% CO2. Storage in 2% 02 plus 15% CO2 resulted in the best score in all visual

quality parameters. These results are in agreement with Aharoni et al. (1996) who

demonstrated that sweetcorn stored in MAP with high CO2 level had better general

appearance quality due to better green color maintenance of the husk, less denting (due to

the reduced denting as a result of the reduction in weight loss), and also reduced decay

incidence.

Dimethyl sulfide is the main characteristic volatile component in sweetcorn and is

responsible for the characteristic aroma of cooked sweetcorn, providing the "corny" smell

during cooking (Flora and Wiley, 1974b; Wiley, 1985; Williams and Nelson, 1973; 1974;

Wong et al., 1994). It was noticed that the levels of this compound is increased by

canning or freezing (Flora and Wiley, 1974a) but there is no information about the effect

of storage in CA storage on the levels of DMS. There were significantly greater

concentrations of DMS in CA-stored sweetcorn compared with the air control at the end

of the 14-d storage period (Fig. 3-4). Storing sweetcorn in 2% 02 plus 15% CO2 resulted









in the highest DMS content followed by air or 2% 02 plus 25% CO2. The results suggest

that elevated CO2 was more efficient than reduced 02 in maintaining higher levels of

DMS after 14 d of storage at 5 OC.

In conclusion, CA storage was beneficial in maintaining most sweetcorn quality

parameters during 14 d of storage at 5 OC. Storage in 2% 02 plus 15% CO2 gave the best

result in terms of quality maintenance since it preserved the highest sugar level, reduced

deterioration in sweetcorn visual quality to the greatest extent, and maintained the highest

DMS content and thus presumably the highest flavor and aroma. Sweetcorn had a high

tolerance level to either 2% 02 or 25% CO2 alone for 2 weeks, but the tolerance was

apparently less when the two gases were combined since 2% 02 plus 25% CO2 provided

less benefits compared with using 2% 02 plus 15% CO2.












Table 3-1. Description of the visual quality ratings used for visual quality evaluation of sweetcorn after storage.


Parameter


Rating Description


Field fresh




Good




Fair


3 Unmarketable


Husk appearance

Field fresh, turgid
appearance



Reasonably fresh,
leaves still green and
flexible

Drying and yellowing
apparent, some leaves
brittle

Portion of the leaves
show dry appearance or
water soaking


Unusable Completely dried out


Silk appearance

Light color, fresh, and
turgid



Not that fresh but still
had a light color and
turgid texture

Starting to show some
brown color and some
drying

Obvious discoloration
and dryness with limp
texture

Limp texture or watery
decay


Kernel appearance

Fresh with bright, shiny
color and very turgid



Still noticeably bright
color and turgid


Dull appearance with
no denting


Severe dull appearance
and some brown or
white color

Kernels showing severe
denting or decay


Kernel denting

No denting with turgid
kernels and bright
appearance

No denting with
slightly dull kernel
appearance

A few kernels showing
denting and more dull
kernels

10-30% of the kernels
showing denting and
dull appearance

More than 30% of the
kernels dented













- -- air (Control)

-- 2% 02 + 0% C02


140



- 120
6)

04
o 100

E
g 80


0
O

60

,.
40


--air + 15% C02


-Aair+ 25% C02


-- 2% 02 + 15%C02 -m- 2% 02 + 25%C02


1 2 3 4 5 6 7 8 9 10 11 12 13 14

Storage duration (d)



Fig. 3-1. Effect of controlled atmosphere storage on respiration rate (mL CO2-kg-h-1) of
sweetcorn stored in air or 2% 02 plus 0, 15, or 25% CO2 at 5 OC. Bars indicate
+ 1 SD value (n=3).







57




11
10
9 b b
8c c


6
8,-


S4
0)

= 3
C'
2
1-

air air + air + 2%02 + 2%02 + 2%02 +
(Control) 15%C02 25%CO2 0%C02 15%CO2 25%CO2

Treatment

Fig. 3-2. Effect of controlled atmosphere storage on the sugar content (%) in sweetcorn
kernels from cobs stored for 14 d in different gas compositions (air or 2% 02
plus 0, 15, or 25% CO2) at 5 C. Columns with the same letter are not
statistically different at the 5% level by LSD.













































air alr+15% air+25% 2%02 +
(Control) C02 C02 0% C02
Treatment


2%02+ 2%02+
15% C02 25% C02


air air+15%
(Control) C02


air+25% 2%02+ 2%02+ 2%02+
C02 0%C02 15% C02 25% C02
Treatment


Fig. 3-3. Effect of controlled atmosphere storage on the different attributes of sweetcorn
visual quality after 14 d of storage at 5 OC in different gas compositions (air or
2% 02 plus 0, 15, or 25% CO2). Husk, silk, and kernel appearance and kernel
denting were evaluated using a scale from 1 to 9, where a score of 9 represents
the best quality and a score of 1 represents the lowest quality. Columns with
the same letter are not statistically different at the 5% level by LSD.


b b b


a a a a a a













a
80 -

S70
E b
60
E- b
50
o -
0
a 40 --
4- e
S30
CO
S20

E 10

0 | -
air(Control) air+ air + 2%02+ 2%02+ 2% 02+
15% C02 25% C02 0% C02 15% C02 25% C02
Treatment



Fig. 3-4. Effect of controlled atmosphere storage on dimethyl sulfide content (mg-L1) in
sweetcorn kernels from cobs stored for 14 d in different gas compositions (air
or 2% 02 plus 0, 15, or 25% CO2) at 5 C. Columns with the same letter are
not statistically different at the 5% level by LSD.














CHAPTER 4
PERFORATION MEDIATED MODIFIED ATMOSPHERE PACKAGING (PM-MAP)
OF SWEETCORN

Introduction

The beneficial effect of atmosphere modification during fresh fruit and vegetable

storage is well documented (Kader et al., 1989; Kader, 1986b; Weichmann, 1986).

Because of the limitation of CA storage to relatively large-scale systems, the modified

atmosphere packaging (MAP) technique was developed to provide the optimal

atmosphere, not for the entire storage facility, but for just the product, thus maintaining

the desired atmosphere during almost all of the postharvest chain even during the retail

display. Modified atmosphere packaging is an atmosphere control system that relies on

the natural process of respiration of the product along with the gas permeability of the

package holding the product. Due to respiration, there is a build up of CO2 and a

depletion of 02 within the package and the package material helps to maintain these

modified gas levels until the package reaches the steady state because of restricted gas

permeability. The steady state levels of 02 and CO2 are a function of the mass of product,

its respiration rate, the package permeability properties, the area of the package surface

available for gas diffusion and temperature. In addition, the time to reach steady state

conditions is largely a function of the void volume in the package. In the steady state, the

02 diffusing into the package equals the 02 being consumed by respiration, and the CO2

diffusing out of the package equals the CO2 being produced by respiration. Besides the

benefits of modifying the 02 and CO2 levels in lowering respiration rate, MAP has the









additional benefits of water loss prevention, produce protection, and brand identification.

(Kader et. al., 1989)

The early work by Spalding et al. (1978) found that sweetcorn appearance and

flavor were not significantly improved under modified storage conditions (CA and low

pressure) and concluded that breeding cultivars that retain quality, in combination with

prompt pre-cooling, offers more potential for success than modified atmospheres. In that

work, sweetcorn cobs were held in 2 or 21% 02 plus 0, 15, or 25% CO2 atmospheres for

3 weeks at 1.7 C. On the other hand, there are many reports stating the beneficial effects

of using MAP for sweetcorn. Deak et al. (1987) demonstrate that using MAP (shrink

wrap film with moisture permeability of 0.1 g-100 cm-224 h-1 and 02 permeability

varying from 0.4 to 40 mL-100 cm-224 h-1) at 10 OC eliminated water loss and

maintained beneficial CO2 and 02 levels of -14-18% 02 plus -4-5% CO2 within the

package. These effects together with lower storage temperature (10 C versus 20 C)

markedly reduced postharvest deterioration and extended the shelf life. But MAP

treatment increased microbial growth due to the water-saturated atmosphere. Similar

results were obtained by Risse and McDonald (1990) who stored sweetcorn at 1, 4, or

10 C for 26 d. either unwrapped or wrapped in stretch or shrink wrap. It was concluded

that film wrapping maintained freshness and reduced moisture loss better than the lack of

wrapping. Also these authors recommended stretch wrap over shrink wrap since stretch

wrap generated higher CO2 levels (4-9% versus 1-3%) and lower 02 levels (14-19%

versus 18-21%), which resulted in maintenance of higher total soluble solids.

Reduced 02 and elevated CO2 have been reported to reduce decay of sweetcorn and

maintain husk chlorophyll levels (Aharoni et al., 1996; Schouten, 1993). Aharoni et al.









(1996) demonstrated that sweetcorn wrapped in either polyvinyl chloride (PVC) or

polyolefin stretch films benefited from the reduction of water loss, but there was more

pathological breakdown on the trimmed ends of the cobs in the case of the PVC film.

Polyolefin stretch film has lower permeability rates for 02 and CO2 than PVC film and

consequently generated higher CO2 levels (-10% versus -3%) and lower 02 levels

(-7% versus -15%) in the packages and this resulted in a significant reduction in the

decay incidence and water loss and a significantly better maintenance of the sweetcorn's

general appearance (Aharoni et al., 1996). These levels of 02 and CO2 did not trigger

anaerobic respiration, and ethanol levels in the packages were very low until the packages

were transferred to 20 OC to simulate retail conditions. Aharoni et al. (1996) suggested

that the increase in microbial growth reported by Deak et al. (1987) and Risse and

McDonald (1990) was probably due to relatively low CO2 levels within their packages, a

consequence of the high ratio of the CO2 versus 02 permeability coefficients of plastic

films as mentioned previously. Schouten (1993) demonstrated that using CA storage (2%

02 plus 10% C02) retained higher sugar content than air storage and CA was more

beneficial when used at a higher temperature of 5-6 C compared with 1-2 OC. Similar

results were obtained in the work of Rumpf et al. (1972), which clearly proved that the

loss in sucrose content, the decisive factor in determining the taste quality of sweetcorn,

may be delayed by both low temperature (0 C versus 5 or 10 C) and low 02 content

since 1% 02 was the best treatment in retaining the sucrose levels after 11 d of storage at

5 C followed by 2% 02, then 4% 02, and finally air storage. All the CA treatments in

this experiment had 0% CO2.









The development of MAP has faced several problems such as the lack of

commodity respiration data under several temperatures and gas compositions, lack of

permeability data for packaging materials at different temperatures and relative

humidities, and also the lack of consistency in respiration data gathered for the same

commodity (Kader, 1987; Kader et al., 1989).

In medium and high respiring commodities, like sweetcorn, using commonly

available films such as low density polyethylene (LDPE), PVC, and polypropylene is not

ideal due to their low gas transmission rates, which may lead to anaerobic respiration

(Morales-Castro et al., 1994). Fonseca at al. (2000) summarized some of the limitations

of using the flexible polymeric films that result from their structure and permeation

characteristics such as 1) Films are not strong enough for large packages; 2) The film

permeability characteristics change unpredictably when films are stretched or punctured;

3) Film permeability may be affected by water condensation; 4) Film permeability is too

low for high respiring products: 5) Products that require high CO2/02 concentrations may

be exposed to anaerobiosis in film packages because of the high ratio of the film CO2 and

02 permeability coefficients. On the other hand, perforation-mediated modified

atmosphere packaging (PM-MAP) is a potential technique for postharvest preservation of

fresh horticultural commodities that overcomes several of the disadvantages associated

with films. In this technique, instead of using the common polymeric films, a package is

used in which the regulation of gas exchange is achieved by single or multiple

perforations or tubes that perforate an impermeable package. (Emond and Chau, 1990a,b;

Emond et al., 1992; Silva et al., 1999a).









For sweetcorn, the recommended atmosphere is 2% 02 plus 10-20% CO2 (Kader at

al., 1989; Riad and Brecht, 2003; Schouten, 1993; Spalding, 1978) and, to get such an

atmosphere, a package with a 3 value (the ratio between CO2 and 02 mass transfer

coefficients) of around 1 is required. Using PM-MAP could be very beneficial in

obtaining such a steady state atmosphere since the 3 value of a perforation is around 1,

which is better than the common polymers used for film formulation. Most of such films

have P values ranging from 2 up to 10 (Exama et al., 1993; Kader et al., 1989), which, for

sweetcorn, would likely lower the package 02 level and increase the CO2 level to the

critical levels that induce anaerobic respiration and accompanying increased ethanol,

acetaldehyde, and off-flavor production.

Using the PM-MAP technique has many potential advantages (Fonseca et al., 1997)

such as 1)The low values of mass transfer coefficients implying that high-respiring

produce can be packed in this system; 2) MAP using perforations can be adapted easily to

any impermeable container, including large bulk packages; polymeric films are not strong

enough for packs much larger than those used for retail, but perforations can be applied to

retail packages as well as shipping boxes because rigid materials can be used; rigid

packages also can prevent mechanical damage to the product; 3) It is a flexible system

due to the ability to change the gas transfer coefficients by selecting the adequate size and

shape of the perforation; 4) Commodities requiring high CO2 concentrations along with

relatively high 02 concentrations can be packed with this system.

The respiratory quotient (RQ) is the ratio between the respiration rate expressed in

terms of CO2 production and respiration expressed as 02 consumption. The RQ has been

observed to depend on both temperature and gas composition. It increases with a decrease









in 02 level or an increase in CO2 level and also with an increase in the temperature

(Beaudry et al., 1992; Joles et al., 1994). Joles et al., (1994) reported that in red raspberry

RQ was around 1 at 0 OC and increased to about 1.3 at 10 or 20 OC. The RQ is assumed to

be around 0.7 when the respiratory substrate is lipid, 1.0 when it is sugar, and up to

1.3 when it is acid. Under stress conditions, when the 02 level decreases or CO2 increases

beyond the critical level and anaerobic respiration takes place, the RQ usually is much

greater than one. In grated carrots and mushrooms, the RQ reached high levels of up to

5-6 when anaerobic respiration started (Beit-Halachmy and Mannheim, 1992; Carlin et

al., 1990; Mannapperuma et al., 1989).

The general objectives of this study were to examine the validity and the

effectiveness of PM-MAP to create the previously predicted atmosphere and also to study

the effect of this treatment on the visual and chemical quality of sweetcorn.



Materials and Methods

Plant Material

Fresh harvested sweetcorn (Zea mays L. rugosa var. Prime Time) cobs were

harvested from a local farm in Florida (Hugh Branch Inc., Pahokee, Fla.) on 24 January

2000 precooled, and delivered overnight by refrigerated truck to a local grocery chain's

distribution facility in Jacksonville (Publix Distribution Center, Jacksonville, Fla.) on

25 January. The sweetcorn was shipped to Gainesville, Fla. on 26 January where it was

then picked up and transferred to the postharvest lab (University of Florida, Gainesville,

Fla.) and the experiment started on the same day. As soon as the sweetcorn arrived at the

lab, the cobs were de-husked and stored at 5 OC until they were distributed among the

PM-MAP treatments (within 6 h).









Perforation Mediated Modified Atmosphere Packaging Treatment

The PM-MAP was established using 3-L glass wire bail jars with glass lids (Village

Kitchen, Redding, Calif). Two holes were drilled in the glass lids; one of them was fitted

with an appropriate size brass tube and the other fitted with a rubber serum stopper to

facilitate gas sample withdrawal. The brass tube size was determined using a computer

program (Chau, 2001) that depends on the equations of Fonseca et al. (2000). This

program uses respiration rate, RQ, commodity weight, and tube diameter and length. In

this program, tube design can be based on the desired 02 or CO2 concentrations. In this

experiment, all the calculations were made based on the final desired CO2 level in the

containers; also, RQ = 1 was used in all these calculations. Brass tubes with the diameters

and lengths shown in Table 4-1 were cut from commercial brass pipes. From previous

work by the author, three gas compositions were chosen (15, 20, and 25% CO2) and two

storage temperatures (1 and 10 C). The average respiration rate in the previous

experiments was used to calculate the appropriate dimensions of the brass tubes for the

PM-MAP.

Sweetcorn Storage

Three cobs of sweetcorn (about 500 g) were placed in each of three replicate

containers per treatment and stored at 1 or 10 OC for 10 d. Kernel samples for sugar

measurement were taken after storage by removing the kernels from the cob using a sharp

stainless steal knife. Kernel samples (about 100 g) were placed in plastic freezer storage

bags and stored at -20 OC until analysis.









Parameters

Gas composition

Oxygen and CO2 treatment levels were measured each 12 h for 10 d using a PBI

Dansensor Checkmate 02 and CO2 analyzer (Topac, Hingham, Mass.) with zirconium

and infrared detectors, respectively. The unit is designed to measure package

atmospheres and automatically withdraw samples through a tube fitted with a needle that

is inserted in the package or through a septum in ajar lid in this experiment and gives a

reading every 5 s.

Respiratory quotient

Respiratory quotient was calculated for each gas composition measurement using

the following formula,

RQ= CO2jar/ (02 air 02jar)

Where:

RQ = Respiratory quotient

CO2 jar = Carbon dioxide concentration in the jar

02 air = Oxygen concentration in the air outside the jar

O2jar = Oxygen concentration in the jar


Weight loss

Cob weight was recorded initially before starting the experiment and also after the

storage period, and percentage of weight loss was calculated.









Sugar content

Total soluble sugars were measured in raw and cooked kernels using the phenol-

sulfuric method described by Dubois et al. (1956). In this experiment, 15 g of sweetcorn

kernels were blended in a commercial blender with 85 mL of 95% ethanol for 2-3 min

then were heated for 20 min in an 85 C water bath. Then the samples were stored

overnight at -20 OC to precipitate ethanol-insoluble materials. The samples were then

filtered through Whatman #2 filter paper in a Buchner funnel attached to a side-arm

Erlenmeyer flask. A vacuum pump was connected to the side arm to expedite the

extraction. The collected filtrates were volumetrically adjusted to 200 mL using 95%

ethanol and 20 mL portions of the extracts were stored in scintillation vials at -20 OC until

the measurements were performed.

To prepare samples for measurement of total soluble sugars, 0.5 mL of 5% phenol

(w/w) was added to 0.5 mL of diluted (1:250) ethanol extract in test tubes, the mixture

was vortexed, then 2.5 mL concentrated sulfuric acid was added and the mixture vortexed

again. The test tubes were let to stand for 10 min then were placed in a 25 C water bath

for another 10 min to stop the reaction. The total soluble sugars were measured by

reading the absorbance at 490 nm using a Shimadzu UV-1201 spectrophotometer

(Shimadzu Scientific Instruments, Inc.; Tokyo, Japan). The absorbance values were

compared to a standard curve of glucose.

Ethanol and acetaldehyde content

Ethanol and acetaldehyde were measured in the storage container headspace.

Measurements were done using an HP 5890 Series II gas chromatograph equipped with

flame ionization detector and a Hewlett Packard Model 3390A integrator (Hewlett

Packard, Avondale, Pa.). A 3 mm by 2 m column of 5% Carbowax 20M on









80/120 Carbopack B (Supelco, Bellefonte, Pa.) was used and the carrier gas was N2

(30 mL.-min1). Oven, injector, and detector temperatures were 80, 110, and 300 C,

respectively.

A 1-mL sample of the container headspace gas was withdrawn using a 1.0 mL

plastic syringe and injected on the gas chromatograph. Calibration was done before

sample analysis using standards prepared from authentic compounds.

Statistical Analysis

The experimental design was a completely randomized design with three replicates

consisting of three sweetcorn cobs in each of three jars per treatment. The data were

analyzed using the Statistical Analysis System (SAS). Analysis of variance (ANOVA)

was used to determine the effect of treatments on the dependent variables. Means were

separated by the least significant difference (LSD) test at P<0.05.

Results and Discussion

Using PM-MAP could be a helpful tool in storing sweetcorn. The high respiration

rate of sweetcorn makes the use of a package with a low 3 value very beneficial in order

to reduce the possibility of the sweetcorn switching to anaerobic respiration.

In this work, the use of PM-MAP to store sweetcorn cobs for 10 d was very

successful. After 10 d of storage, the visual quality was very high in all treatments with

no sign of microbial growth or deterioration even at the higher temperature (10 C),

which might be due to the fungistatic effect of high CO2 (El-Goorani and Sommer, 1981).

Similar results were obtained in a study by Aharoni et al. (1996) with sweetcorn MAP

using polyolefin stretch film in which the high CO2 and low 02 levels resulted in a

significant reduction in both decay and water loss and hence maintained significantly

better appearance quality than that of unpackaged controls.









The weight loss was very slight in all treatments, which is a common observation

for MAP treatment of sweetcorn. Many researchers have demonstrated that using film

wrapping reduces water loss compared with unwrapped sweetcorn and furthermore, using

films with lower water permeability further reduces water loss compared with films with

high water permeability (Aharoni et al., 1996; Deak et al., 1987; Risse and McDonald

1990). In this work, weight loss ranged between 0.05 and 0.08% at 1 OC with no

significant differences among the MAP treatments, while at 10 OC it ranged between

0.12 and 0.22% with significant differences among MAP treatments that corresponded to

tube size (i.e., treatments utilizing smaller tubes lost less water). The higher weight loss at

the higher temperature was also probably due to the larger brass tubes used for PM-MAP

at 10 C than at 1 OC. The low weight loss at both temperatures might be one of the

factors that helped in maintaining the high quality of the sweetcorn with shiny kernels

and no denting, both of which are affected by low humidity levels around the sweetcorn

(Wiley et al., 1989).

The expected gas composition was successfully obtained in all treatments at 1 C

(Fig. 4-1) while at 10 OC the levels obtained were less than expected by about 3-5% CO2.

This might have been due to overestimation of the respiration rate while designing the

tubes for PM-MAP, which led to steady state CO2 levels around 10, 17, and 22% CO2

instead of 15, 20, 25% CO2, respectively (Fig. 4-2). Sweetcorn stored at 1 OC reached the

expected CO2 level in about 5 d with slightly more time required for the higher

concentrations (Fig. 4-1). The sweetcorn stored at 10 OC was faster in reaching the steady

state (about 2 d.) and this was expected due to higher CO2 production at the higher

temperature (Fig. 4-2).









There was a noticeable change in the RQ during storage that was most noticeable at

1 C. There was a gradual increase in the RQ during the storage period and the RQ was

generally higher at 10 OC (Fig. 4-3). Similar results were observed in blueberry and red

raspberry, for which the RQ values varied between 1.0 and 1.3 and there was also an

increase with increasing storage temperature. Also, when the blueberry and raspberry

packages reached a very low steady state 02, the RQ increased to -4-5 (Beaudry et al.,

1992; Joles et al., 1994). In this experiment, RQ values never exhibited such a sudden

increase, which usually indicates the switching to anaerobic respiration, but rather ranged

between 0.9 and 1.4.

Ethanol production was very low in all treatments, around 1-3 ppm, and there were

no significant differences among the treatments. This is in agreement with Aharoni et al.

(1986) and Rodov et al. (2000) who reported that no significant production of ethanol

was detected when sweetcorn was stored in MAP at 1 or 2 OC although higher ethanol

production (200-500 ppm) was detected when the MAP packages were later transferred

to 20 C.

Sugar content was higher in the sweetcorn stored at the lower temperature, but

there were no significant differences among the treatments at 1 OC (Fig. 4-4). The only

difference in sugar concentrations that was observed was in the sweetcorn stored at 10 C

with 15% CO2, which had significantly lower sugar content than sweetcorn stored in

20 or 25% CO2. This finding is in agreement with previous work with CA or MAP

storage of sweetcorn in which it was reported that there were higher sugar concentrations

in kernels stored in high CO2 and/or low 02 (Risse and McDonald, 1990; Rumpf et al.,

1972; Schouten, 1993)









This work showed that using PM-MAP could be a very useful tool in sweetcorn

storage at different storage temperatures. Perforation mediated modified atmosphere

packaging was mostly successful in generating the desired and expected gas

compositions and maintaining them during a 10-d. storage period. The PM-MAP system

was also successful in maintaining sweetcorn quality for 10 d of storage. On the other

hand, using active gas modification could help reduce the time required to reach

equilibrium, especially at the lower temperature. More work needs to be done to collect

improved respiration data for different temperatures and also to determine the switching

point from aerobic to anaerobic respiration in sweetcorn, which will lead to a better

understanding of possible 02 and CO2 levels that could be used for sweetcorn storage.






73




Table 4-1. Diameters and lengths of brass tubes used for creating PM-MAP with desired
CO2 levels at different temperatures.
Temperature (C) Desired CO2 (%) Diameter (mm) Length (mm)
15 4 27.1
1 20 4 43.8
25 4 63.7
15 8 19.4
10 20 8 31.4
25 8 45.7







74



A -02 -- C02
25






15


10






0
25


20


o 15


S10
0
C












15

10
25

20




10





0 0.5 1 1.5 2 2.5 3 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10
Storage duration (d)


Fig. 4-1. Gas composition in perforation-mediated MAP originally designed to generate
15% CO2 (A), 20% CO2 (B) or 25% CO2 (C) in sweetcorn packages stored for
10 d at 1 C. Bars indicate + 1 SD (n = 3).














25
A -A-02 --C02O

20















B
20


15


10
5









0










0 C
25


20









15




10
o
O
5











0
25


20 -


15


10







0 0.5 1 1.5 2 2.5 3 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10

Storage duration (d)

Fig. 4-2. Gas composition in perforation-mediated MAP originally designed to generate
15% CO2 (A), 20% CO2 (B) or 25% CO2 (C) in sweetcorn packages stored for
10 d at 10 C. Bars indicate + 1 SD (n = 3).








76






1.5
-*- 15% -20% 25%
1.4

1.3

1.2

1.1

1.0

0.9

0.8------------------------------------------------------------, I I
A)

Cy 1.5

0
S 1.4


w 1.3

1.2

1.1

1.0


0.9

0.8
0.5 1 1.5 2 2.5 3 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10
Storage duration (d)

Fig. 4-3. Effect of different levels of CO2 generated through PM-MAP on the respiratory
quotient (RQ) of sweetcorn stored for 10 d at 1 OC (A) or 10 'C (B).













Z15% C02 L~2O% C02 *25% C02


9.0


a


a


b


Storage temperature


Fig. 4-4. Effect of temperature and different CO2 levels built up through perforation-
mediated MAP on total sugars (%) in sweetcorn stored for 10 d at 1 or 10 OC.
Means with the same letter in each storage temperature are not significantly
different at P <0.05 by LSD.


O 15% C02


O 20% C02


S25% C02














CHAPTER 5
SELECTION OF MATURITY STAGE, STORAGE TEMPERATURE, AND
ATMOSPHERE FOR OPTIMUM POSTHARVEST QUALITY OF FRESH-CUT
SWEETCORN KERNELS

Introduction

Shelf life of perishable fruits and vegetables can be extended by a variety of

techniques. The most widely used techniques are refrigeration and controlled atmosphere

(CA) storage or modified atmosphere packaging (MAP; Chinnan, 1989; Kader et al.,

1989). For sweetcorn, the consensus recommendation is to keep it as close to 0 OC as

possible during transportation and storage (Brecht, 2002) and this may be of even more

importance for fresh-cut sweetcorn. On the other hand, CA storage and MAP benefits

with regard to extension of fruit and vegetable shelf life are well documented (Anzueto

and Rizvi, 1985; Brecht, 1980; Nakashi et al., 1991; Zagory and Kader, 1988), including

sweetcorn (Aharoni et al., 1996; Deak et al., 1987; Risse and McDonald, 1990; Schouten,

1993)

Sweetcorn shelf life is one of the shortest among fruits and vegetables. This is due

to its high metabolic and respiration rates, which lead to severe sugar loss and rapid

quality deterioration. Although sweetness is the most important quality characteristic in

sweetcorn (Evensen and Boyer, 1986; Showalter and Miller, 1962), the importance of

other flavor components in consumer acceptance is also well documented (Flora and

Wiley, 1974; Wiley, 1985; Williams and Nelson, 1973). These other flavor components

include texture and aroma.









With the increasing acceptability and demand for fresh-cut or ready-to-eat

products, fresh-cut sweetcorn kernels could be a valuable new product. Previous work on

fresh-cut sweetcorn showed that intact kernels (prepared by splitting the ears lengthwise

and then pushing the kernels off the cob) were much better than cut kernels (which were

prepared similarly to the canned or frozen sweetcorn by cutting the whole kernels from

the cob ) in maintaining higher sugar content and better flavor. Intact kernels showed

little or no after cooking brown discoloration but were very difficult to prepare (Brecht,

1999).

Browning during cooking is a common occurrence during the preparation of many

foods. One of its causes is the non-enzymatic browning reaction usually associated with

thermal processing that is known as the Maillard reaction. In this reaction, brown

pigments are generated from the reaction between free sugars and basic amino acids at

elevated temperatures (Bayindirli et al., 1995; Gogus et al., 1998). Sweetcorn has a high

background of sugars (Courter et al., 1988) and amino acids (Grunau and Swiader, 1991).

Protein degradation in sweetcorn kernels may also occur during storage, increasing the

concentration of free amino acids, which could increase the probability of occurrence of

Maillard reaction. Unlike many other foods in which browning during cooking is a

desirable occurrence, browning of fresh-cut sweetcorn kernels is a negative quality factor

that would make the product unsalable.

The main objective of this work was to determine the effects of maturity stage,

storage temperature, and atmosphere modification on the quality of fresh-cut sweetcorn

in order to develop recommendations for commercial development of this as a fresh-cut

product.









Materials and Methods

Plant Material

Fresh harvested sweetcorn (Zea mays L. rugosa var. Prime Time) cobs were

harvested from a local farm in Florida (Wilkinson-Cooper Produce Inc., Belle Glade,

Fla.) on 11 January 2002 precooled, and delivered overnight by refrigerated truck to a

local grocery chain's distribution facility in Jacksonville (Publix Distribution Center,

Jacksonville, Fla.) on 12 January where it was picked up and transferred to the

postharvest lab (University of Florida, Gainesville, Fla.) and the experiment started on

the same day. As soon as the sweetcorn arrived at the lab, the cobs were de-husked and

divided into two groups according to the maturity stage. The more mature sweetcorn had

larger kernels and the kernels were more compact on the cob, while the less mature

kernels were smaller and there were still some spaces between the kernel rows (Fig. 5-1).






















Fig. 5-1. The differences between the freshly harvested more mature (A) and less mature
(B) sweetcorn cobs.









The cobs were held at 5 OC while the kernels were cut. The kernels were cut from

the cobs using a sharp stainless steel knife. The kernels in the first row were popped out

first using the edge of the knife and discarded, then the adjacent rows were cut from the

cob near the kernel base and the cut kernels were collected. Special care was taken during

cutting in order to minimize damage to the kernels. After cutting the kernels, they were

dipped in chlorinated (1.34 mM NaOC1), 5 C water at pH 7 for 1 min. for sanitation and

kept covered at 5 OC until distributed among the treatments.

Controlled Atmosphere Treatment

Controlled atmosphere treatment was used to determine the effect of atmosphere

modification on fresh-cut sweetcorn quality and the potential application of MAP

technology for fresh-cut sweetcorn handling. The CA mixtures were established using a

system of pressure regulators, manifolds and needle valve flowmeters to blend air, N2,

and CO2 from pressurized cylinders. The CA was a flowing system with a flow rate set to

maintain respiratory CO2 accumulation under 0.5%. The air-flow was humidified by

bubbling through water before introducing it to the containers. Three replicates from each

treatment were prepared by putting 300 g of cut kernels in 1.2-L plastic containers

(Tupperware; Orlando, Fla.) fitted with 2 plastic connections that attached to the plastic

tubing and served as in and out ports for the gas flow.

Cut kernels were exposed to three gas compositions: Air, 2% 02 plus 10% C02, or

2% 02 plus 20% CO2. Gas composition was monitored daily using a Servomex 02 and

CO2 analyzer (Servomex, Norwood, Mass.).

Fresh-Cut Kernel Storage and Cooked Sample Preparation

The cut kernels were stored at 1 or 5 OC for 10 d with subsamples taken at 4 and

7 d. The other treatments (maturity stage and CA) were distributed randomly in each









storage room. After the storage period, the cut kernels were cooked by placing 50 g of

kernels in 250 mL boiling water for 5 min. Both fresh and cooked samples were stored at

-20 C until analysis.

Parameters

Respiration rate

Carbon dioxide levels were measured daily using a Gow Mac series 580 gas

chromatograph (Gow Mac Instruments Co., Bridgewater, N.J.) equipped with thermal

conductivity detector (TCD) and a Hewlett Packard Model 3390A integrator (Hewlett

and Packard Co. Avondale, Pa.). The respiration rate was measured using the static

method in which the gas flow was blocked using metal clips and the first sample was

withdrawn. The containers were then incubated for 2 h before taking the second sample

and removing the metal clips. The difference in CO2 concentration in the samples

withdrawn before and after the incubation period was used to calculate the respiration

rate. The gas samples were withdrawn from the container headspace using 1.0 mL BD

plastic syringes and a 0.5-mL sample was injected on the gas chromatograph. Calibration

was done before sample analysis using a certified standard mixture.

Sugar content

Total soluble sugars were measured in raw and cooked kernels using the

phenol-sulfuric method described by Dubois et al. (1956). In this experiment, 15 g of

sweetcorn kernels were blended in a commercial blender with 85 mL of 95% ethanol for

2-3 min then were heated for 20 min in an 85 C water bath. Then the samples were

stored overnight at -20 OC to precipitate ethanol-insoluble materials. The samples were

then filtered through Whatman #2 filter paper in a Buchner funnel attached to a side-arm

Erlenmeyer flask. A vacuum pump was connected to the side arm to expedite the









extraction. The collected filtrates were volumetrically adjusted to 200 mL using 95%

ethanol and 20 mL portions of the extracts were stored in scintillation vials at -20 OC until

the measurements were performed.

To prepare samples for measurement of total soluble sugars, 0.5 mL of 5% phenol

(w/w) was added to 0.5 mL of diluted (1:250) ethanol extract in test tubes, the mixture

was vortexed, then 2.5 mL concentrated sulfuric acid was added and the mixture vortexed

again. The test tubes were let to stand for 10 min then were placed in a 25 C water bath

for another 10 min to stop the reaction. The total soluble sugars were measured by

reading the absorbance at 490 nm using a Shimadzu UV-1201 spectrophotometer

(Shimadzu Scientific Instruments, Inc.; Tokyo, Japan). The absorbance values were

compared to a standard curve of glucose.

Free amino acids

The ethanol extracts prepared for soluble sugar measurements were used to

measure free amino acids colorimetrically using the ninhydrin method described by Lee

and Takahashi (1966). In this method, a 0.1-mL aliquot of the ethanol extract was added

to 1.9 mL of reaction mixture [0.5 mL 1% ninhydrin solution (1 g ninhydrin in 100 mL

0.5 M citrate buffer) with 1.2 mL glycerol and 0.2 mL 0.5 M citrate buffer (0.5 M citric

acid and 0.5 M sodium citrate)], mixed well, heated in a boiling water bath for 12 min,

then cooled in a water bath at room temperature. Samples were mixed once more then

measured at 570 nm on a Shimadzu UV-1201 spectrophotometer (Shimadzu Scientific

Instruments, Inc.; Tokyo, Japan). Total free amino acid concentration was calculated

from a standard curve oftryptophan (Sigma Chemical Co.; St. Louis, Mo.).









Statistical Analysis

The experimental design was a completely randomized design with three replicates.

The data were analyzed using the Statistical Analysis System (SAS). Analysis of variance

(ANOVA) was used to determine the effect of treatments on the dependent variables.

Means were separated by the least significant difference (LSD) test at P<0.05.

Results and Discussion

Fresh-cut sweetcorn is a potential product for the fresh-cut produce industry. Fresh-

cut sweetcorn was stored up to 10 d without severe loss in visual quality even at 5 OC.

Also, dipping the kernels in chlorinated water (1.34 mM NaOC1) for 1 min after cutting

was apparently effective in controlling decay since there was no visual appearance of any

microbial growth in these experiments.

Mature sweetcorn kernels had a respiration rate that was about 50% lower than the

less mature kernels (Fig. 5-2). Also, the respiration rate of kernels stored at 5 OC was

about 25% higher than those stored at 1 OC (Fig. 5-2), which is in agreement with

previous work by Brecht (1999). Using CA with 2% 02 plus 10% CO2 significantly

reduced the respiration rate by about 25%, but using 2%02 plus 20% CO2 nearly doubled

the respiration rate through most of the storage period (Fig. 5-2), which may have been

due to initiation of anaerobic respiration. Despite the increase in the respiration rate when

kernels were held in 2% 02 plus 20% C02, this treatment did not seem to result in a

significant difference in kernel taste compared with the air control (personal assessment

by the author, no taste panel conducted).

Kernels stored at 1 OC had 28% higher sugar content at the end of the storage

period than kernels stored at 5 OC (Fig. 5-3). Storing sweetcorn kernels in 2% 02 plus

10% or 20% CO2 maintained about 30% higher sugar content than air storage (Fig. 5-3).









On the other hand, less mature kernels had about 25% more sugar at the beginning of the

experiment but levels decreased more rapidly than in the mature kernels such that there

was no significant difference between the two maturities by the end of the experiment.

This may have been due to the higher respiration rate of the less mature kernels, which

led to a greater sugar loss. These results are in agreement with several research reports

indicating that temperature control is the most important factor in the preservation fruits

and vegetables generally and in sweetcorn specifically due to its high metabolic rate,

which makes sweetcorn more prone to rapid deterioration (Brecht, 2002; Brecht and

Sargent, 1988; Evensen and Boyer, 1986). It is also in agreement with work on the

importance of gas modification (CA and/or MA) in maintaining quality in fruits and

vegetables (Anzueto and Rizvi, 1985; Nakashi et al., 1991; Zagory and Kader, 1988)

including sweetcorn cobs (Aharoni et al., 1996; Deak et al., 1987; Risse and McDonald,

1990; Schouten, 1993)

Visual quality was not greatly affected by any of the storage treatments (Fig. 5-5, 7,

9, 11, and 13). The main problem in all treatments was loss of flavor and the kernels

becoming more watery with a bland taste which is in agreement with Kader (2002) that

the post-cutting life based on flavor is shorter than that based on appearance. Fresh-cut

sweetcorn stored in air at 5 C did not show a severe loss in visual quality after storage

for 10 d (Fig. 5-9), but there was severe browning of the kernels after cooking (Fig. 5-10)

compared with the appearance of kernels cooked before storage (Fig. 5-6). While after

cooking browning appeared in cooked kernels stored in air at 5 OC, it did not occur in

cooked kernels after storage in air at 1 OC (Fig. 5-8), and did not appear in the kernels

stored in CA at either 1 or 5 C (Fig. 5-12 and 14). This browning also was more severe