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Forced-Air Cooling of Strawberries in Reusable Plastic Containers

Permanent Link: http://ufdc.ufl.edu/UFE0011867/00001

Material Information

Title: Forced-Air Cooling of Strawberries in Reusable Plastic Containers
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: UFE0011867:00001

Permanent Link: http://ufdc.ufl.edu/UFE0011867/00001

Material Information

Title: Forced-Air Cooling of Strawberries in Reusable Plastic Containers
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: UFE0011867:00001

Full Text












FORCED-AIR COOLING OF STRAWBERRIES
IN REUSABLE PLASTIC CONTAINERS















By

MELVIN BERNABE MEANA


A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF ENGINEERING

UNIVERSITY OF FLORIDA


2005

































Copyright 2005

by

Melvin Bemabe Meana






















This thesis is dedicated to my mother, Josephine.
To my sister and brother, Melanie and Manzel,
and to Grace
for their love and prayers.















ACKNOWLEDGMENTS

I would like to express my gratitude to Dr. Khe V. Chau, my supervisory

committee chair, for giving me the opportunity to work on postharvest packaging and for

his time and guidance throughout my master's program.

I thank Dr. Jean-Pierre Emond for his invaluable support and Dr. Steven A. Sargent

for serving on my supervisory committee and their careful perusal of this thesis.

I thank Dr. Michael Talbot for his valuable time and extra effort during the conduct

of this thesis. I also want to thank Bob Tonkinson for all the help he provided and the

skills he shared.

I greatly appreciate the staff of the Philippine-American Educational Foundation

(Fulbright-Philippine Department of Agriculture Scholarship Program), Institute of

International Education and the Department of Agricultural and Biological Engineering

of the University of Florida.

I also want to extend my gratitude to my friends, Filipinos, Americans and other

internationals, for their company during my stay here in Gainesville.

And to God Almighty for providing me with everything I needed, all praise and

honor are due Him.
















TABLE OF CONTENTS



ACKNOW LEDGM ENTS ........................................ iv

LIST OF TABLES ............... ................. ......................... ...... .......... vii

LIST OF FIGURES ...................................... ix

LIST OF SYMBOLS ............. ........................................xi

ABSTRACT.............................. .........................xiii

CHAPTER

1 INTRODUCTION ................... ............................ ......... .. .......... 1

2 LITERATURE REVIEW .................................................. ...............5

Im portance of Fruits and V egetables..........................................................................5
Postharvest Biology and Technology .............................. ...............6
Precooling......................................................... ........................... 9
Packaging...................................... .................. .............. .........13

3 COOLING CALCULATIONS........................................................17

T ransient H eat C onduction .................................................................................... 17
Precooling Schedules............ .... ...............20

4 EXPERIMENTAL SET-UP AND PROCEDURES ...................................... 23

F orced-air C cooling U nit......................................................................................... 2 3
E xperim mental Product ........................ ... ......... .. ....................24
Experimental Cooling Test at a Commercial Cooling Facility ................................24
Determination of the Seven-Eights Cooling Time ...........................................24
Cooling Rate as a Function of the Distance from the Entrance of the Cold Air .31
Uniformity of Cooling as a Function of the Location of the Strawberry Inside
the Clam shell Container................................................... 32

5 RESULTS AND DISCUSSION ................................................... 36

Determination of the Seven-Eights Cooling Time ................................................36


v









Cooling Rate as a Function of the Distance from the Entrance of the Cold Air ........39
Uniformity of Cooling as a Function of the Location of the Strawberry Inside the
Clamshell Container.............. ... ................. ......49

6 CON CLU SION S .................................. ......................64

APPENDIX

A AIRFLOW RATE CALCULATION ....................................................... 66

B TEMPERATURE GRADIENT OF STRAWBERRIES IN A CLAMSHELL
CONTAINER.. .. ................ .... ... .................. 69

LIST OF REFERENCES ........................ ....... .... ........73

BIOGRAPHICAL SKETCH .................................................. ............... 78
















LIST OF TABLES


Table page

2-1. Recommended temperature, relative humidity, and approximate storage life for
berries. ....................................................... ........ 8

4-1. Percent vent area of the clamshell container. ....................................................25

4-2. Specifications of the GP 6411 reusable plastic container (RPC) tray....................25

4-3. Percent vent area of the RPC................................ ..................... ............... 27

4-4. Summary of treatments used in the cooling test. ............................................28

4-5. Distance and location of the clamshell containers inside the RPC and inside the
forced-air cooling unit. ............................. ....... .......................... 32

4-6. Distance and location of the RPC's inside the forced-air cooling unit....................32

5-1. The 7/8th cooling times and cooling coefficients of every treatment....................38

5-2. 7/8th cooling times as a function of the location of the clamshell containers
inside the RPC and in the forced-air cooling unit. ...................................... 39

5-3. 7/8th cooling times as a function of the location of the clamshell containers
inside the RPC and in the forced-air cooling unit. ...................................... 40

5-4. 7/8th cooling times as a function of the location of the RPC's in the forced-air
cooling unit. ...................................... ....... ........... .48

5-5. Average temperature gradient between strawberries in the entire layer and each
clam shell container................... .............................. ....... .. ..... ... .. ............ 50

B-1. Temperature gradient of strawberries in each clamshell container for treatment
1, replication 1....................................... ......... 69

B-2. Temperature gradient of strawberries in each clamshell container for treatment
1, replication 2 ........................... ...........................69

B-3. Temperature gradient of strawberries in each clamshell container for treatment
1, replication 3 .............. ......... .......... .. .......... .. ........... ......... .. 70









B-4. Temperature gradient of strawberries in each clamshell container for treatment
2, replication 1....................................... ......... 70

B-5. Temperature gradient of strawberries in each clamshell container for treatment
2, replication 2 ........................... ...........................70

B-6. Temperature gradient of strawberries in each clamshell container for treatment
2, replication 3 ........ ..... ........... .......... ...... .... .................71

B-7. Temperature gradient of strawberries in each clamshell container for treatment
3, replication 1....................................... ......... 71

B-8. Temperature gradient of strawberries in each clamshell container for treatment
3, replication 2 ........................... ...........................71

B-9. Temperature gradient of strawberries in each clamshell container for treatment
3, replication 3 ................... .............................................. ...............72
















LIST OF FIGURES


Figure page

1-1. Typical strawberry clamshell container packaging ............................................4

2-1. Typical tunnel type forced-air cooling room..................... ............... 11

2-2. Typical cooling curve for perishable products with constant air temperature. ........13

4-1. Isometric view of the forced-air cooling unit with cover................ ...............23

4-2. The forced-air cooling unit fitted with blower and an exhaust pipe with flow
meter and a valve.............. ....... ......... .............. 24

4-3. Top view of pallet arrangement for a 60 x 40 cm reusable plastic container
(RPC) tray with 0.454 kg clamshell containers in "5-down configuration". ..........26

4-4. Cross section of the 3 treatments used in the cooling test...............................28

4-5. Locations of individual thermocouples inside each clamshell container..............29

4-6. Measured distances of clamshell containers in cm. .....................................31

4-7. Location of clamshell containers instrumented with thermocouples....................34

5-1. Sample cooling curve from one of the replicates with 11.30% side and 16.70%
end by-pass area. .....................................................37

5-2. Cooling times of strawberries packed in clamshell containers as a function of
their distance to the entrance of cold air with 34.30% side by-pass area..............40

5-3. Cooling times of strawberries packed in clamshell containers as a function of
their distance to the entrance of cold air with 37.80% end by-pass area. .............42

5-4. Cooling times of strawberries packed in clamshell containers as a function of
their distance to the entrance of cold air with 11.30% side by-pass area..............43

5-5. Cooling times of strawberries packed in clamshell containers as a function of
their distance to the entrance of cold air with 16.70% end by-pass area. .............44

5-6. Cooling times of strawberries packed in clamshell containers as a function of
their distance to the entrance of cold air with 1.80% side by-pass area...................45









5-7. Cooling times of strawberries packed in clamshell containers as a function of
their distance to the entrance of cold air with 2.70% end by-pass area. ..................46

5-8. Air temperature entering the forced-air cooling unit during the cooling process. ...49

5-9. Sample cooling curve showing the temperature gradient during cooling of
individual strawberries inside clamshell container 2 with 34.30% side by-pass
area. ............................................................... 50

5-10. Sample cooling curve showing the temperature gradient during cooling of
individual strawberries inside clamshell container 2 with 11.30% side by-pass
area. ............................................................... 52

5-11. Sample cooling curve showing the temperature gradient during cooling of
individual strawberries inside clamshell container 2 with 1.80% side by-pass
area. ............................................................... 53

5-12. Cooling curve of the 4 clamshell containers instrumented with thermocouples
placed on RPC's wherein the side of the RPC was perpendicular to the entering
cold air. ............... ......... ......... ......................... 56

5-13. Cooling curve of the 4 clamshell containers instrumented with thermocouples
placed on RPC's wherein the end of the RPC was perpendicular to the entering
cold air. ............... ......... ......... ......................... 57

5-14. Cooling curve of the 4 clamshell containers instrumented with thermocouples
placed on RPC's wherein the side of the RPC was perpendicular to the entering
cold air. ............... ......... ......... ......................... 5 8

5-15. Cooling curve of the 4 clamshell containers instrumented with thermocouples
placed on RPC's wherein the end of the RPC was perpendicular to the entering
cold air. ............... ......... ......... ......................... 59

5-16. Cooling curve of the 4 clamshell containers instrumented with thermocouples
placed on RPC's wherein the side of the RPC was perpendicular to the entering
cold air. ............... ......... ......... ......................... 60

5-17. Cooling curve of the 4 clamshell containers instrumented with thermocouples
placed on RPC's wherein the end of the RPC was perpendicular to the entering
cold air.............................. ..... ........... .61
















LIST OF SYMBOLS

A: area, m2

Bi: Biot number

c: specific heat, J/ kg K

c,: specific heat at constant pressure, J/ kg K

Fo: Fourier number

h: convection heat transfer coefficient, W/ m2 K

m: mass, kg

k: thermal conductivity, W/ m K

L: characteristic length of a body, m

r : spherical coordinate, m

ro: sphere radius, m

T: temperature, K

t: time, s

V.: volume, m3

x: coordinate, m



Greek Letters

a : thermal diffusivity, m2/ s

p: mass density, kg/ m3










Subscripts

f : refers to the difference in initial temperature to the ambient temperature

f, : refers to the temperature after reaching the half cooling time
2

f,: refers to the temperature after reaching the 7/8th cooling time
8

i: refers to the initial temperature

itr : refers to the instantaneous temperature ratio

s : refers to the surface area

0o: refers to the ambient condition

co,: refers to the ambient temperature at time t


- refers to the degree of temperature to be taken out to reach half cooling
2


7: refers to the degree of temperature to be taken out to reach 7/8th cooling
8



Superscripts

* : dimensionless quantity
















Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Engineering

FORCED-AIR COOLING OF STRAWBERRIES
IN REUSABLE PLASTIC CONTAINERS

By

Melvin Bernabe Meana

August 2005

Chair: Khe V. Chau
Major Department: Agricultural and Biological Engineering

Strawberries are highly perishable fruit and need to be cooled immediately after

harvest. Strawberries are typically packed in the field in individual containers and then

placed on trays for further postharvest handling. The thermoformed plastic clamshell

design with tapered side, hinged lid and less venting is now a popular packaging for

strawberries. Reusable plastic containers (RPC's) where clamshell containers are placed

are also in demand in the agricultural industry as a substitute to the corrugated fiberboard

trays.

Cooling times of strawberries packed in clamshell containers and placed in RPC's

were studied. The data taken during the cooling tests were cooling times at different fruit

location inside each clamshell container, at different locations of the clamshell containers

within each RPC, at different locations of the RPC's within a layer in a pallet and the

effect of blocking off the by-pass areas in the RPC's to force more air through the fruit in









the clamshell containers. To determine the cooling times during the tests, both the 7/8th

cooling time and the cooling coefficient were calculated.

A portable forced-air cooling unit was designed and constructed to have the same

footprint of a standard pallet. The airflow rate through the unit can be controlled and the

unit can accommodate 5 layers of RPC's. The cooling tests were conducted at a

commercial cooling facility.

The results showed that minimizing the by-pass areas inside the RPC's can

significantly hasten the cooling time of strawberries in the clamshell containers.

Minimizing the by-pass areas inside the RPC's can also lower the temperature gradient

existing between strawberries in a layer of a pallet and each clamshell container. The

results also showed that cooling time can also be a function of the location of the

clamshell container to the entrance of the cold air. Cooling times of clamshell containers

located at the middle portion of an RPC tend to have longer cooling time when by-pass

areas were left open. But when by-pass areas were covered, the cooling times of

clamshell containers tend to increase as they were located farther away from the entrance

of the cold air. The results clearly emphasize that clamshell containers and RPC's should

be designed together to maximize air-to-product contact during precooling.














CHAPTER 1
INTRODUCTION

Fresh fruits and vegetables are living tissues that will deteriorate after harvest

(Wilson et al., 1995; Kader, 2002). The deterioration rate becomes a concern when

marketing of fresh fruits and vegetables is far from the growing areas and these products

need to be transported (Wills et al., 1998; Mitcham and Mitchell, 2002). If fruits and

vegetables are not handled and transported properly, a high value, nutritious product can

deteriorate and rot in a matter of days. The magnitude of postharvest losses in fresh fruits

and vegetables is an estimated 5 to 25% in developed countries and 20 to 50% in

developing countries (Kader, 2002). Production and management practices including

effective postharvest handling greatly affect the deterioration rate, the final quality, and

the market of fruits and vegetables (Liu, 1999; Goldman et al., 1999; Bachmann and

Earles, 2000; Suslow, 2000; Kader, 2001; Mitcham and Mitchell, 2002).

The most effective method to reduce the deterioration rate of fruits and vegetables

after harvest is to have a good temperature management. Time and temperature

management is very important in slowing the deterioration rate of fruits and vegetables

(Wilson et al., 1995; Bachmann and Earles, 2000; Suslow, 2000; Kader, 2002; Nunes and

Emond, 2003).

A supplemental method in slowing the deterioration rate of fruits and vegetables is

having the right choice of packaging. Important factors to be considered in the selection

of any packaging system for fruits and vegetables are how quickly, efficiently, and

uniformly the fruits and vegetables can be cooled (Talbot et al., 1995). Packaging should









be designed to protect the product from injury, to allow rapid cooling of the field heat and

to allow for continual removal of heat generated by the product (Boyette et al., 1996;

Bachmann and Earles, 2000; Thompson and Mitchell, 2002).

Small fruits, like strawberries, sent to the consumer market are typically packed in

individual containers (Talbot et al., 1995; Thompson and Knutson, 1997; Anderson et al.,

2003). These individual containers are then placed on trays for precooling, temporary

storage, and transportation. Many different materials, sizes, and shapes used in individual

containers and trays had been introduced over time (Vigneault and Goyette, 2002). The

thermoformed plastic clamshell design with a tapered side, a hinged lid, and less venting

is becoming a popular container for small fruits (Talbot et al., 1995; Anderson et al.,

2003). This kind of material and design leads to an increased protection against physical

injury when handling the product (Anderson et al., 2003). There are still few works that

had been published on precooling with the use of these containers especially in

combination with different kind of trays. Test conducted by Arifin and Chau (1988),

Talbot et al. (1995), Emond et al. (1996), and Anderson et al. (2003), all pointed out that

the containers' design, particularly the vent hole design and its orientation inside a tray

have a significant effect on the precooling time of the product.

The reusable plastic containers (RPC's), where individual clamshell containers can

be placed, are also increasing in demand. The RPC's can be a substitute to the commonly

used fiberboard trays. For the last two decades, Europe has been using RPC's for

handling fruits and vegetables (Chonhenchob and Singh, 2003). According to

Chonhenchob and Singh (2003), Wal-Mart stores in North America is leading the change

from the traditional corrugated boxes to RPC's and it is expected that over 150 million









RPC's will be needed to serve the North American market for fruits and vegetables.

Different design for RPC's were introduced but most of these plastic trays were designed

for multipurpose use and have narrow wall openings to accommodate different methods

of precooling (Vigneault and Goyette, 2002; Castro et al., 2004b). These RPC trays also

have the advantage of retaining their strength after several use and wetting (Vigneault

and Goyette, 2002) as compared to the corrugated fiberboard trays. Several international

and national standards for size and vent alignment of RPC's are already in effect but the

designs are concentrated more on the structural part than their influence on the cooling

efficiency of the product (Castro et al., 2004b). Thus, despite the advantages of using

RPC's in terms of its strength and reusability, it is still important to choose the RPC's

that is best suited for a particular product in terms of physical protection and temperature

management.

Vigneault and Goyette (2003) studied the effect of using tapered walled trays

against the vertical walled trays during precooling. Their study showed that the vertical

walled trays had a faster cooling time. The tapered walled trays created spaces between

each tray where by-pass of cold air was observed and have slowed the cooling time

(Vigneault and Goyette, 2003). The thermoformed plastic clamshell containers basically

have a slightly tapered wall and extended hinged lid that creates open spaces when placed

side by side as shown in Figure 1-1. The open spaces created between clamshell

containers could introduce air by-pass and can increase the precooling time.

Longer cooling time can cause delay in cooling of fruits and vegetables especially

during peak season and can reduce product quality. Delay in cooling of strawberries for

just 6 h at 30 'C resulted in fruit that had a significant decrease in firmness, soluble solid









content, and sugars and ascorbic acid levels as well as less attractive appearance (Nunes

et al., 1995). Thus, rapid and uniform cooling time of fruits and vegetables to their

optimal temperature will not only potentially extend their postharvest life and preserve

their nutritive values but will also allow more throughputs at a cooling facility and avoid

longer delay of cooling for the harvested products.




(a) n






(b) L




Figure 1-1. Typical strawberry clamshell container packaging: (a) side view of the
clamshell container when placed side by side and (b) end view of the
clamshell container when placed side by side.

The overall objectives of this work are to determine the time of cooling by forced-

air using the commercial standard 7/8th cooling time, the uniformity of cooling of

strawberries packed in thermoformed plastic clamshell containers and placed on vertical

walled RPC trays, and the effects of the by-pass areas created by the spaces between the

clamshell containers and the headspace inside the RPC's.














CHAPTER 2
LITERATURE REVIEW

Importance of Fruits and Vegetables

Fruits and vegetables play an important part in human nutrition (Kader, 2001).

They are the natural source of energy, protein, vitamins, minerals, and dietary fibers

(Burton, 1982; Quebedeaux and Bliss, 1988; Quebedeaux and Eisa, 1990; FAO, 1995;

Wargovich, 2000; Kader, 2001). Daily diet that includes fruits and vegetables has been

proven to reduce risk for some forms of cancer, heart diseases, and other chronic diseases

(Quebedeaux and Bliss, 1988; Quebedeaux and Eisa, 1990; Produce for Better Health

Foundation, 1999; Wargovich, 2000; Kader, 2001). Fruits and vegetables are so

important that the Dietary Guidelines for Americans 2005 (HHS and USDA, 2005)

encourage consumers to consume sufficient amount of fruits and vegetables. A "5 a Day

for Better Health" program promoted by the National Cancer Institute (NCI) and the

Produce for Better Health (PBH) also promotes the importance of eating fruits and

vegetables (Produce for Better Health Foundation, 1999).

The promotion done by private and government institutions on the benefit of eating

fruits and vegetables had a significant effect on the consumption of fruits and vegetables.

As a proof, there was an increase overall in the consumption of fruits and vegetables over

the past years (Cook, 2002). This trend is observed to be in effect for the coming years as

consumers are becoming more concerned about their diet combined with the promotional

programs by private and government institutions about the importance of eating fruits and

vegetables. Several factors that also contributed to the consumption growth of fruits and









vegetables aside from new consumer awareness of the nutritional benefits are improved

varieties and greater variety selection, introduction of convenient fresh-cut forms,

development of year-round availability, and new uses through food service channels

(Bertelsen, 1995; Kader, 2002).

Fruits and vegetables not only contribute to the maintenance of a healthy life, but

they also generate income to growers and employment for people in many parts of the

world. The production of fruits and vegetables also contribute to the regional and national

economy. In Florida alone, cash receipts from vegetable and melon crops amounted to

$1.41 billion in 2003 and accounted for 21.8% of the total cash receipts for agricultural

products (FASS, 2004).

Postharvest Biology and Technology

Fresh fruits and vegetables typically have high moisture content, tender texture, and

high perishability (Wilson et al., 1995; Liu, 1999); they continue to respire even after

harvest .They are living tissues that are subject to continuous change (Kader, 2002; Bartz

and Brecht, 2003). Changes are inevitable and some are desirable but most are not from

the consumers' standpoint (Kader, 2002). Postharvest changes in fresh fruits and

vegetables cannot be hindered, but they can be slowed to certain limits (Kader, 2002).

Deterioration of fresh fruits and vegetables are influenced by biological and

environmental factors. The biological factors involved are respiration, ethylene

production, compositional changes, growth and development, transpiration, physiological

breakdown, physical damage and pathological breakdown (Kader, 2002). The

environmental factors involved are temperature, relative humidity, atmospheric

composition, and exposure to ethylene (Kader, 2002).









C6H1206 + 602 6CO2+ 6H20 + 2818 kJ (2.1)

Respiration, shown in equation (2.1), is the process by which stored organic

materials are broken down into simple end products with a release of energy. Oxygen is

used in this process, and carbon dioxide, water, and energy is produced. The loss of

stored food reserves in the commodity during respiration means the hastening of

senescence as the reserves that provide energy to maintain the commodity's living status

are depleted (Kader, 2002). It also results in reduced food value for the consumer, loss of

flavor quality, especially sweetness, and loss of salable dry weight (Kader, 2002).

The rate of deterioration of fruits and vegetables is generally proportional to their

respiration rate (Wilson et al., 1995; Bachmann and Earles, 2000). A high respiration rate

means faster rate of deterioration. But the respiration rate of fruits and vegetables can be

slowed by lowering the temperature to their optimal storage temperature.

Temperature is the environmental factor that has the greatest influence on the

deterioration rate of fruits and vegetables (Fraser, 1998; Kader, 2002; Nunes and Emond,

2003), but it is also the factor that can be easily and promptly controlled (Nunes and

Emond, 2003). Temperature also greatly influences the other biological and

environmental factors mentioned above. Thus, having a good temperature management

would be the most effective tool for extending the shelf life and preserving the nutritive

values of fruits and vegetables (Kader, 2002; Nunes and Emond, 2003).

Products harvested from the field often carry field heat and have high rates of

respiration. Rapid removal of field heat by precooling is so effective in quality

preservation that this procedure is widely used for most fruits and vegetables. Currently









used precooling methods include room cooling, forced-air cooling, water cooling,

vacuum cooling, and package icing.

Desirable storage and transportation temperatures for major berry crops have been

identified and published (Kader, 2002) as shown in Table 2-1.

Table 2-1. Recommended temperature, relative humidity, and approximate storage life
for berries.

Product Temperature Relative humidity Approximate
OF 0C % storage life
Blackberry 31-32 -0.5-0 98-100 3-6 days
Blueberry 31-32 -0.5-0 98-100 10-18 days
Cranberry 35-41 2-5 90-95 8-16 weeks
Dewberry 31-32 -0.5-0 90-95 2-3 days
Elderberry 31-32 -0.5-0 90-95 5-14 days
Loganberry 31-32 -0.5-0 90-95 2-3 days
Raspberry 31-32 -0.5-0 90-95 3-6 days
Strawberry 32 0 90-95 7-10 days
Source: Adapted from Kader, 2002.

Strawberries are fragile and a highly perishable fruit (Talbot and Chau, 1991). They

have a high respiration rate, 12 to 18 mg C02/kg-h at 0 oC and 127 mg C02/kg-h at 20 oC

(Talbot and Chau, 1991) and a shelf life of only few days. The recommended optimum

storage temperature of strawberries is near 0 'C as shown in Table 2-1 and this

temperature must be maintained throughout the handling process until the product

reaches the market (Talbot and Chau, 1991).

The marketing and types of packaging of berries differ depending on the

destination of the strawberries. Direct marketing of strawberries is used when

strawberries are sold directly to consumers either through roadside stands or nearby

farmers' markets. Packaging for this kind of market generally uses mesh baskets in pints

or are sold by flats on a new or reused corrugated fiber boards. Selling strawberries in

fresh wholesale markets is more complicated and more demanding than direct local

marketing, and normally only suitable for large producers or growers' associations.









Grading, packaging, storage, and transportation must meet certain standards, thus the

need for packing or cooling facility. Thus, having a successful wholesale marketing

requires expertise in postharvest refrigeration, packaging, transportation, and distribution.

The strawberries are graded for shipping and inspection purposes. The available

grades are: U.S. 1, Combination, and U.S. 2 (USDA, 1997). The principal grade is U.S.

1; the fruit should be firm, not overripe or undeveloped, free from mold or decay and

damage from dirt, moisture, foreign materials, disease, and insects (Talbot and Chau,

1991; USDA, 1997). The fruit must have at least 3 of its surface showing a pink or

reddish color and the minimum diameter of each strawberry must not be less than 1.90

cm (Talbot and Chau, 1991; USDA, 1997).

Precooling

Initial cooling of fruits and vegetables to their optimal storage temperature can be

achieved with several cooling methods but few are used with a wide range of

commodities (Thompson et al., 1998; Thompson et al., 2002). The most used precooling

methods are room cooling, forced-air cooling, and water cooling (Vigneault and Goyette,

2002). The selection of a particular precooling method according to Talbot and Chau

(1991) is determined by several factors which include: the rate of cooling required,

compatibility of the method with the product to be cooled, subsequent storage and

shipping conditions, and equipment and operating cost.

Room cooling is relatively the simplest method which needs only a refrigerated

room with an adequate cooling capacity. It is commonly used for products with relatively

long storage life and can be stored in the same room in which they are cooled (Thompson

et al., 1998; Thompson et al., 2002). The product is packed in containers which are

loosely stacked in the cooling room, leaving enough space between containers for each









one to be exposed to circulating cold air (Thompson et al., 1998; Thompson et al., 2002).

The rate of cooling is rather slow compared to other methods of cooling because the heat

inside each container needs to be transferred to the surface of the container by means of

conduction before being carried away by the refrigerated air. It may take hours or even

days to cool a product, depending on the kind of product, the size and nature of the

container, and the temperature and velocity of the circulating air. The total fan airflow

should be at least 0.3 m3/min per ton of product storage capacity for adequate heat

removal (Thompson et al., 2002).

Forced-air cooling is a more rapid way of using air to cool products and it is

adaptable to a wider range of commodities than any other cooling method (Fraser, 1998;

Thompson et al., 2002). Products are air cooled rapidly by a difference in air pressure on

opposite faces of stacks of vented containers and fans create the pressure difference,

which is also called static pressure or pressure drop (Talbot et al., 1992; Talbot and

Fletcher, 1996; Vigneault and Goyette, 2002). Cold air is forced to flow through the

inside of each container to carry away the heat directly from the surface of the product by

forced-convective contact rather than from the surface of the container (Talbot and Baird,

1990; Talbot et al., 1992; Fraser, 1998). The key principle when forced-air cooling is

considered is the path of least resistance (Talbot et al., 1992). For a forced-air cooler to

be effective, the path of least resistance must be through the product and not around the

product (Talbot et al., 1992). The study by Vigneault and Goyette (2003) showed that

tapered walled trays had longer cooling time since they created by-pass of air and cold air

traveled through the tapered areas between the trays rather than through the product mass.

































Figure 2-1. Typical tunnel type forced-air cooling room. A tunnel is formed by pallets of
products placed in pairs against the duct with a fan inside it. A tarp is pulled
over the tunnel created by the two rows of pallet to force air to travel through
the sides of the pallet.

Various forced-air cooling designs can be used, depending on the specific needs

(Thompson et al., 2002), but the tunnel-type forced-air cooling is the most common. A

typical commercial forced-air cooling room is shown in Figure 2-1. For a tunnel-type

forced-air cooling, two rows of pallets are placed inside a room equipped with an exhaust

fan and the tunnel is created under a tarp between the two rows of pallets. Highly

perishable and high-value products such as grapes, strawberries and raspberries may be

cooled in less time using this method.

All fruits and vegetables cool quickly at first and then it slowly cools over time

(Fraser, 1998). Factors that affect the rate of forced-air cooling are: size and density of

the product in the container, container type, orientation and venting characteristics,









volume to surface area of the product, travel distance of the air, and airflow capacity

(Baird et al., 1988; Fraser 1998). The study conducted by Castro et al. (2004a) showed

that higher airflow rate applied in the cooling process, the faster was the cooling. But

according to Talbot and Chau (1991) and Thompson et al. (2002), maximum feasible

cooling requires about 0.001 to 0.002 m3 per sec per kg of product and rates greater than

this only slightly reduce cooling time. Vigneault and Goyette (2003) also demonstrated

the importance of increasing the quantity of air circulating through the container

compared to the air circulating around them, thus increasing the cooling rate and

uniformity of cooling through the product mass.

The study by Baird et al. (1988),Talbot and Chau (1991), Talbot and Fletcher

(1996), and Castro et al. (2004a) showed that cooling time is a function of the location of

the fruit in the container with respect to the entrance of the cold air, where the more

downstream the location the longer the cooling time.

Most users are concerned with the time to reach the desired temperature before

transfer to storage or transport. Cooling times are usually reported as "half-cooling" or

"seven-eights-cooling" times. Half-cooling is the time to cool the product halfway from

its initial temperature to the temperature of the refrigerated air (Mitchell et al., 1972;

Sargent et al., 1991; Fraser, 1998). Seven-eights cooling time is three times longer than

half cooling and is the time needed for the product temperature to drop by seven-eights of

the difference between the initial product temperature and the temperature of the

refrigerated air (Fraser, 1998; Thompson et al., 2002; Anderson et al., 2003). The 7/8th

cooling time is a standard industry term and it is a convenient method of indicating when

the product has come as close as practical to the temperature of the refrigerated air










(Fraser, 1998), while the final 1/8th can be removed during temporary cold storage or

transport. The 7/8th cooling time is measured from the time the product is first placed in

the forced-air cooler (Fraser, 1998). A typical cooling curve is shown in Figure 2-2.



22 Initial product temperature
20 a
18
0 16
I 14
1 /2 cool time
$ 12
E 10
-5 Average product temperature
3 8
2 6
Q 7/8" cool time

SAir temperature

0
0 30 60 90 120 150 180 210 240 270

Time (minutes)


Figure 2-2. Typical cooling curve for perishable products with constant air temperature.

Packaging

Packaging is a primary element in the storage and transportation of perishable

commodities (Dincer, 2003). Most fruits and vegetables ready for market is composed of

large numbers of small units which must be moved in amounts conveniently handled by

at least one person (FAO, 1981). This is best achieved by using packaging either by

individual containers, trays, baskets, and sacks. Bartz and Brecht (2003) described the

world of vegetable postharvest technology as dynamic and so as the world of product

packaging. Different kind of materials, sizes, and shapes were used in packaging of fruits

and vegetables over time (Thompson and Mitchell, 2002; Vigneault and Goyette, 2002)









and each has its own advantages and disadvantages. The packages could either be made

out of wood, styrofoam, corrugated fiberboard, or plastic. Originally, wooden boxes were

used in the packaging industry then corrugated cartons were used as a substitute to the

wooden boxes (Janick, 1963; Vigneault and Goyette, 2003). Corrugated cartons were

then coated to resist moisture damage but this limited the recycling and secondary use of

such material. As a substitute to the coated corrugated cartons, reusable and recyclable

plastic containers were introduced and now gaining popularity in the packaging and

agricultural industry (Vigneault and Goyette, 2003).

Fruits and vegetables have different requirements for handling and storage, thus

different type of containers are made to fit them (Wills et al., 1998). For large

commercial quantities of products, right packaging is needed in order to minimize losses

and achieve the most economical use of transport (FAO, 1981). The purpose of using the

right choice of packaging is to protect the product from damage in handling, transport,

and storage (FAO, 1981). Packages also convey information about their contents and

should be attractive when used for display.

Soft fruits and many vegetables are usually packed directly in the field. The

packing of products directly into marketing packages in the field during harvest reduces

the damage caused by multiple handling and also reduces the time between harvest and

precooling (FAO, 1981; Thompson et al., 2002). Since the product came directly from

the field, they usually carry field heat, especially fruits exposed to direct sunlight. The

control of the exchange of heat, moisture and gases with the environment should be

highly considered for the quality conservation of packed fruits and vegetables (Wills et

al., 1998; van der Sman, 2002). Since package containers and trays are often used in









precooling, they must accommodate the temperature management requirement of the

product (Wills et al., 1998; Thompson and Mitchell, 2002; Vigneault and Goyette, 2002).

Thus packaging systems are frequently provided with vent holes to accommodate the

exchange processes allowing cold air to flow in and out of the package (Chau et al., 1985;

Talbot, 1988; Wills et al., 1998; van der Sman, 2002). A good temperature management

relies on good contact between the product in the package and the cold air (Thompson

and Mitchell, 2002).

An important factor in the efficiency of containers used for forced-air cooling is the

percentage of openings necessary for adequate air circulation through the containers

(Baird et al., 1988; Vigneault and Goyette, 2002) both for the RPC trays and individual

clamshell containers. Regarding the RPC trays, Vigneault and Goyette (2002) determined

that 3.2 to 12.7 mm opening widths on the surface of the containers have negligible effect

on pressure drop during cooling but the relative surface area of the wall openings should

cover approximately 25%. Opening area less than 25% can cause significant restriction of

airflow and the possibility of increasing the fan power requirement (Vigneault et al.,

2004). Stacking orientation of trays can also affect the cooling times of products.

Anderson et al. (2003) suggested that cross-orientation of RPC trays having a 5-down

configuration can slow the cooling time.

For the clamshell containers, forced-air cooling of 0.454 kg clamshell container for

strawberries was observed to be faster with a 7% sidewall venting as compared to lesser

percentage of venting, but the cooling time is not reduced with venting greater than 13%

(Thompson and Mitchell, 2002). Castro et al. (2004b) also observed that increasing the









opening area increased the cooling rate and they recommended a 6% opening area as a

function of the containers' structural restrictions.

The percentage of vent holes both on tray walls and individual clamshell containers

should not only be satisfied but should also be aligned to avoid disruption of flow. The

recommendation of Talbot et al. (1995) and Anderson et al. (2003) is for the clamshell

container and RPC tray to be designed together to maximize air-to-product contact during

cooling. It was also recommended by Talbot et al. (1992) and Mitchell (1992) that having

few larger vent holes are more advantageous than having smaller vent holes. Larger vent

areas can also lower the power required by the fan and compressor and can result in more

uniform cooling (Baird et al., 1988).














CHAPTER 3
COOLING CALCULATIONS

Transient Heat Conduction

For the cooling of strawberries, transient heat transfer principles must be

considered. Solutions in describing heat transfer between solid objects and their

environment have been developed for homogenous regular shapes, for slab, cylinder, and

sphere that have constant thermal properties and do not change with temperature

(Gaffney et al., 1985). But fruits and vegetables are irregular in shape, not perfectly

homogenous, and their thermal properties changes with temperature, thus using the

solutions developed will only give approximate answers to the cooling and temperature

distribution that will occur within the fruits and vegetables (Gaffney et al., 1985).

During heat transfer analysis, some bodies are observed to behave like a "lump"

where the internal temperature remains essentially the same at all time during a heat

transfer process (Incropera and DeWitt, 1996; Cengel, 2003). The temperature of lump

system bodies are a function of time only, T(t) and do not change much with the location

inside the body. During a differential time interval dt, the temperature of the body

changes by a differential amount dT. The energy balance of the body for the time interval

dt can be expressed as (Incropera and DeWitt, 1996; Cengel, 2003):

hA, (T T)dt = mc,dT (3.1)









If the thermal properties do not change with temperature, and the surrounding air

temperature is constant, Equation (3.1) can be integrated with T = T, att = 0, thus, the

result will be (Gaffney et al., 1985; Incropera and DeWitt, 1996; Cengel, 2003):


T(t) = e or T(t) e (3.2)
T 'T T Tk

Equation (3.2) is used to determine the temperature T(t) of an object at time t or can

be used to determine the time t required for the temperature to reach a specified value

T(t). But equation (3.2) is valid only if the Biot number is less than 0.1. The Biot number

is a ratio of the external resistance to heat transfer to the internal resistance of an object.

Thus, for low Biot numbers (
much greater than its internal resistance, and therefore there is negligible temperature

gradient within the product. Thus, it is considered a lumped capacitance system as

mentioned above. The equation for the Biot number is:

hL
Bi = h(3.3)
k

When the surface temperature changes faster than the interior of an object and

Equation (3.2) cannot be applied but the thermal conductivity is constant with time, the

rate of change of temperature (in one-dimensional problem) can be described by

Fourier's Law (Gaffney et al., 1985):

dT k 82T
T -k 2x 0 at pC ax2

Equation (3.4) is called the one-dimensional heat diffusion equation (Leinhard and

Leinhard, 2003). The heat diffusion equation includes a new property which is important

to transient heat conduction. This is the thermal diffusivity, a, where:










a = (3.5)
pc

The thermal diffusivity is a measure of how quickly a material can carry heat away

from the source (Leinhard and Leinhard, 2003). Equation (3.4) and similar equations for

other shapes can be solved analytically in regularly shaped objects subject to the

following restrictions (Gaffney et al., 1985):

* The material is homogenous;

* The initial temperature of the object is uniform;

* The temperature of the surrounding air is constant with time;

* The surface heat transfer coefficient, h, is constant with time and position of the
obj ect;

* The thermal properties of the material are constant with time and temperature;

* There is no internal heat generation; and

* There is no mass transfer at the surface.

If these restrictions are met, the temperature response as function of time at any

position within an object undergoing transient heat transfer can be described by infinite

series solutions to the governing heat conduction equations (Gaffney et al., 1985). The

exact solution for sphere will then become (Incropera and De Witt, 1996):


T(t)- T. 4[sin($ ) cos(s)] -$ sinsm(cy)
e e (3.6)
T, T n= 2; sin(2;n )

Where, '; are the positive roots of the transcendental equation:

1 -' cot()= Bi (sphere) (3.7)

and










r r (3.8)
ro


The combination from Equation (3.6) forms a dimensionless grouping called
r

the Fourier number (Fo), often referred to as dimensionless time:

at kt
Fo =a t=k (3.9)
r pcr

For Fourier numbers greater than 0.2, the solution for the center temperature can be

accurately estimated by (Incropera and DeWitt, 1996):

T(t)- T 4[sin(41)-C cos()l-)]e (3.10)
T, T 2C, sin(2z$)

Precooling Schedules

In determining the speed of cooling for strawberries, the standard method used in

the industry is to calculate the 7/8th cooling time. The concept of "half-cooling" and

"7/8th,, cooling times assume a lumped capacitance system (Bi< 0.1). Most fruits and

vegetables are not lumped systems (Bi > 0.1), but the industry still uses 7/8th cooling time

as approximation of the process.

The determination of the 7/8th precooling schedules for a specific crop involves

three steps: (1) determine the average initial pulp temperature, (2) calculate the exit pulp

temperature which should be achieved by the precooling treatment to accomplish 7/8th

cooling, and (3) measuring the actual 7/8th cooling time using the forced-air cooler

(Sargent et al., 1991). The exit pulp temperature required to achieve 7/8th cooling can be

calculated using the following formulas (Sargent et al., 1991):










T = T, (3.11)


T7x, T, (3.12)
8


T,- T, = Tf (3.13)
8 8

However, this method assumes that a constant air temperature is used throughout

the cooling process, thus, producing a log-linear cooling curve (Anderson et al., 2003). In

cases where constant air temperature and log-linear cooling curve is assumed and the

cooling of produce is very slow, V2 cooling time can be used to determine the 7/8th

cooling time. Half cooling time was also discussed in the previous chapter and it has the

same steps with the determination of 7/8th cooling presented above.


T, x = T, (3.14)
2 2


T, = T, (3.15)
2 2

Since 7/8th cooling time is three times longer than half cooling time, multiplying

the half cooling time by three will yield the 7/8th cooling time. A graphical representation

of an ideal cooling curve with constant air temperature and log-linear cooling profile is

shown in Figure 2-2.

When air temperature changes greatly during the cooling process, which is

expected during the actual cooling, calculating the cooling coefficient using

instantaneous air temperature can be used (Anderson et al., 2003). By plotting the natural

logarithm of the instantaneous temperature ratio, the slope of the line could be

determined. The air temperature at a given time should be used in the temperature ratio

(Anderson et al., 2003):






22


T(t) T.
Ttr (3.16)


The value of the slope is the cooling coefficient having a unit of inverse time. An

"apparent" 7/8th cooling time can be calculated from the cooling coefficient by dividing

the natural logarithm of 1/8 by the cooling coefficient. This "apparent" 7/8th cooling time

is the 7/8th cooling time that would be expected based on the assumed constant air

temperature and a log-linear cooling curve (Anderson et al., 2003).















CHAPTER 4
EXPERIMENTAL SET-UP AND PROCEDURES

Forced-air Cooling Unit

A forced-air cooling unit was designed with a footprint of a typical pallet 1.016 x

1.220 m and to fit inside a refrigerated room. An isometric view of the forced-air cooling

unit is shown in Figure 4-1. The height of the forced-air cooling unit was designed to fit 5

layers of 60 x 40 x 12.7 cm reusable plastic container (RPC) trays.










1.5 m












Figure 4-1. Isometric view of the forced-air cooling unit with cover.

The unit was fitted with a blower (4C108, 25.40 cm diameter wheel Dayton

blower) and an exhaust pipe holding a flow meter (Annubar, Dieterich Standard Corp.,

Wallingford, CT) and a valve to adjust the airflow rate during the cooling process as

shown in Figure 4-2.



































Figure 4-2. The forced-air cooling unit fitted with blower and an exhaust pipe with flow
meter and a valve.

Experimental Product

The commodity chosen for the cooling test was strawberries. Strawberries are

usually available in the state of Florida for the months of November to April (FSGA,

2005). The state of Florida is the second largest producer of strawberries in the United

States second only to the state of California and produces about 15% of the nation's

strawberries (Bertelsen, 1995; Hinton et al., 2004). The characteristics of strawberries

have been previously discussed in Chapter 2.

Experimental Cooling Test at a Commercial Cooling Facility

Determination of the Seven-Eights Cooling Time

Strawberry cooling tests were done on a commercial forced-air cooling facility. The

commercial forced-air cooling facility is located at Walden Sheffield Road, Dover,











Florida managed by BBI Produce, Inc. The forced-air cooling unit that was constructed

was used in the tests. Strawberries of the variety "Strawberry Festival" were harvested,

sorted, and packed on 0.454 kg clamshell containers by trained pickers then placed on a

0.41 x 0.51 m corrugated fiberboard trays (8-1, 0.454 kg wireless corrugated fiberboard

tray, Weyerhaeuser, Tampa, FL). The type and percent vent area of the clamshell

container used is shown in Table 4-1. Seventeen 0.41 m x 0.51 m corrugated fiber board

trays containing 136 clamshell containers were transported to the forced-air cooling

facility and then transferred in the reusable plastic container (RPC) trays. A total of 135

0.454 kg clamshell containers were used in the tests.

Table 4-1. Percent vent area of the clamshell container.
% Vent Area
Clamshell Container Side End
Side End
Pactiv 9762 5.60 6.90

Prior to the insertion of thermocouples on the strawberries for the cooling tests, the

strawberries packed in the clamshell containers were weighed and the strawberries per

clamshell containers were counted to determine the average weight of strawberries and

the average number of strawberries per 0.454 kg clamshell container. Nine clamshell

containers can fit in one RPC and 5 RPC's can fit in a layer on a pallet, thus the term "5-

down configuration" as shown in Figure 4-3. The model and dimension of the RPC is

given in Table 4-2.

Table 4-2. Specifications of the GP 6411 reusable plastic container (RPC) tray.
Outside dimensions Inside dimensions Collapsed Return Capacity Weight
Return Capacity Weight
_cm Cm Height ratio liters kg
cm
L W H L W H cm
60.00 40.00 12.70 57.46 37.46 11.02 3.56 3.6 to 1 24.04 1.47
Source: Adapted from Smartcrate.











RPC1 RPC2 RPC 3


Side of



Direction of airflow




End of


RPC


RPC


1.016 m



~E~U L B BE BEBEBE


RPC 4 RPC 5
1.220 m


Figure 4-3. Top view of pallet arrangement for a 60 x 40 cm reusable plastic container
(RPC) tray with 0.454 kg clamshell containers in "5-down configuration".

Five layers of RPC trays were stacked inside the forced-air cooling unit and the 3

middle layers contained the packed strawberries. The bottom and the topmost layers of

the RPC's were filled with styrofoams and covered with a plastic tarpaulin to force air

through the middle layers. The middle most layer containing the packed strawberries

were instrumented with thermocouples.

Sixty-two individual thermocouples were inserted approximately in the center of

the strawberry from its calyx and the strawberries were placed strategically inside the

clamshell containers, at least one thermocouple per clamshell container. The 2 layers

containing packed strawberries served as a buffer.

Strawberry core temperatures were measured using type T, gauge-30 thermocouple

wires. Linear calibration of the thermocouples was done in ice water and the accuracy

was 0.30 'C. The air temperatures entering and exiting the forced-air cooling unit were


Ot
L 'A L 'A L 'A


~i~r~i~r~i~










measured using type T, gauge-24 thermocouple wire. Linear calibration of the

thermocouples was done in ice water and the accuracy was 0.50 'C.

The forced-air cooling unit was installed with a blower (4C 108, 25.40 cm diameter

wheel Dayton blower) powered by a 1-Hp. GE Air Compressor motor at 3,450 revolution

per minute. Once the forced-air cooling unit was placed inside the refrigerated room, the

blower was turned on to start the cooling process.

The percent vent areas of the RPC trays and the clamshell containers were

measured as shown in Tables 4-1 and 4-3. The uncovered vent areas for the clamshell

containers inside the RPC's both for the side and end orientation of the clamshell

containers were also measured for the 3 treatments used as shown in Table 4-4. The by-

pass areas created inside the RPC's in combination with the clamshell containers for the 3

treatments including the headspace between the top of the clamshell containers and the

top of the RPC's were also measured as shown in Figure 4-4.

Table 4-3. Percent vent area of the RPC.
% Vent Area
Reusable Plastic Container S end Boto
Side End Bottom
GP 6411 12.70 11.30 6.20

There were 3 treatments and 3 replications per treatment used for the tests as shown

in Table 4-4 and Figure 4-4. The treatments were the following: 1) Treatment 1

(Control): the clamshell containers were placed in RPC's with no modifications, 2)

Treatment 2: the headspace between the top of the clamshell container and the bottom of

the RPC directly above was blocked off with 2.54 cm thick styrofoam, and 3) Treatment

3: all the by-pass areas headspacee and spaces between the sides of the clamshells) were

blocked off with 2.54 styrofoam and urethane foam.










Table 4-4. Summary of treatments used in the cooling test.
Combination ofRPC
Clamshell Container Percent Vent Area a Cinaimph
and Clamshell
Treatment Side of RPC End of RPC Container Percent By
pass Area
Side of End of Side of End of Side of End of
Clam. Clam. Clam. Clam. RPC RPC
1 5.60 6.90 5.60 6.90 34.30 37.80
2 5.60 6.90 5.60 6.90 11.30 16.70
3 5.60 2.00 1.60 6.90 1.80 2.70




S1 .Bypass areas
(a) ---- p---- o --- U-

Top vent hole
(Clamshell)

(b) Bottom vent hole
S 1 1 1 1 (Clamshell)

Styrofoam


(c) E Urethanefoam




Figure 4-4. Cross section of the 3 treatments used in the cooling test: (a) standard
configuration (b) clamshell containers with 2.54 cm thick styrofoam at the top
of the clamshell containers (c) clamshell containers with 2.54 cm thick
styrofoam at the top of the clamshell containers and the sides of the trays
covered with urethane foam.

Once the blower was turned on, the temperatures of the strawberries and the air

entering and exiting the forced air cooling unit were monitored. The temperatures sensed

by the thermocouples were recorded at an interval of 60 seconds using a data logger

(CR10, Campbell Scientific, Inc. Logan, UT). A total of 66 thermocouples were used

during the tests as shown in Figure 4-5, thus, there was a need to use 2 multiplexers (AM

416 Relay Multiplexer, Campbell Scientific, Inc. Logan, UT).












































Airflow


D 6(b ...)14) cl





H3(bs)1 F .1.(base)22

E3(m p )65 E8( containe










Fiue4-.Lctin f niiua hroculs nieeahcamhl onanr









The pallet inside the forced-air cooling unit was cooled in an orientation such that

air flowed parallel to the 1.220 m dimension of the pallet as shown in Figure 4-5. Air

flow parallel to the 1.016 m was not possible with the constructed forced-air cooling unit

because the unit was originally designed to fit the refrigerated room located the Frazier-

Rogers Hall at the University of Florida, Gainesville, Florida.

The pressure drops during the tests were measured using a handheld digital

manometer (Dwyer instruments, Inc., Series 475, Michigan City, IN). The pressure drops

were measured at the open space between the end of the pallet and the intake of the

blower. The air flow rates were calculated by using the measured pressure at the flow

meter. The calculation of the air flow rate was described in Appendix A. The valve

attached to the exhaust pipe was adjusted to have a uniform air flow rate for all the

treatments during the cooling process.

After the desired temperature was reached, the forced-air cooling unit was brought

out of the forced-air cooling facility for data downloading and conditioning. Conditioning

of the strawberries was done by running the blower outside the forced-air cooling facility

under the ambient temperature (18-27 C). After a desired strawberry core temperature

was reached, which was near the ambient temperature (18-27 C), the forced air cooling

unit was placed to the forced-air cooling facility for another cooling test. These

procedures were done for all the cooling tests.

The determination of the 7/8th cooling time and the calculation of the cooling

coefficient were done for each treatment. The 7/8th cooling time was used since it was the

time commonly used in the commercial cooling industry. The calculation of the 7/8th

cooling time as was described in the previous chapter assumes constant temperature










throughout the cooling process. During the actual cooling process, air temperature in the

precooling room was expected to vary with time, thus, the air temperature after a

determined amount of time shall be averaged (at least 30 minutes) and this will be used as

the constant air temperature. The calculation procedure for the 7/8th cooling time and the

cooling coefficient was previously discussed in Chapter 3.

Cooling Rate as a Function of the Distance from the Entrance of the Cold Air

Individual thermocouples were used to measure strawberry core temperature. A

single thermocouple was placed in the center of a strawberry from its calyx and was

placed at strategic positions inside the clamshell containers, at least one thermocouple per

clamshell container. The summary of treatments can be seen in Table 4-4 and Figure 4-4.

The measured temperatures and the 718th cooling times of the 3 clamshell containers

placed on the same RPC and located at the same axis perpendicular to the entering cold

air were averaged. The measured distances can be seen in Figure 4-6 and Table 4-5.











8763 89 2 7

59 94 5






Figure 4-6. Measured distances of clamshell containers in cm. The measured distances
were from the cold air entry point.









Table 4-5. Distance and location of the clamshell containers inside the RPC and inside
the forced-air cooling unit.
Location and distance from entering cold air (cm)
RPC. Clam. Clam. Clam. Clam. Clam. Clam. Clam. Clam. Clam.
1 2 3 4 5 6 7 8 9
Side 7.62 20.07 32.51 47.50 59.94 72.39 87.63 100.08 112.52
End 10.92 29.97 49.02 70.87 89.22 108.97 -

The 7/8th cooling time per RPC was also determined by averaging the temperatures

and the 7/8th cooling time of the 9 clamshell containers placed on the same RPC and

instrumented with thermocouples. The position of the center of each RPC measured from

the entry point of cold air is given in Table 4-6. Since there were 2 different orientations

of the RPC on a pallet, the locations for both orientations were given.

Table 4-6. Distance and location of the RPC's inside the forced-air cooling unit.
RPC. Location and distance from entering cold air (cm)
RPC 1 RPC 2 RPC 3
Side 20.07 59.94 100.08
End 29.97 89.22

Uniformity of Cooling as a Function of the Location of the Strawberry Inside the
Clamshell Container

Another set of strawberry cooling tests were done on the same commercial forced-

air cooling facility. Strawberries of the variety "Carmine" were harvested, sorted, and

packed on a 0.454 kg clamshell container by trained pickers then placed on a 0.41 x 0.51

m corrugated fiber board trays (8-1, 0.454 kg wireless corrugated fiberboard tray,

Weyerhaeuser, Tampa, FL). Seventeen 0.41 x 0.51 m corrugated fiber board trays

containing 136 clamshell containers were transported to the forced-air cooling facility

and then transferred to the RPC's. A total of 135 0.454 kg clamshell containers were used

in the tests. The same type and model of clamshell containers and RPC's were used for

these particular tests.









Prior to the insertion of the thermocouples on the strawberries for the tests, the

strawberries packed in the clamshell containers were again weighed and the strawberries

per container were counted to determine the average weight and the average number of

strawberries per 0.454 kg clamshell container. Five layers of RPC trays were stacked

inside the forced-air cooling unit and the 3 middle layers contained the packed

strawberries. The bottom and the topmost layers of the RPC's were filled with styrofoams

and covered with plastic tarpaulin to force cold air through the middle layers. The middle

most layer containing the packed strawberries were instrumented with thermocouples.

Sixty individual thermocouples were inserted approximately in the center of the

strawberry from the calyx and were placed strategically inside the clamshell containers,

at least 6 thermocouples per clamshell containers. Some clamshell containers have 7

thermocouples (clamshell containers 3 and 4) and some have 8 thermocouples (clamshell

containers 1, 2, 5, 6, and 7). The 2 layers of RPC's containing packed strawberries served

as a buffer. Figure 4-7 shows the location of the clamshell containers instrumented with

thermocouples.

At least 2 strawberries instrumented with thermocouples were located at the base of

the clamshell container. After filling the base of the clamshell containers with

strawberries, at least 2 strawberries instrumented with thermocouples were then located at

the top of the strawberries that filled the base of the clamshell containers. After the

middle layers were filled with strawberries, at least 2 strawberries instrumented with

thermocouples were then placed on top of all the strawberries inside the clamshell

containers.

















Airlow





End of RPC





Figure 4-7. Location of clamshell containers instrumented with thermocouples. The
numbered clamshell containers were the ones instrumented with
thermocouples.

The 3 determined strategic locations were the base, the middle, and the top of the

clamshell containers. The location was considered as a base when the strawberries

instrumented with thermocouples were located at any part of the base of the clamshell

containers. The location was considered as the middle when the strawberries were located

above one or 2 strawberries at any position inside the clamshell containers. And the

location was considered as top when the strawberries were above all other strawberries at

any position inside the clamshell containers. Three treatments with 3 replications each

were also used in this study and the summary of the treatments were shown in Table 4-4

and Figure 4-4.

After the desired temperatures were reached during the cooling process, the forced-

air cooling unit was brought out of the forced-air cooling facility for data downloading

and conditioning. Conditioning of the strawberries was also done under the ambient air

temperature (18-27 'C). After a desired strawberry core temperature was reached, which









was near the ambient temperature (18-27 'C), the forced air cooling unit was placed to

the forced-air cooling facility for precooling. These procedures were done for all the

tests.

After all the treatments were done, the strawberries in the clamshell containers

instrumented with thermocouples were brought to Frazier-Rogers Hall, University of

Florida, Gainesville, Florida for dissection. The strawberries were dissected to determine

if the tip of the thermocouples were approximately in the center of the strawberries.

Strawberries that were instrumented with thermocouples whose tip were located near the

edge of the strawberries were neglected.














CHAPTER 5
RESULTS AND DISCUSSION

Determination of the Seven-Eights Cooling Time

The strawberries packed in thermoformed clamshell containers that came from the

field had an average weight of 0.537 kg and contained an average number of 23

strawberries per clamshell container. The minimum weight taken for packed strawberries

was 0.454 kg and the maximum weight was 0.619 kg. The minimum number of

strawberries counted was 19 strawberries and the maximum number was 27 strawberries

per clamshell container. The least number of strawberries counted per clamshell container

did not necessarily corresponded to the minimum weight of one clamshell container since

there was variability in terms of the size and weight of each strawberry packed in every

clamshell container. The average weight of strawberries would give an estimated total

weight of 72.43 kg that was placed inside the constructed forced-air cooling unit.

The 7/8th cooling times and the cooling coefficients were calculated and averaged

for each treatment using the procedure described in Chapter 3. A sample cooling curve

was shown in Figure 5-1.

The cooling tests give a comparison of the effects of the by-pass areas created by

the spaces between clamshell containers inside the RPC's as shown in Table 5-1.

Treatment 2 shows the effect of making the clamshell containers fit tightly inside the

RPC in terms of its height by eliminating the by-pass area created by the extended height

of the RPC. The test results using the Dunnet's method of statistical analysis comparing

Treatment 1 against the two treatments showed that Treatment 2 against Treatment 1 was









not statistically significant at 5% level; that is, at the 5% significance level, the data does

not provide sufficient evidence to conclude that lowering the by-pass area from 34.30%

side and 37.80% end to 11.30% side and 16.70% end would lower the cooling time of

packed strawberries. It also indicates that even if the height of the RPC was reduced to

level it with the height of the clamshell container and minimizing the by-pass area was

not enough to hasten the cooling time.


0 10 20 30 40 50
Time (min)


60 70 80 90


Figure 5-1. Sample cooling curve from one of the replicates with 11.30% side and
16.70% end by-pass area.

Treatment 3 shows the effect of making the clamshell containers fit tightly inside

the RPC's in terms of its height by minimizing the by-pass area created by the extended

height of the RPC and minimizing the by-pass area created between clamshell containers

as shown in Table 5-1. For Treatment 3 against Treatment 1, where the by-pass areas









were lowered to 1.80% side and 2.70% end, the test results were statistically significant

at 5% level. It means that at 5% significance level, the data provides a sufficient evidence

to conclude that lowering the by-pass area from 34.30% side and 37.80% end to 1.80%

side and 2.70% end would lower the cooling time of packed strawberries by 21%.

Table 5-1. The 7/8th cooling times and cooling coefficients of every treatment.
7/8th Cooling Cooling Pressure Drop Calculated
Pressure Drop
Treatment Time Coefficient Airflow Rate
(min)[a] (minm) (Pa) (L s -' kg -1)
1 82.5a -0.0202 17.44 1.71- 1.78
2 76.3ab -0.0241 44.84 1.71- 1.79
3 65.0b 0.0316 191.80 1.75- 1.83
[a] Times in the same column followed by the same lowercase letter are not significantly
different at an alpha level of 5% based on Tukey-Multiple-Comparison Method.

The test results supported the conclusion of Vigneault and Goyette (2003) that

tapered walled containers create spaces causing by-pass of air instead of air going

through the strawberries. The problem was aggravated for the clamshell containers in the

GP 6411 RPC trays since aside from the tapered walls of the clamshell containers, the

clamshell containers have an extended hinged lid that increases the by-pass area when

placed side by side. An added issue was the height of the RPC trays wherein they do not

exactly fit in terms of the height of clamshell containers adding more air by-pass area.

The volume of air by-passing the strawberry during the cooling process was very critical

because it has a major impact on the precooling of the strawberries. Thus, aside from the

design and uniform distribution of vent holes for clamshell containers (Arifin and Chau,

1988), additional way to improve the cooling process of packed strawberries in RPC's is

for the percent by-pass area perpendicular to the entering cold air be minimized to the

lowest possible. That is the path of least resistance must be through the product and not

around the clamshell containers (Talbot et al. 1992). Concerning the use of the RPC's,

Vigneault and Goyette (2003) suggested that when using vertical-walled trays such as the









one used in the tests, they must be placed side-by-side tightly to minimize the by-pass of

air.

Table 5-1 also shows the effect of covering the by-pass area with respect to the

pressure drop. The results showed that the pressure drop increased as the by-pass area

was decreased. Since the by-pass areas were blocked off, especially in the third treatment,

the air circulation around and between clamshell containers were restricted. Vigneault

and Goyette (2002) also observed that pressure loss were affected during forced-air

cooling when surface openings on the wall of a trays are less than 25%, and looking at

Table 4-2, surface openings were below 25%.

Cooling Rate as a Function of the Distance from the Entrance of the Cold Air

Cooling rates as a function of the distance of the clamshell container from the

entrance of the cold air was studied. There were two orientations considered for this

study, they were the side and the end of the RPC perpendicular to the entering cold air.

Tables 5-2 and 5-3 show the calculated 7/8th cooling times of the different clamshell

containers as a function of their distance to the entrance of the cold air.

Table 5-2. 7/8th cooling times as a function of the location of the clamshell containers
inside the RPC and in the forced-air cooling unit. The side of the RPC is
perpendicular to the flow of air.
7/8th cooling time (min)
Trtm. Clam. Clam. Clam. Clam. Clam. Clam. Clam. Clam. Clam.
1 2 3 4 5 6 7 8 9
1 66.5 88.0 72.0 83.5 98.0 83.5 90.5 101 84.5
2 54.0 75.0 74.0 76.3 80.7 88.7 77.3 92.7 89.7
3 39.0 53.3 56.0 60.3 65.7 73.7 71.30 82.0 81.3

Considering Treatment 1 with the side of the RPC perpendicular to the entering

cold air, clamshell containers 1, 2, and 3 were all placed in one RPC and so were

clamshell containers 4, 5, and 6 and clamshell containers 7, 8, and 9. Figure 5-2 showed

that clamshell containers located at the middle part of the RPC's had a longer cooling






40


time compared to one or 2 clamshell containers farther from them. This result was

comparable to the observation done by Castro et al. (2004a) where the experimental balls

located in the middle cooled more slowly than the ones located in the last layer.

Table 5-3. 7/8th cooling times as a function of the location of the clamshell containers
inside the RPC and in the forced-air cooling unit. The end of the RPC is
perpendicular to the flow of air.
Trtm. 7/8th cooling time (min)
Clam. 1 Clam. 2 Clam. 3 Clam. 4 Clam. 5 Clam. 6
1 75.0 79.0 97 82 100.0 83.0
2 59.3 79.0 78.7 72.0 84.7 82.7
3 41.7 60.0 73.3 68.0 77.3 79.0


Clam. 1 Clam. 2 Clam. 3 Clam. 4 Clam. 5 Clam. 6 Clam. 7 Clam. 8 Clam. 9
Clamshell Container Location


Figure 5-2. Cooling times of strawberries packed in clamshell containers as a function of
their distance to the entrance of cold air with 34.30% side by-pass area.
Columns with the same shade have the same boundary conditions.









The result shown in this particular treatment did not actually coincide with the

results gathered by Castro et al. (2004b) and the observations by Baird et al. (1988),

Talbot and Chau (1991), and Talbot and Fletcher (1996) that product cooling time

increases as their distance away from the entrance of cold air increases.

But the results showed that having the same boundary conditions inside the RPC's,

the cooling time increases with distance from the entrance of the cold air. Clamshell 4

cooled faster than clamshell 2 and so was clamshell 7 as compared to clamshell 5. But

clamshell containers 4 and 7 were farther away from clamshell containers 2 and 5

respectively. This effect could be attributed to the location of the clamshell containers

inside the RPC and the percentage of the by-pass area (Emond et al., 1996). Clamshell

containers 4 and 7 were located at the entrance of the RPC's as compared to the clamshell

containers 2 and 5 which were located at the middle part of the RPC's. Air passing

through the by-pass areas may have an effect on the air going though the second and third

RPC's and lowering to some degree the air entering the clamshell containers, particularly

the clamshell containers located at the entrance of the RPC's (Emond et al., 1996). The

results also showed that clamshell 9 which was located at the end of the forced-air

cooling unit and exposed to the exiting air had faster cooling as compared to clamshell

container 7 which was located at the entrance of the RPC.

Considering the end of the RPC perpendicular to the entering cold air, clamshell

containers 1, 2, and 3 were all placed in one RPC and so were clamshell containers 4, 5

and 6. The orientation of the clamshell containers in this particular side of the RPC was

different with the orientation of clamshell containers previously discussed. Figure 5-3

showed that clamshell containers located at the end of the first RPC and the clamshell









containers located at the middle of the second RPC had a longer cooling time. But the

result also showed that having the same boundary conditions inside the RPC, the cooling

time of strawberries in clamshell containers increases with the distance from the entrance

of the cold air, except for the clamshell containers located at the end of the RPC's.

Clamshell container 6 cooled relatively faster as compared to clamshell container 3 and

both were located at the end of the RPC's. The result might have been affected by the

location of clamshell container 6 where it was exposed to the exiting air as was discussed

previously. The result might also been affected by the presence of a large percentage of

by-pass area.


Clam. 1 Clam. 2 Clam. 3 Clam. 4 Clam. 5 Clam. 6
Clamshell Container Location


Figure 5-3. Cooling times of strawberries packed in clamshell containers as a function of
their distance to the entrance of cold air with 37.80% end by-pass area.
Columns with the same shade have the same boundary conditions.






43


For the treatment with 11.30% side by-pass area and considering the side of the

RPC perpendicular to the entering cold air, Figure 5-4 showed that the clamshell

containers located in middle part of each RPC, with the exception of the clamshell

containers in the second RPC, had longer cooling times. The result also showed that

having the same boundary conditions inside the RPC, the cooling time of strawberries

increases with the distance from the entrance of the cold air. The result was comparable

to the graph shown in Figure 5-2. This effect could still be the result of the by-pass areas

between clamshell containers and their location inside the RPC's.


Clam. 1 Clam. 2 Clam. 3 Clam. 4 Clam. 5 Clam. 6 Clam. 7 Clam. 8 Clam. 9
Clamshell Container Location


Figure 5-4. Cooling times of strawberries packed in clamshell containers as a function of
their distance to the entrance of cold air with 11.30% side by-pass area.
Columns with the same shade have the same boundary conditions.









For the end of the RPC with 16.70% end by-pass area, as shown in Figure 5-5,

clamshell containers 2 and 5 which were located at the middle part of the RPC's had the

longest cooling time. The results also showed that having the same boundary conditions

inside the RPC, the cooling times of strawberries increases with the distance from the

entrance of the cold air. Clamshell 4 which was located at the entrance of the second RPC

had a faster cooling time as compared to clamshell containers 2 and 3. Clamshell

container 4 was located at the entrance of the second RPC and the cooling time might

have been affected also by the by-pass area and the location of the clamshell containers

inside the forced-air cooling unit.


Clam. 1 Clam. 2 Clam. 3 Clam. 4 Clam. 5 Clam. 6
Clamshell Container Location


Figure 5-5. Cooling times of strawberries packed in clamshell containers as a function of
their distance to the entrance of cold air with 16.70% end by-pass area.
Columns with the same shade have the same boundary conditions.









For Treatment 3 with 1.80% side by-pass area, the cooling rates of the clamshell

containers tend to follow the result taken by Castro et al. (2004b) and observations by

Baird et al. (1988), Talbot and Chau (1991), and Talbot and Fletcher (1996) wherein the

farther the distance of products to the entrance of cold air, the longer the cooling time.


Clam. 1 Clam. 2 Clam. 3 Clam. 4 Clam. 5 Clam. 6 Clam. 7 Clam. 8 Clam. 9
Clamshell Container Location


Figure 5-6. Cooling times of strawberries packed in clamshell containers as a function of
their distance to the entrance of cold air with 1.80% side by-pass area.
Columns with the same shade have the same boundary conditions.

Looking at Figure 5-6, even within the clamshell containers inside the same RPC's,

the cooling time increases with distance, with the exception of clamshell container 7,

which was located at the entrance of the third RPC. Since majority of the by-pass areas

were blocked off for this treatment, the cold air entering the forced-air cooling unit that









carried heat away from the strawberries have increased in temperature as it goes further

from the entrance of the cold air. By blocking off the by-pass areas, cold air was forced to

pass through the vents of the clamshell containers carrying heat away from the

strawberries starting upstream and going downstream.


90


Clam. 1 Clam. 2 Clam. 3 Clam. 4 Clam. 5
Clamshell Container Location


Clam. 6


Figure 5-7. Cooling times of strawberries packed in clamshell containers as a function of
their distance to the entrance of cold air with 2.70% end by-pass area.
Columns with the same shade have the same boundary conditions.

For the end of the RPC with 2.70% by-pass area, as shown in Figure 5-7, the

cooling time also increases with distance except for clamshell container 4 which was

located at the entrance of the second RPC. The results shown in Figures 5-6 and 5-7

which had quite different results from the two previous treatments might be the result of

blocking off most of the by-pass areas and forcing the air to pass through the vents of the









clamshell containers. But the same result was also observed with the cooling times of

clamshell containers having the same boundary conditions wherein the farther their

location from the entrance of the cold air, the longer the cooling time.

Considering all the treatments, the clamshell containers located at the entrance of

the cold air had the fastest cooling time as compared to all the other clamshell containers.

This result agrees with the claim of Alvarez and Flick (1999) about the first layers

exposed to the entering cold air to cool faster. Cooling times of the clamshell containers

for the first and second treatment did not actually follow the observations by Baird et al.

(1988), Talbot and Chau (1991), Talbot and Fletcher (1996), and Castro et al. (2004a)

that the farther the distance of the clamshell containers from the entrance of cold air, the

longer was the cooling time. This could be the effect of the clamshell containers exposed

to different boundary conditions inside each RPC and the effect of the percent by-pass

areas. The consistent observation for the first and second treatment was that the clamshell

containers located at the middle part of the RPC's were the slowest to cool. The clamshell

containers located at the entrance of the RPC's cooled faster compared to the clamshell

containers preceding them.

The cooling times for the third treatment with the least percent by-pass area follows

the claim of Baird et al. (1988), Talbot and Chau (1991), Talbot and Fletcher (1996), and

Castro et al. (2004a) which states that the farther the clamshell containers from the

entrance of the cold air, the longer was the cooling time. The result might be the effect of

lowering the percent by-pass areas and forcing air through the vents of the clamshell

containers and not on the by-pass areas.









Looking at the data from Table 5-2 and 5-3 and considering the 7/8th cooling times

of each clamshell container as a function of their distance to the entrance of the cold air,

the third treatment with the least percent by-pass area still had the fastest cooling time as

compared to the two other treatments.

Table 5-4. 7/8th cooling times as a function of the location of the RPC's in the forced-air
cooling unit.
7/8th cooling time (min)
Treatment Side of RPC End of RPC
RPC 1 RPC 2 RPC 3 RPC 1 RPC 2
1 75.5 88.3 91.8 83.7 88.3
2 67.7 81.9 86.8 72.3 79.8
3 49.4 66.7 78.2 58.3 74.8

Considering the average cooling times of clamshell containers per RPC's, Table 5-

4 shows the averaged 7/8th cooling times for every RPC's. The results showed that the

RPC's cooling times were increased as they were located farther away from the entrance

of the cold air. Treatment 3 with the least percent by-pass area still cooled faster as

compared to the other two treatments.

An added issue was observed during the cooling process at the commercial forced-

air cooling facility. The air temperature kept on varying with time; thus it was difficult to

achieve a constant air temperature. Air temperature entering the forced-air cooling unit

varied by as much as 8.50C during the cooling process as shown in Figure 5-8. Air

temperature even went up as high as 110C during one cooling process. Warm

strawberries were constantly brought inside the cooler during the test and forklifts were

also operated inside the cooler. The main door of the forced-air cooling facility was also

open most of the time to allow the operation of forklifts in and out of the facility and this

could significantly increase the air temperature of the forced-air cooling facility.












AAA* Treatment 1 Rep 1

LA A Treatment 1 Rep 2
10 Xx-- Treatment 1 Rep 3 -
X XS* X *** x Treatment 2 Rep 1
xxx Treatment2 Rep 2
8 *. ''^xxXXxxxx *A Treatment 2 Rep 3
0 ^ + "@' XXhx + Treatment 3 Rep 1
| 6 .* + it %* Treatment3 Rep 2
S ... -.. s" + ^ it, 't Treatment 3 Rep 3
E XX +- 0 A




2 .


0 -
0 10 20 30 40 50 60 70 80 90 100 110
Time (min)



Figure 5-8. Air temperature entering the forced-air cooling unit during the cooling
process.

Uniformity of Cooling as a Function of the Location of the Strawberry Inside the
Clamshell Container

The cooling tests showed that a temperature gradient existed between individual

strawberries inside each clamshell container and in the entire layer after reaching the 7/8th

cooling time as shown in Table 5-6. The mean temperature differential gradient between

the strawberries instrumented with thermocouples at the start of the cooling process

where the strawberries were still fresh from the field was 3.99 'C. This could be the

result of some strawberries being exposed to direct sunlight and some strawberries

covered by leaves or where shaded during the time of harvest.










Table 5-5. Average temperature gradient between strawberries in the entire layer and
each clamshell container.
Mean temperature gradient in the Mean temperature gradient in each
Treatment entire layer (C) clamshell container (C)
After 7/8th Cool After 7/8th Cool
Start of Cooling After 7Start of Cooling After 7h Cool
Time Time
1 4.0 6.8 2.0 3.2
2 4.5 5.2 2.2 1.9
3 3.0 4.3 1.8 1.3


0 10 20 30 40 50 60 70 80 90
Time (min)


Figure 5-9. Sample cooling curve showing the temperature gradient during cooling of
individual strawberries inside clamshell container 2 with 34.30% side by-pass
area. The time was the average cooling time of strawberries per location. The
clamshell container was located at the center of the 2nd RPC. The side of the
RPC was perpendicular to the entering cold air.

Figure 5-9 shows a sample cooling curve and the temperature gradient existing

between strawberries as a function of their location inside the clamshell containers with

34.30% side and 37.80% end by-pass area. The average temperature gradient of the

monitored strawberries for every clamshell container was 2.0 'C during the start of the









cooling process. After reaching the 7/8th cooling time, the mean temperature difference

went up to 3.20C.

Considering the entire layer, the mean temperature gradient of individual

strawberries instrumented with thermocouples at the start of the cooling process was 4.0

'C while the mean temperature gradient after reaching the 7/8th cooling time went up to

6.8 oC.

Sixty-three percent of the monitored strawberries observed to have the lowest

temperature after reaching the 7/8th cooling time were located at the base of the

clamshell containers while 25% were located at the top of the clamshell containers.

Eighty-eight percent of the monitored strawberries and observed to have the highest

temperature after reaching the 7/8th cooling time were located at the middle part of the

clamshell containers.

Figure 5-10 shows a sample cooling curve and the temperature gradient existing

between individual strawberries as a function of their location inside the clamshell

container with 11.30% side and 16.70% end by-pass area. The average temperature

gradient on the monitored strawberries for every clamshell container was 2.2 oC during

the start of the cooling process. After reaching the 7/8th cooling time, the difference in

temperature went down 1.9 oC.

Considering the entire layer, the temperature gradient of individual strawberries

instrumented with thermocouples at the start of the cooling process was 4.5 OC while the

temperature gradient after reaching the 7/8th cooling time went up to 5.2 oC.

Seventy-nine percent of the monitored strawberries observed to have the lowest

temperature after reaching the 7/8th cooling time were located at the base of the









clamshell containers. Seventy-nine percent of the monitored strawberries and observed to

have the highest temperature after reaching the 7/8th cooling time were located at the

middle part of the clamshell containers.


0 10 20 30 40 50 60 70 80 90
Time (min)


Figure 5-10. Sample cooling curve showing the temperature gradient during cooling of
individual strawberries inside clamshell container 2 with 11.30% side by-pass
area. The time was the average cooling time of strawberries per location. The
clamshell container was located at the center of the 2nd RPC. The side of the
RPC was perpendicular to the entering cold air.

Figure 5-11 shows the temperature gradient existing between individual

strawberries as a function of their location inside the clamshell containers with 1.80%

side and 2.70% end by-pass area. The average temperature gradient of the monitored

strawberries for every clamshell containers was 1.8 'C during the start of the cooling

process. After reaching the 7/8th cooling time, the difference in temperature went down

to 1.3 'C.









Considering the entire layer, the temperature gradient of individual strawberries at

the start of the cooling process was 3.0 'C while the temperature gradient after reaching

the 7/8th cooling time went up to 4.3 oC.

Seventy-five percent of the monitored strawberries for Treatment 3 observed to

have the lowest temperature after reaching the 7/8th cooling time were located at the base

of the clamshell containers while 17% were located at the top of the clamshell containers.

Seventy one percent of the monitored strawberries and observed to have the highest

temperature after reaching the 7/8th cooling time were located at the middle part of the

clamshell containers.


0 10 20 30 40 50 60 70 80 90
Time (min)


Figure 5-11. Sample cooling curve showing the temperature gradient during cooling of
individual strawberries inside clamshell container 2 with 1.80% side by-pass
area. The time was the average cooling time of strawberries per location. The
clamshell container was located at the center of the 2nd RPC. The side of the
RPC was perpendicular to the entering cold air.









Considering the 3 treatments used in the cooling process, the results showed that a

temperature gradient existed between individual strawberries as a function of the 3

identified locations inside the clamshell containers. A summary of the temperature

gradient between strawberries was shown in Appendix B.

The strawberries located at the middle part of the clamshell containers tend to have

the highest temperature after reaching the 7/8th cooling time. The strawberries located at

the base of the clamshell containers were the fastest to cool and had the lowest

temperature after reaching the 7/8th cooling time followed by the strawberries located at

the top portion of the clamshell containers. Temperature gradients between strawberries

in one clamshell container can go as high as 5.7 'C after reaching the 7/8th cooling time.

The temperature gradient can be attributed to the location and percentage of vent

areas in the clamshell containers. The vent area at the bottom part of the clamshell

containers was measured to have a larger percentage as compared to the vent area at top

portion of the clamshell containers.

The results also showed that the percent by-pass area created inside the RPC's

affected the temperature gradient existing between strawberries in each clamshell

container and for the entire layer, wherein the lower the by-pass area the lower was the

temperature gradient as shown in the summary of the temperature gradients between

strawberries in Appendix B.

There was no significant trend observed with the effect of the location of the

clamshell container inside the RPC with the temperature gradient between strawberries

inside the clamshell containers.









Figures 5-12 to 5-17 show the difference in temperature and cooling times of

strawberries on clamshell containers as a function of their location inside the forced-air

cooling unit. These results can also verify the results gathered earlier in the determination

of the 7/8th cooling time of clamshell containers as a function of their distance from the

entrance of the cold air.

Figures 5-12 and Figure 5-13 show that there was not much difference between

cooling times of the eight clamshell containers. But one thing was consistent with the

result previously discussed, wherein the first layers of clamshell containers exposed to

the entering cold air cooled faster and the clamshell containers located at the middle part

of the RPC's had higher temperatures as it approaches the 7/8th cooling time. The test

results were also comparable to the results shown in Figures 5-2 and 5-3.

Considering the 2nd treatment as shown in Figures 5-14 and Figure 5-15, the results

also coincided with the result shown in Figures 5-4 and Figure 5-5, where the clamshell

containers located at the end of the 2nd RPC cooled slower than the clamshell containers

in the middle part of the RPC. The clamshell containers located at the entrance of the

RPC's were still the fastest to cool. The test results were also comparable to the results

shown in Figures 5-4 and 5-5.

Considering the third treatment, the results shown in Figures 5-16 and 5-17 also

coincided with the results shown in Figures 5-6 and 5-7. The clamshell container located

at the entrance of the forced-air cooling unit cooled faster and the cooling times of the

other clamshell containers increased as their location went farther from the entrance of

the cold air.















26
24
22
20
18


I 14

E 12
10
8
6

4
2
0


10 20 30


40 50 60 70 80 90


Time (min)


Figure 5-12. Cooling curve of the 4 clamshell containers instrumented with thermocouples placed on RPC's wherein the side of the
RPC was perpendicular to the entering cold air. The time was the average cooling time of the strawberries per clamshell
containers. The percent by-pass area was 34.30%.















26
24

22

20

18
16

| 14

E 12
I--
10

8

6

4
2
0


10 20 30 40 50 60 70 80 90


Time (min)


Figure 5-13. Cooling curve of the 4 clamshell containers instrumented with thermocouples placed on RPC's wherein the end of the
RPC was perpendicular to the entering cold air. The time was the average cooling time of the strawberries per clamshell
containers. The percent by-pass area was 37.80%.

















22 -Case conaie
SClamshell container 2
20 -EI- A Clamshell container 3
18 x Clamshell container 4
18* "
S* Ax Air temperature entering the unit
1 6 ------
0* X-N
S14 xx<:

I 12 xx A
I XX:::______AA.


10
aX X:X X 7/8th cooling







TimeXx time at 79 m(m)



containers. The percent by-pass area was 11.30 .
*1 x X ***AAAAAAAAA
-X'K # X*0 M N.

4 _____I__6__1L,__xxxxxxxxxx *



0 10 20 30 40 50 60 70 80 90

Time (min)


Figure 5-14. Cooling curve of the 4 clamshell containers instrumented with thermocouples placed on RPC's wherein the side of the
RPC was perpendicular to the entering cold air. The time was the average cooling time of the strawberries per clamshell
containers. The percent by-pass area was 11.30%.













24

22 Clamshell container 5
Clamshell container 6
20 Clamshell container 7
oPt x Clamshell container 8
18 C)
z i Air temperature entering the unit




12 10 Xx xx x
a)
10 **, Ia Xx 7/8th cooling
Sxxxxx time at79 min
I-xI







0
0 10 20 30 40 50 60 70 80 90
Time (min)


Figure 5-15. Cooling curve of the 4 clamshell containers instrumented with thermocouples placed on RPC's wherein the end of the
RPC was perpendicular to the entering cold air. The time was the average cooling time of the strawberries per clamshell
containers. The percent by-pass area was 16.70%.















24 Gliigx. Clamshell container 1
*1 -* Clamshell container 2
22 -
I* "f A Clamshell container 3
20 in x Clamshell container 4

18 *, g xAir temperature entering the unit -


14 *-s

S14-
2 /8th coo12ling-



10 **t timeat64min





0
0 10 20 30 40 50 60 70 80 90
Time (min)


Figure 5-16. Cooling curve of the 4 clamshell containers instrumented with thermocouples placed on RPC's wherein the side of the
RPC was perpendicular to the entering cold air. The time was the average cooling time of the strawberries per clamshell
containers. The percent by-pass area was 1.80%.
















24 Ii Clamshell container 5
x^xx_ Clamshell container 6
22 -
S ** A Clamshell container 7
20 x Clamshell container 8

18 Al l, x Air temperature entering the unit

16 *- a x
14 30 A
A3 *X
CI No, .^ A'' A x, 7/8th cooling
SAAAtime at64 min




X K rAAAXXXX
SMEMO I *** -AAAXXXX
xxxxx;:x:z*^ MO.- -





0 10 20 30 40 50 60 70 80 90

Time (min)



Figure 5-17. Cooling curve of the 4 clamshell containers instrumented with thermocouples placed on RPC's wherein the end of the
RPC was perpendicular to the entering cold air. The time was the average cooling time of the strawberries per clamshell
containers. The percent by-pass area was 2.70%.









The results taken for the determination of the 7/8th cooling time and the uniformity

of cooling had been significantly affected by the by-pass area created inside the RPC's.

The 7/8th cooling times were decreased as the percent by-pass area inside the RPC's was

reduced to 1.80% side and 2.70% end by-pass area. The percent by-pass area also

affected the uniformity of cooling between individual strawberries inside each clamshell

container and for the entire layer, where the lesser the by-pass area, the lesser was the

temperature gradient between strawberries. The location of the clamshell containers

inside the RPC's and inside the forced-air cooling unit did not necessarily affected the

uniformity of cooling of individual strawberries. But the location of the clamshell

containers inside each RPC and inside the forced-air cooling unit had an effect on the

cooling times of strawberries. The results showed that for by-pass areas of 11.30% side

and 16.70% end or greater, it did not necessarily followed the claim of Baird et al.

(1988), Talbot and Chau (1991), Talbot and Fletcher (1996), and Castro et al. (2004a)

that the farther the produce from the entrance of the cold air, the longer was the cooling

time. But one observation was consistent for first two treatments and was comparable to

the observations of Castro et al. (2004a) that the clamshell containers located at the

middle part of every RPC's had longer cooling time as compared to the clamshell

containers located at the end of the RPC which was farther from the entrance of the cold

air. Treatment 3 with the least by-pass area followed the claims of Baird et al. (1988),

Talbot and Chau (1991), Talbot and Fletcher (1996), and Castro et al. (2004a), where the

farther the produce form the entrance of the cold air, the longer was the cooling time.

There were also critical issues observed during the conduct of the test. Aside from

the varying inlet temperature existing on a commercial forced-air cooling facility,









subsequent conditioning, different strawberry size, insertion of thermocouples, location

of clamshell containers inside the RPC's, and the percent by-pass area as factors that

affected the cooling times collected, it was also observed the vent holes of the clamshell

containers and the RPC's were not aligned. This orientation will not only affect the

cooling time but will also affect the pressure drops during the cooling process.

Therefore, considering the results taken from all the test, it greatly supports the

suggestions of Talbot et al. (1995) and Anderson et al. (2003) that clamshell container

and trays should be designed together to maximize air-to-product contact during cooling.














CHAPTER 6
CONCLUSIONS

Forced-air cooling of packed strawberries placed on reusable plastic containers

(RPC's) was conducted on a forced-air cooling facility. The results taken for the

determination of the 7/8th cooling times showed that minimizing bypass areas created

inside the RPC's could significantly reduce the cooling times of strawberries. Cooling

times as a function of the distance and location of the clamshell containers inside a

forced-air cooling unit were also affected by the by-pass areas. Clamshell containers

located on the middle part of the RPC's with percent by-pass areas equal or greater than

11.30% side and 16.70% end tend to cool longer compared to one or two clamshell

containers farther from them. For a percent bypass area of 1.80% side and 2.70% end,

cooling times of clamshell containers tend to increase as their locations were farther from

the entrance of the cold air. Cooling times of clamshell containers also followed this

trend if boundary conditions were the same throughout the cooling process.

Blocking off the by-pass areas also affected the pressure drop. The measured

pressure drop during the cooling process for the standard configuration of the clamshell

containers in the RPC's was 17.44 Pa; but when the headspace was blocked off, the

pressure drop increased to 44.84 Pa. A dramatic increase was observed when the by-pass

areas between the clamshell containers were also blocked off, the pressure drop increased

from 17.44 Pa to 191.80 Pa.

The test results also showed that a temperature gradient exist between strawberries

in each clamshell container and for the entire layer after reaching the 7/8th cooling time.









Strawberries located in the middle part of clamshell containers cooled longer while the

strawberries located at the base of the clamshell containers cooled faster. This could be

attributed to the design and location of vent holes of the clamshell containers. The

temperature gradient existing between strawberries in a clamshell container were not

affected by their location inside the RPC and inside the forced-air cooling unit but were

affected by the by-pass area created inside the RPC's. The temperature gradient was

observed to be minimized when the by-pass area was minimized to 1.80% side and

2.70% end.

Therefore, considering the results taken from the entire test, it is highly

recommended that clamshell containers and trays should be designed together to

maximize air-to-product contact during cooling. Design consideration for the clamshell

container in RPC's for maximum air to product contact are: to minimize the bypass area

created when the clamshell containers are placed side by side and consider vent holes in

the middle part of the clamshell containers or equally distributing vent holes on the walls

of the clamshell containers. Design consideration for RPC's as a tray for clamshell

containers are to level the height of the RPC with the height of the clamshell container to

maximize stacking and avoid headspace and to align vent holes of the RPC's to the vent

holes of the clamshell containers.














APPENDIX A
AIRFLOW RATE CALCULATION

Standard "Plant" Equation for Gas Volume Flow Rate at Standard Condition:


Y2i
Q, = 7.9 SND2 f (B.1)


Where:

Q, = Volume Flow Rate in cubic feet per minute (CFM)

S = Constant factor for element at specific flow

S = kg F, (B.2)

Where:

kg = Geometrical constant

for Element types 710 and 720 for Element types 730 and 740

2 /2 inches Pipe, kg = 0.863 2 to 2 1/2" inches pipe, kg = 0.876

F, = Velocity distribution factor

= 0.82 for transition and turbulent flow

Therefore:

S = 0.863 x 0.82 = 0.70766 (for element types 710 and 720)

S = 0.876 x 0.82 = 0.71832 (for element types 730 and 740)

D = Inside diameter of pipe in inches (exact)

= 2.469 inches (from the annubar tag)










N = Grouped constant including 2g (gravity acceleration), (circular area), and
4

conversion constants which depends on units chosen for Q,

= for a unit of CFM at inches of water = 0.7576

h,, = Differential pressure output of annubar element in convenient unit (inches of

water for the manometer.

y, = Specific weight of gas at base conditions times the weight of air (lb/ cubic

feet) at base conditions.

7, = 0.0765 lb/ cubic foot at standard base conditions (60 oF, 14.73 psia)

yf = Specific weight at flowing conditions in pounds per cubic foot including

compressibility

Note:

To determine flow at flowing conditions set y, = y = 0.0765 lb/ cubic foot

Therefore:

Element types 710 and 720


Q = 7.9 x 0.70766 x 0.7576 x 2.4692 xJh ~
0.0765

= 93.3476023877 x F


= 93.35 x

Element types 730 and 740


Q = 7.9 x 0.71832 x 0.7576 x 2.4692 x 765 h,
0.0765

= 94.7537655754 x^






68


= 94.75 xJh7

Since the flow rate derived from the equation was in English unit, we can convert

the flow rate in to S.I. unit by using the following conversion factor:

1 cubic foot per minute = 0.47 liters per second
















APPENDIX B
TEMPERATURE GRADIENT OF STRAWBERRIES IN A CLAMSHELL
CONTAINER

Table B-1. Temperature gradient of strawberries in each clamshell container for treatment
1, replication 1.
Start of Cooling After Reaching the 7/8th Cooling Time
DLocation of Location of difference Location of Location of Difference in
lowest highest in lowest highest temperature
los h s temperature tmeare
temperature temperature pa) temperature temperature (0C)
1 top top 1.21 base top 3.07
2 base top 2.31 base mid 3.42
3 top base 1.32 top mid 1.67
4 base base 2.00 base top 2.16
5 mid base 1.47 base mid 5.40
6 top top 2.45 top mid 1.54
7 base top 1.54 base mid 3.94
8 mid mid 1.51 mid mid 2.42
Mean 1.73 2.30



Table B-2. Temperature gradient of strawberries in each clamshell container for treatment
1, replication 2.
Start of Cooling After Reaching the 7/8th Cooling Time
Clamshell Location of Location of Difference in Location of Location of Difference in
lowest highest temperature lowest highest temperature
temperature temperature (0C) temperature temperature (0C)
1 top base 1.33 base top 3.17
2 mid base 2.48 base mid 3.98
3 mid base 1.67 top mid 1.20
4 mid base 3.08 base mid 2.52
5 mid base 3.24 base mid 5.72
6 base top 2.33 top mid 1.72
7 base base 3.23 base mid 4.40
8 mid mid 1.81 mid mid 2.47
Mean 2.40 3.15










Table B-3. Temperature gradient of strawberries in each clamshell container for treatment
1, replication 3.
Start of Cooling After Reaching the 7/8t Cooling Time
Clamshell Location of Location of Difference in Location of Location of Difference in
lowest highest temperature lowest highest temperature
temperature temperature (oC) temperature temperature (OC)
1 top mid 0.41 base top 3.25
2 top base 2.54 base mid 4.01
3 mid base 2.43 top mid 1.88
4 mid base 3.16 base mid 2.36
5 mid base 1.79 base mid 5.74
6 base top 1.81 top mid 2.07
7 base base 2.71 base mid 4.24
8 mid top 0.35 mid mid 2.35
Mean 1.90 3.24



Table B-4. Temperature gradient of strawberries in each clamshell container for treatment
2, replication 1.
Start of Cooling After Reaching the 7/8th Cooling Time
Clamshell Location of Location of Difference in Location of Location of Difference in
lowest highest temperature lowest highest temperature
temperature temperature (oC) temperature temperature (OC)
1 base top 1.41 base top 1.02
2 mid base 3.51 base mid 2.96
3 mid base 3.87 bae mid 1.33
4 mid base 5.00 base mid 1.52
5 mid base 2.50 base mid 3.91
6 base top 3.13 base top 1.87
7 base top 4.14 base mid 3.03
8 base mid 3.37 mid mid 2.46
mean 3.37 2.26



Table B-5. Temperature gradient of strawberries in each clamshell container for treatment
2, replication 2.
Start of Cooling After Reaching the 7/8th Cooling Time
Clamshell Location of Location of Difference in Location of Location of Difference in
lowest highest temperature lowest highest temperature
temperature temperature (oC) temperature temperature (OC)
1 base mid 1.04 base mid 0.61
2 mid base 2.07 base mid 2.47
3 mid base 1.86 base mid 1.24
4 mid base 2.55 base mid 1.31
5 mid base 2.26 base mid 3.00
6 base top 1.58 top top 1.67
7 base top 1.45 base mid 2.41
8 mid mid 2.04 mid mid 2.41
mean 1.86 1.89







71


Table B-6. Temperature gradient of strawberries in each clamshell container for treatment
2, replication 3.
Start of Cooling After Reaching the 7/8w Cooling Time
Clamshell Location of Location of Difference in Location of Location of Difference in
lowest highest temperature lowest highest temperature
temperature temperature (oC) temperature temperature (OC)
1 top top 0.69 base top 0.38
2 top top 1.09 base mid 1.88
3 mid base 1.04 base mid 0.93
4 base base 1.32 base mid 1.02
5 mid base 1.46 base mid 2.19
6 base top 1.18 top top 1.38
7 base base 1.30 base mid 1.81
8 base mid 1.86 mid mid 1.96
mean 1.24 1.44



Table B-7. Temperature gradient of strawberries in each clamshell container for treatment
3, replication 1.


Clamshell Location of
lowest
temperature
1 base
2 mid
3 mid
4 base
5 mid
6 top
7 top
8 mid
mean


Start of Cooling
Location of
highest
temperature
top
top
base
top
mid
base
base
mid


Difference in
temperature
(oC)
2.01
1.17
0.60
0.88
3.12
1.52
1.25
1.31


After Reaching the 7/8' Cooling Time
Location of Location of Difference in
lowest highest temperature
temperature temperature (OC)
base top 0.37
base mid 2.19
bae mid 0.67
base mid 1.10
base mid 1.59
base base 1.42
top mid 1.58
ton mid 0.72


Table B-8. Temperature gradient of strawberries in each clamshell container for treatment
3, replication 2.
Start of Cooling After Reaching the 7/8th Cooling Time
Clamshell Location of Location of Difference in Location of Location of Difference in
lowest highest temperature lowest highest temperature
temperature temperature (oC) temperature temperature (OC)
1 base top 1.66 base top 0.49
2 mid base 0.68 base mid 2.41
3 mid base 1.20 base mid 0.68
4 mid base 2.15 base mid 0.90
5 mid mid 1.90 base mid 1.47
6 top base 1.44 base base 1.36
7 top base 1.16 top mid 1.60
8 mid mid 1.28 mid mid 1.17
mean 1.43 1.26







72


Table B-9. Temperature gradient of strawberries in each clamshell container for treatment
3, replication 3.
Start of Cooling After Reaching the 7/8w Cooling Time
Clamshell Location of Location of Difference in Location of Location of Difference in
lowest highest temperature lowest highest temperature
temperature temperature (oC) temperature temperature (OC)
1 base top 2.48 base top 0.95
2 mid base 2.67 base mid 1.89
3 mid base 1.40 base mid 0.89
4 mid base 2.42 base top 1.21
5 top mid 2.16 base mid 1.89
6 top base 2.49 base base 1.67
7 mid base 2.36 top mid 2.02
8 mid mid 2.81 mid mid 1.30
mean 2.35 1.48
















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BIOGRAPHICAL SKETCH

Melvin Bernabe Meana was born in La Trinidad, Benguet, Philippines, in June,

1977. In March, 1999, he completed a five year degree in agricultural engineering from

Benguet State University, La Trinidad, Benguet, Philippines. A month after taking his

licensure examination for agricultural engineers in the Philippines in September, 1999 he

worked as a technical support staff for the Department of Agriculture Cordillera

Administrative Region Field Unit under the Regional Agricultural Engineering Division

(Philippines). During his service with the department, he was involved in socio-economic

and detailed engineering/ topographic survey and preparation of plan and design of

agricultural infrastructure/ postharvest facilities. In March, 2002, he applied to the

Philippine-American Educational Foundation for a chance to earn a Fulbright

scholarship. He was granted a Fulbright scholarship and was accepted at the University of

Florida, Gainesville, Florida, United States of America. In August, 2003, he enrolled at

the University of Florida under the Agricultural and Biological Engineering Department,

to work towards a Master of Engineering in agricultural engineering.