Group Title: Food technology and nutrition mimeo report
Title: Factors affecting quality maintenance of fresh sweet corn
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 Material Information
Title: Factors affecting quality maintenance of fresh sweet corn
Series Title: Food technology and nutrition mimeo report - Florida Agricultural Experiment Station ; 60-1
Physical Description: 13 leaves : ; 28 cm.
Language: English
Creator: Showalter, R. K.
University of Florida -- Agricultural Experiment Station
University of Florida -- Dept. of Food Technology and Nutrition
Publisher: Department of Food Technology and Nutrition, Florida Agricultural Experiment Station
Place of Publication: Gainesville, Fla.
Publication Date: 1960
Copyright Date: 1960
Subject: Corn -- Handling -- Florida   ( lcsh )
Corn -- Postharvest technology -- Florida   ( lcsh )
Genre: bibliography   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
non-fiction   ( marcgt )
Bibliography: Includes bibliographical references (leaf 13).
General Note: Cover title.
General Note: "June, 1960."
Statement of Responsibility: by R.K. Showalter.
 Record Information
Bibliographic ID: UF00094969
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: oclc - 435493649

Full Text

June, 1960

Food Technology and Nutrition
Mimeo Report 60-1



R. K. Showalter


Department of Food Technology and Nutrition
Florida Agricultural Experiment Station
Gainesville, Florida


;tiup ~?'.




R. K. Showalter


The material contained herein summarizes certain handling
practices used during the marketing of Florida sweet corn and
the effects of these practices on the sweet corn quality. This
information was presented before a Hearing on a proposed State
Marketing Order on January 7, 1960. This publication is intended
to furnish those who grow and market sweet corn with similar
information which may be useful to the sweet corn industry.

The material presented is divided as follows:


The Importance of Sweet Corn Eating Quality 2
Sweet Corn Precooling is Essential 3
Principles of Precooling 4
Precooling Methods 4
Hydrocooling 5
Vacuum Cooling 6
Effects of Precooling on Quality 7
Containers for Florida Sweet Corn 9
Quality of Florida Sweet Corn as Affected by
Present Marketing Practices 10
Literature Cited 13



The quality of sweet corn on the consumer's table depends

upon the quality at harvest and the handling after harvest. The

methods of handling sweet corn after harvest are as important as

the practices used to produce high quality corn. The big increase

in fall, winter, and spring production, particularly in the

Everglades, has made sweet corn available for twelve months of

the year in many markets where it was previously available only

during the summer season. The per capital consumption of fresh

sweet corn is steady after rising for several years. Why should

this situation exist when the supply often exceeds the demand?

The answer is probably due partly to consumer dissatisfaction

over poor eating quality.

Consumers prefer sweet corn with fresh, green husks and ears

well filled with plump, milky kernels which are free from worm

damage, discoloration and mechanical injury. However, the real

test of consumer satisfaction is made on the basis of eating

qualities. The "garden fresh" eating qualities of sweet corn

are much more perishable than the grade and appearance factors.

Many consumers receive sweet corn with a good appearance but not

good eating qualities.

3 -

The physical and chemical changes associated with sweet corn

quality deterioration have been studied at various times during the

past 40 years. One of the most significant results of these studies

is the fact that changes in sweet corn quality after harvest are

influenced by temperature and moisture conditions. At high temper-

tures the toughness of the pericarp, or kernel skin, increases and

the sugar changes to starch. Under dry conditions the succulence

and moisture content of the kernels decrease.


For many years the major portion of the sweet corn for fresh

markets was grown near the consuming centers. Little emphasis was

placed on precooling when the corn was marketed within 24 hours

after harvest. When sweet corn was handled in bulk or in burlap

bags heat developed rapidly, and the corn often became so hot that

the husks dried and became yellow or brown. A number of studies

(1) were made on the effects of icing sweet corn in retail stores.

In all cases the quality of the iced sweet corn was superior and

resultant sales far exceeded the sales of non-refrigerated corn.

Refrigeration during long distance shipping of fresh produce

has been used for over 70 years. During this period it was found

that better quality was maintained in some produce if the tempera-

ture was reduced before shipment. This practice of precooling as

used in the fruit and vegetable business refers to the rapid

cooling before shipment. In this statement precooling shall be


defined technically as "the rapid removal of field heat from sweet

corn soon after harvesting." Obviously, refrigeration only during

display in a retail store can hardly be considered precooling.


Precooling can be accomplished by several methods which result

in the transfer of heat from the corn to the air, water, ice, or

the direct evaporation of water from the corn by vacuum cooling.

The rapidity of precooling according to Ryall (2) depends upon:

(a) the temperature differential between the commodity and the

refrigerating medium; (b) the velocity of the refrigerating medium;

(c) the accessibility of the product to the cooling medium; (d) and

the nature of the cooling medium.

These principles may be illustrated by (a) field heat will be

removed faster by water or air at 330 F than at 430; (b) rapidly

circulating water or air will precool faster than non-moving water

or airy (c) individual ears will cool faster than packed crates of

ears, and ..crates of corn will cool more rapidly than bulk loads,

(d) vacuum cooling is more rapid than hydrocooling.


Forced-air cooling in a rcom, car, or tunnel has been used

successfully for those commodities which maintain freshness longer

if they are not wet. Precooling with water, which is commonly

known as hydrocooling, has been used for many years. Commodities

such as sweet corn benefit from the water as well as the temperature

- 5-

reduction. The newest method of precooling is by vacuum, where

the temperature is reduced by evaporation of moisture at greatly

reduced pressures.

Some of the early shipments of Florida sweet corn were packed

in mesh bags and loaded in rail cars with alternate layers of

cracked ice. This method was unsatisfactory because the ice melted

before the corn was adequately cooled, and no air could circulate

through the load to prevent reheating. Florida growers have found

that they must not only precool their corn rapidly to handle large

volumes, but they must provide sufficient refrigeration to remove

the field heat and the heat of respiration during the long transit


The larger hydrocoolers use mechanical refrigeration and

pump the water over cooling coils and the crates of partially

submerged corn. The crates are moved through a large, rectangular

tank 55 to 65 feet long on a conveyor belt beneath the shower of

cold water. The smaller units use cracked ice for refrigeration

and move the crates through a shower of cold water for about 25

feet without partially submerging them. Both types usually maintain

satisfactory water temperatures of 310 to 34F. The time required

for the corn to pass through the hydrocooler and the rate of water

circulation vary considerably and affect the precooling process

much more than slight differences in water temperature.

6 -

Vacuum Cooling

At normal atmospheric pressure of approximately 30 inches,

water boils at 2120F. If the pressure over water is reduced to

0.18 inch, the water boils at 320F. The water which "boils" or

evaporates from the vegetables cools them to a temperature corres-

ponding to the temperature of the water. To secure the reduced

pressure, sweet corn is placed in a large sealed tube and the re-

quired vacuum is obtained by a pump or a steam jet. The precooling

is measured with a recording thermometer, and the vacuum is released

when the corn reaches the desired temperature.

Research on the vacuum cooling of Florida sweet corn (5) has

shown that the ear temperatures can be reduced rapidly to 320 to

400 in wirebound crates or fiberboard cartons. Hydrocooling individ-

ual ears (8) is more rapid than hydrocooling ears in containers,

but Florida sweet corn is almost entirely field packed in wirebound

crates, and it would not be feasible to precool individual ears.

Precooling of crated corn is more rapid by the type of hydrocoolers

used in Florida than by submersion in a tank of non-circulating

water of the same temperature. Precooling of crated corn is also

more rapid by vacuum than by hydrocooling. In both instances the

differences in cooling rates are related to differences in access-

ibility of the corn to the cooling medium. The ears are more

accessible to the cooling medium of evaporation in a vacuum than

they are to the flow of water in a hydrocooler.

7 -

Another basic difference between the two precooling methods

results from the fact that water changing from the liquid to the

vapor phase during vacuum cooling removes more heat from the pro-

duct than when ice changes to the liquid state. Thus, the vapor-

ization of one pound of water and the melting of about 7 pounds

of ice accomplish the same precooling. A single container or a

whole load of packed containers can be readily vacuum cooled

provided the containers are not sealed to prevent the escape of

water vapor.


Recently a number of studies have been made of precooling

methods for sweet corn delivered directly from local growers to

retail stores. Winter (8) in Minnesota compared the temperatures

and quality of corn after two methods of precooling and no precool-

ing. Cob temperatures decreased more rapidly when husked ears were

submerged in a small tank of ice water than when unhusked ears in

burlap bags were placed in a truck with cracked ice. The non-pre-

cooled sweet corn lost 8 percent moisture after 20 hours and 10 to

15 percent after 54 hours, while there was no appreciable loss in

the precooled lots. One day after harvest the non-precooled corn

had lost 50 percent of its sugar compared with a 16 percent loss in

the two precooled lots.

In Florida sweet corn stored at 750 F without hydrocooling

there was a decrease of 51 percent in sugars after 18 hours (3).

8 -

After hydrocooling to 400 and storing at 32, there was still a

slow decrease in sugars. Without hydrocooling and refrigeration

the pericarp content increased 43 percent in 3 days. Hydrocooling

to 400 and storing at 400 prevented any increase in pericarp for

7 days.

In a study of vacuum cooling and hydrocooling as related to

quality (6), the Florida sweet corn vacuum cooled to 350-400 F had

significantly higher total sugars after 2, 5, and 8 days storage

than the sweet corn hydrocooled to 55-60o during the same 30 min-

ute precooling period. Both lots were top iced and stored at 380,

but the temperature of the hydrocooled corn decreased gradually

for two days before reaching the temperature of the vacuum cooled

ears. Hydrocooling could be used to reduce the corn temperature

to the same level as that secured by vacuum cooling. However,

Winston (7) found that when sweet corn was held in a large com-

mercial hydrocooler for two hours the temperature was reduced to

only 420 from a field temperature of 90. Since sweet corn is

harvested continuously throughout the day, and not just during a

few hours in the morning or evening the large volume to be pre-

cooled prohibits lengthy periods of hydrocooling for any particular


Time is a very important factor in marketing sweet corn. Pre-

cooling has been stressed as a means of maintaining sweet corn

quality. However, most of the changes in quality are not reversible,

9 -

and once the sugars have decreased, the pericarp increased, and

the husks discolored, no amount of precooling can return the fresh

qualities. Quality decreases very rapidly during the interval be-

tween harvest and precooling. After precooling, delay in loading

for shipment and top icing should also be avoided.

When the sweet corn temperature was reduced about 500 F by

vacuum cooling there was a loss in moisture of approximately 5 per-

cent. Theoretically there is a one percent water loss for each

100 F decrease in temperature (2). Recent studies (6) on precooling

of Florida sweet corn showed that the moisture loss during vacuum

cooling resulted in dented kernels after 5 and 8 days storage if

the ears were not wet with water before and after the cooling. Wet-

ting the ears with water eliminated the moisture loss, and top ice

during storage prevented denting of kernels. Ryall (2) also found

that wetting before vacuum cooling reduced the weight loss.


Bulk handling of sweet corn in Florida is usually limited to

those harvesting operations where the corn is hauled in tractor-

drawn wagons to packing bins in the field or to a packing house.

Sweet corn should not be handled in bulk (1), except for very short

periods, because of its tendency to heat throughout the load.

Mesh bags were used by the early shippers of Florida sweet

corn. The ears in bags were not protected from mechanical damage,

and proper refrigeration was almost as difficult as in bulk loads

10 -

because of the lack of channels for ice or air circulation. Most

Florida sweet corn is now packed in corn container No. 3730 with

4 1/2 to 6 dozen ears per crate. Some varieties with larger ears

are packed in vegetable crate No. 5007 which is about two inches

wider than No. 3730. Winston (7) showed that the rate of cooling

was influenced by the crate size. Better hydrocooling occurred

in the smaller crate where the corn had better accessibility to the

cold water.

Fiberboard cartons have been introduced but not generally

accepted for sweet corn. New fiberboard cartons may withstand

top icing, but maximum accessibility of the corn to cold water

during hydrocooling and to top ice during shipment are also nec-


Sweet corn needs a rigid shipping container for protection

against mechanical damage. Bruising of the ears, particularly at

the tips, results in dark, discolored kernels with a very poor

appearance. Long ears protruding from the wirebound crate are

especially susceptible to bruising damage. When handled in bulk

or mesh bags the entire ear is susceptible to severe bruising which

becomes apparent when the ear is husked.


Although nearly all Florida sweet corn is hydrocooled, Winston

(7) found the average cob temperature was only 580 after hydrocool-

ing. During an eight week period in April and May, 1955, a study

11 -

(4) was made of sweet corn quality changes between Florida shipping

points and retail stores in Baltimore. All of the sweet corn was

precooled for periods of 10 to 45 minutes in ice or mechanically

refrigerated hydrocoolers. The shorter cooling cycles were very

inadequate, and the corn temperatures ranged from 460 to 730

(average 600) after precooling. Although the temperature of 100

lots of sweet corn at wholesale averaged only 500, most of the

corn appeared to be in excellent or good condition. The corn

temperatures at 6 retail stores averaged 570, and the appearance

varied from excellent to very poor.

The eating qualities of sweet corn are measured principally

by the tenderness, sweetness, and succulence of the kernels. At'

wholesale in Baltimore the corn had lost about 40 percent of the

initial sugars and at retail the loss varied from 40 to 60 percent.

The total time from harvesting in Florida until the corn was offered

for retail sale varied from 3 to 15 days and averaged 7 days. The

succulence of the corn changed very little during marketing, but

there was a marked increase in toughness. The pericarp content

increased 61 percent in the store retailing 5 day old corn compared

with an increase of 84 percent in the store selling 10 day old corn.

If Florida growers desire to place high quality sweet corn on

distant markets adequate precooling and continuous refrigeration

are essential. The production problems have been solved to the

extent that Florida has surpassed all other states in growing large

12 -

quantities of worm free, fancy grade sweet corn. The trade and

consumers have purchased Florida corn because it looks good and

often far surpasses the appearance of locally grown corn in other

areas. However, Florida corn must also have good flavor in order

to stimulate repeat purchases by consumers.

Since sweet corn with good eating qualities brings premium

prices and repeat purchases, one solution to the problem of excess

supplies and low prices appears to center around limiting the hand-

ling practices to those which yield the highest consumer quality.

Considerable advancement has been made in recent years in our

knowledge and facilities for producing and shipping high quality

sweet corn to distant markets. It is very doubtful that all of

those who handle Florida sweet corn know of the exacting require-

ments needed to maintain high quality. It appears that an educa-

tional or regulatory program is needed to prevent any one group---

growers, shippers, transportation agencies, wholesalers, or re-

tailers---from falling down on their part of the job of moving

sweet corn with good eating and appearance qualities to distant

and nearby markets.

- 13


1. Roberts, Elise W. Sweet Corn. Fruit and Vegetable Facts
and Pointers. United Fresh Fruit and Vegetable Associa-
tion. Pages 1, 7-9. Revised December 1954.

2. Ryall, Lloyd A. Precooling Principles and Methods. Pro-
ceedings 10th Short Course on Causes and Prevention of
Economic Waste in Marketing Fruits and Vegetables.
Pages 76-81. 1958.

3. Showalter, R. K. Precooling as Related to Quality of Sweet
Corn. Proceedings Sweet Corn Quality Conference Sponsored
by Cornell University. Pages 21-23. 1958.

4. Showalter, R. K., W. Smith Greig, C. S. Parsons, and K. D.
Demaree. Quality of Florida Sweet Corn as Affected by
Marketing Practices. Proceedings Florida State Horticul-
tural Society. 68: 182-185. 1955.

5. Showalter, R. K. and B. D. Thompson. Vacuum Cooling of
Florida Vegetables. Proceedings Florida State Horti-
cultural Society. 69: 132-135. 1956.

6. Showalter, R. K. Effect of Wetting and Top Icing upon the
Quality of Vacuum Cooled and Hydrocooled Sweet Corn.
Proceedings Florida State Horticultural Society.
70: 214-219. 1957.

7. Winston, J. R., Randall Cubbedge and Jacob Kaufman.
Effect of Hydrocooling and of Top Icing on Temperature
Reduction in Car lot Shipments of Crated Green Sweet
Corn. Handling, Transportation, and Storage Office
Report No. 270. U. S. Department of Agriculture Bureau
of Plant Industry, Soils, and Agricultural Engineering.
April 1952.

8. Winter, J. D., R. E. Nyland, and R. W. Cox. Marketing
Fresh Sweet Corn in the Midwest. Minnesota Agricultural
Experiment Station Bulletin 427. 1954.

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