Food Technology and Nutrition
Mimeo Report 60-1
FACTORS AFFECTING QUALITY MAINTENANCE OF FRESH SWEET CORN
R. K. Showalter
Department of Food Technology and Nutrition
Florida Agricultural Experiment Station
FACTORS AFFECTING QUALITY MAINTENANCE OF FRESH SWEET CORN
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
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 IMPORTANCE OF SWEET CORN EATING QUALITY
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.
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.
SWEET CORN PRECOOLING IS ESSENTIAL
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.
PRINCIPLES OF 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
reduction. The newest method of precooling is by vacuum, where
the temperature is reduced by evaporation of moisture at greatly
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.
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.
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
EFFECTS OF PRECOOLING ON QUALITY
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).
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
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,
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.
CONTAINERS FOR FLORIDA SWEET CORN
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
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
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.
QUALITY OF FLORIDA SWEET CORN AS AFFECTED BY PRESENT
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
(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
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.
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.
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.