Postharvest Cooling and Handling
of Sweet Corn
Freshness Facts
Preferred cooling method: ... Hydrocooling, icing
Optimum temperature: ....................... 32 F
Freezing temperature: ...................... 31 F
Optimum humidity: ..................... 90 to 98%
Storage life: .......... 5 to 7 days for standard
varieties; 8 to 12 days for supersweet varieties
Sweet corn is a highly perishable crop. When produced for commercial markets, it must be
cooled immediately and thoroughly after harvest to protect its quality. It must be kept cool until
it reaches the consumer. Mishandling sweet corn causes serious and irreversible deterioration in
quality and loss of sweetness and tenderness. Because temperatures are usually high in
midsummer when sweet corn is harvested in North Carolina, growers, shippers, and processors
must have access to cooling equipment and must have knowledge of proper cooling and handling
methods. Only careful attention to postharvest handling procedures can ensure buyer
satisfaction and marketing success.
This publication acquaints growers, shippers, and processors with energy-efficient cooling and
handling methods useful in preserving the quality of fresh sweet corn.
Harvesting and Handling
Careful supervision of harvesting -- whether done by hand or machine -- results in fewer
problems at the packing shed. Whenever possible, sweet corn should be harvested early in the
morning when its moisture content is high. Also, the pulp temperature may be as much as 30 F
lower at dawn than at midday. Harvesting in early morning is thus a good way to reduce cooling
loads and save energy.
After harvest, the ears are normally transported to the packing shed in bulk trucks or trailers.
Pulp temperatures at harvest are often higher than 90 F. At these temperatures, bulk lots of
uncooled sweet corn will rapidly overheat because of the heat produced by respiration. (The
respiration rate of sweet corn is among the highest of common fruits and vegetables; in fact, its
rate is about eight times higher at field temperatures than at 32 F.)
If harvested ears are to be left in bulk trucks or trailers for more than an hour, they should be
kept from direct sunlight and cooled with a steady flow of well water to remove as much of the
field heat and heat of respiration as possible.
Water should be evenly distributed throughout the load with sprinklers to ensure complete
coverage at the rate of approximately 1 gallon of water per 4 pounds of corn per hour. For
example, a truck containing 10,000 pounds of freshly harvested sweet corn would require
approximately:
10,000 / 4 = 2,500 gallons of water per hour.
Well water temperatures during the summer in North Carolina average about 60 F, low enough
to reduce markedly the total cooling load and thus the cost of subsequent refrigeration.
The care given sweet corn during harvesting should be matched by the attention it receives
during packing. At the packing shed, sweet corn should be trimmed uniformly to eliminate flag
leaves and long shanks. If left on the ear, they will cause packaging problems and induce further
moisture loss. Objectionable kernel denting may occur with a moisture loss of 2 percent or less.
Only first-quality sweet corn devoid of defects and of uniform maturity, color, shape, and size
should be selected and packed. Any ears exhibiting signs of disease or mechanical or insect
damage should be discarded along with any ears that lack adequate shuck coverage.
Most sweet corn is packed in wirebound crates holding 42 pounds net weight. Cabbage bags
containing 4 1/2 to 5 dozen ears and wax-impregnated cartons containing 50 pounds net are also
used. New growers should always consult with buyers to determine what type of shipping
container they prefer. Buyers often perceive produce shipped in non-standard containers to be of
lower quality than that shipped in standard containers.
Cooling and Storage
The taste and quality of sweet corn depends heavily upon its sugar content, which rapidly
decreases after harvest if ears are allowed to remain at field temperatures. By lowering the
temperature, the conversion of sugar to starch may be substantially slowed but not completely
stopped. Loss of sugar is about four times as rapid at 50 F as at 32 F.
Newer supersweet cultivars contain over twice as much sugar as standard varieties and are more
forgiving of delayed cooling and mishandling. For maximum quality and value, however, sweet
corn must be continuously and properly refrigerated from harvest until it reaches the consumer.
Figure 1 demonstrates the effect of various storage temperatures on loss of sweetness. The ideal
cooling process for sweet corn removes field heat rapidly and reduces the temperature to near 32
F. This is done in a two-stage process starting with hydrocooling of either packaged or loose
corn. Hydrocooling by drenching or immersing in near-freezing water is effective in removing
the critical highest heat (the upper two-thirds, or three-quarters of the difference between the
harvest temperature and the water temperature). Because the rate of heat transfer is proportional
to the temperature difference, hydrocooling removes heat much faster at the start of the cycle
than at the end.
Figure 1. Loss of sugar during storage at four different temperatures. (Source: USDA
handbook No. 66: The Commercial Storage of Fruits, Vegetables, and Nursery Stocks.)
The nomograph in Figure 2 illustrates this point. For example, assume that the temperature of a
load of sweet corn packed in wirebound crates before cool-ing is 80 F and the temperature of the
hydrocooler water is 36 F. Draw a straight line from the 80-degree mark on the left side of the
chart to the 36-degree mark on the right side. The nomograph shows that it will take 25 minutes
to remove the first 20 degrees of field heat and nearly 65 additional minutes to remove the
second 20 degrees of heat. Thus it would be unduly expensive and time-consuming to hydrocool
sweet corn for longer than 20 to 30 minutes.
Figure 2. Hydrocooling nomograph for sweet corn in wirebound crates. (Source: USDA
Marketing Research Report No. 637. Hydrocooling Vegetables.)
An alternative is to top ice the packaged sweet corn during shipment and store it with large
amounts of crushed ice to remove the remaining heat and minimize respiration heat. Enough ice
should be placed in each crate to remove the remaining field heat above 32 F and adsorb the heat
of respiration plus the heat infiltrating into the transport vehicle or storage building. One pound
of ice for each 5 pounds of sweet corn is usually adequate.
Under optimum storage conditions (low temperature and high humidity), sweet corn may not be
stored for longer than 5 to 8 days without significant loss of quality. Supersweet varieties seem
to maintain an acceptable level of sweetness for a longer period in storage, but reliable
information on the rate of quality loss is not available. Loads should be inspected frequently and
a suitable fungicide material should be added to the hydrocooler water to prevent disease
infestation. For fungicide recommendations, see your county Extension Service agent or consult
the latest edition of the North Carolina Agricultural Chemicals Manual.
For complete information on hydrocooling, refer to Agricultural Extension Service publication
AG-414-4, Maintaining the Quality of North Carolina Fresh Produce: Hydrocooling. For
information on top icing, refer to Agricultural Extension Service publication AG-414-5,
Maintaining the Quality of North Carolina Fresh Produce: Top and Liquid Ice Cooling.
Prepared by
M. D. Boyette, Extension Agricultural Engineering Specialist L. G. Wilson, Extension
Horticulture Specialist E. A Estes, Extension Marketing Specialist
This publication was produced by the North Carolina Agricultural Extension Service with
support provided by the Energy Division, North Carolina Department of Economic and
Community Development from petroleum violation escrow funds. The opinions, findings,
conclusions, and recommendations expressed herein are those of the authors and do not reflect
the views of the Energy Division, North Carolina Department of Economic and Community
Development.
Published by
THE NORTH CAROLINA AGRICULTURAL EXTENSION SERVICE
North Carolina State University at Raleigh, North Carolina Agricultural and Technical State
University at Greensboro, and the U.S. Department of Agriculture, cooperating. State University
Station, Raleigh, N.C., R.C. Wells, Director. Distributed in furtherance of the Acts of Congress
of May 8 and June 30, 1914. The North Carolina Agricultural Extension Service is an equal
opportunity/affirmative action employer. Its programs, activities, and employment practices are
available to all people regardless of race, color, religion, sex, age, national origin, handicap, or
political affiliation.
11/90--3M--DED
AG-413-4