Southeastern U.S. Vegetable Crop Handbook - 2023 - 39

7/8 Cooling. Forced-air cooling is recommended for most of the
fruit-type vegetables and is especially appropriate for vegetables
such as peppers and tomatoes but would not be appropriate for
leafy crops.
HYDROCOOLING
Hydrocooling removes heat at a faster rate than forced-air cooling.
The heat capacity of refrigerated water is greater than that
for air, which means that a given volume of water can remove
more heat than the same volume of air at the same temperature.
Hydrocooling is beneficial in that it does not remove water from
the commodity. It is most efficient (and, therefore, most rapid)
when individual vegetables are cooled by immersion in flumes or
by overhead drench, since the water completely covers the product
surfaces. Cooling becomes less efficient when the commodity
is hydrocooled in closed containers, and even less efficient when
containers are palletized and hydrocooled. It is important to continuously
monitor the hydrocooler water and product temperatures
and adjust the amount of time the product is in the hydrocooler
accordingly to achieve thorough cooling.
Sanitation of the hydrocooling water is critical since it is
re-circulated. Decay organisms and foodborne illness pathogens
present on the vegetables can accumulate in the water, inoculating
subsequent product being hydrocooled. Cooling water
should be changed frequently. Commodities that are hydrocooled
must be sufficiently resistant to withstand the force of the water
drench. The container must also have sufficient strength to resist
the application of water. Crops recommended for hydrocooling
include sweet corn, snap beans, cucumbers, and summer squash.
CONTACT ICING
Contact icing has been used for both cooling and temperature
maintenance during shipping. Heat from the product is absorbed
by the ice, causing it to melt. Provided the contact between the
ice and produce is maintained, cooling is rapid, and the melted
ice serves to maintain a very high humidity level in the package,
which keeps the produce fresh and crisp. Non-uniform distribution
of ice reduces the cooling efficiency. There are two types of
contact icing: top icing and package icing.
Top icing involves placement of crushed ice over the top layer
of product in a container prior to closure. Although relatively
inexpensive, the cooling rate can be slow since the ice only directly
contacts the product on the top layer. For this reason, it is
recommended that top icing be applied after precooling to crops
with lower respiration rates such as celery but is not appropriate
for chilling sensitive crops. Prior to shipping, ice is blown on top
of containers loaded in truck trailers to aid in cooling and maintenance
of higher relative humidity. However, care should be
taken to avoid blockage of vent spaces in the load; this restricts
air- flow, which results in warming of product in the center of
the load during shipment. Ice should also be " tempered " with
water to bring the temperature to 32°F (0°C) to avoid freezing of
the product.
Package Icing. Crushed ice distributed within the container is
known as package icing. Cooling is faster and more uniform
than for top icing, but it can be more labor intensive to apply.
2023 Vegetable Crop Handbook for Southeastern United States
A modified version of package icing utilizes a slurry of
refrigerated water and finely chopped ice drenched over either
bulk or containerized produce or injected into side hand holds.
This " slush ice " method has been widely adopted for commodities
tolerant to direct contact with water and requiring storage
at 32°F (0°C). The water acts as a carrier for the ice so that the
resulting slush, or slurry, can be pumped into a packed container.
The rapidly flowing slush causes the product in the container
to float momentarily until the water drains out the bottom. As
the product settles in the container, the ice encases the individual
vegetables by filling air voids, thus providing good contact
for heat removal. Slush icing is somewhat slower than forcedair
cooling, but it does reduce pulp temperatures to 32°F (0°C)
within a reasonable amount of time and maintains an environment
of high relative humidity. Container selection is critical.
The container must be oversized to accommodate sufficient ice
to provide cooling. Corrugated fiberboard cartons must be resistant
to contact with water (usually impregnated with paraffin
wax) and must be of sufficient strength so as not to deform.
Shipping operations must also tolerate water dripping from the
melting ice during handling and storage. Package icing is successfully
used for leafy crops, broccoli, sweet corn, green onions,
and cantaloupes.
VACUUM COOLING
Vacuum cooling is a very rapid method of cooling and is most
efficient for commodities with a high surface-to-volume ratio
such as leafy crops. This method is based on the principle that,
as the atmospheric pressure is reduced, the boiling point of water
decreases. Containerized or bulk product is thoroughly wetted,
placed in a vacuum chamber (tube) and sealed. The pressure in
the chamber is reduced until the water on the product surface
evaporates at the desired precooling temperature. As water on
the product surface evaporates, it removes field heat; the resultant
vapor is condensed on evaporator coils within the vacuum
tube to increase cooling efficiency. Any water that evaporates
from the vegetable tissue is removed uniformly throughout the
product. Therefore, it does not tend to result in visible wilting in
most cases.
Precautions must be taken so as not to cool the products
below their chilling temperature threshold. Vacuum coolers are
costly to purchase and operate and are normally used only in
high volume operations or are shared among several growers.
Commodities that can be cooled readily by vacuum cooling include
leafy crops, such as spinach, lettuce, and collards.
SUMMARY
When selecting an appropriate cooling method, several factors
must be considered, including: the maximum volume of product
requiring precooling on a given day, the compatibility of the
method with the commodities to be cooled, subsequent storage
and shipping conditions, and fixed/variable costs of the system.
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Southeastern U.S. Vegetable Crop Handbook - 2023

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