Southeastern U.S. Vegetable Crop Handbook - 2021 - 48

their existing facilities. Simple placement of packed vegetables
in a refrigerated cooler is not sufficient to maintain quality for
product destined for distant markets. Neither should non-cooled
vegetables be loaded directly into refrigerated trailers. In both of
these situations, the product cools very slowly, at best. Refrigerated
trailers are designed to maintain product temperature during
transport, and they do not have the refrigeration capacity to quickly
remove field heat. Therefore, only produce that has been properly
cooled should be loaded, and only into trailers that have been
cooled prior to loading.
STORAGE REQUIREMENTS
Horticultural crops may be grouped and stored into two broad categories
based on sensitivity to storage temperatures. The degree of
chilling sensitivity, and therefore the lowest safe storage temperature,
is crop-specific. Those crops that are chilling sensitive should
be held at temperatures generally above 50°F (10°C). Storage below
this threshold will give rise to a physiological disorder known as
chilling injury. Chilling injury symptoms are characterized by development
of sunken lesions on the skin, increased susceptibility to
decay, increased shriveling, and incomplete ripening (poor flavor,
texture, aroma, and color). Vegetables most susceptible to chilling
injury include cucumber, eggplant, melons, okra, peppers, potatoes,
summer squash, sweetpotatoes, and tomatoes.The extent of chilling
symptoms is also dependent on the length of exposure to low temperatures.
Short exposure times will result in less injury than longer
exposure to chilling temperatures. Those crops not as sensitive to
chilling injury may be stored at temperatures as low as 32°F (0°C).
In addition to maintaining storage rooms at proper storage
temperatures, the relative humidity should also be controlled to
reduce water loss from the crop. Optimal storage recommendations
and precooling methods are included for a wide range of vegetable
commodities in Table 14.
OPTIMIZING COMMERCIAL COOLING
COOLING CONCEPTS
Cooling is a term that is often used quite loosely. In order to be effective
and significantly benefit the shipping life of the product, an
appropriate definition of commercial cooling for perishable crops is:
the rapid removal of at least 7/8 of the field heat from the crop by a
compatible cooling method. The time required to remove 7/8 of the
field heat is known as the 7/8 Cooling Time. Removal of 7/8 of the
field heat during cooling is strongly recommended to provide adequate
shipping life for shipment to distant markets; also, 7/8 of the
heat can be removed in a fairly short amount of time. Removal of
the remaining 1/8 of the field heat will occur during subsequent refrigerated
storage and handling with little detriment to the product.
The rate of heat transfer, or the cooling rate, is critical for efficient
removal of field heat in order to achieve cooling. As a form
of energy, heat always seeks equilibrium. In the case of cooling,
the sensible heat (or field heat) from the product is transferred to
the cooling medium. The efficiency of cooling is dependent on
time, temperature, and contact. In order to achieve maximum cooling,
the product must remain in the precooler for sufficient time to
remove heat. The cooling medium (air, water, crushed ice) must be
maintained at constant temperature throughout the cooling period.
The cooling medium also must have continuous, intimate contact
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with the surfaces of the individual vegetables. For reasonable cooling
efficiency, the cooling medium temperature should be at least
at the recommended storage temperature for the commodity found
in Table 14. Inappropriately designed containers with insufficient
vent or drain openings or incorrectly stacked pallets can markedly
restrict the flow of the cooling medium, increasing cooling time.
COOLING METHODS
The cooling rate is not only dependent upon time, temperature, and
contact with the commodity; it is also dependent upon the cooling
method being employed. The various cooling media used to cool
produce have different capacities to remove heat.
ROOM COOLING
The simplest, but slowest, cooling method is room cooling, in
which the bulk or containerized commodity is placed in a refrigerated
room for several days. Air is circulated by the existing fans
past the evaporator coil to the room. Vented containers and proper
stacking are critical to minimize obstructions to air flow and ensure
maximum heat removal. Room cooling is not considered precooling
and is satisfactory only for commodities with low respiration
rates, such as mature potatoes, dried onions, and cured sweetpotatoes.
Even these crops may require precooling, when harvested
under high ambient temperatures.
FORCED-AIR COOLING
The cooling efficiency of refrigerated rooms can be greatly improved
by increasing the airflow through the product. This principle
led to the development of forced-air, or pressure cooling, in
which refrigerated room air is drawn at a high flow rate through
specially stacked containers or bins by means of a high capacity
fan. This method can cool as much as four times faster than room
cooling. Forced-Air cooling is an efficient method for precooling.
In many cases, cold storage rooms can be retrofitted for forced-air
cooling, which requires less capital investment than other cooling
methods. However, in order to achieve such rapid heat removal,
the refrigeration capacity of the room may need to be increased to
be able to maintain the desired air temperature during cooling. Portable
systems can be taken to the field.
With either room cooling or forced-air cooling, precautions
must be taken to minimize water loss from the product. The refrigeration
system actually dehumidifies the cold-room air as water vapor
in the air condenses on the evaporator coil. This condensation
lowers the relative humidity in the room. As a result, the product
loses moisture to the air. To minimize water loss during cooling
and storage, the ambient relative humidity should be maintained at
the recommended level for the particular crop (commercial humidification
systems are available) and the product should be promptly
removed from the forced-air precooler upon achieving 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.
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. Hy2021
Vegetable Crop Handbook for Southeastern United States
Southeastern U.S. Vegetable Crop Handbook - 2021

Table of Contents for the Digital Edition of Southeastern U.S. Vegetable Crop Handbook - 2021
