Southeastern U.S. Vegetable Crop Handbook - 2023 - 38

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 noncooled
vegetables be loaded directly into refrigerated trailers. In
both 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. 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 can mean different things, depending
on your market. To be effective and significantly benefit the
shipping life of the product, commercial cooling for perishable
crops is defined as: 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 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. However, even for
local markets, cooling is the most important step in ensuring
quality and shelf-life of the product.
The rate of heat transfer, or the cooling rate, is critical for
efficient removal of field heat to achieve cooling. As a form of
energy, heat always seeks equilibrium. In the case of cooling, the
field heat from the product is transferred to the cooling medium.
The efficiency of cooling is dependent on time, temperature, and
contact. To achieve maximum cooling, the product must remain
38
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 contact 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 and increase 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 used. The various cooling methods 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 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 generally used for
long storage crops 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. It is important
to maintain high humidity in the room, as room cooling can also
cause significant water loss. More information can be found on
this in the forced-air cooling section below.
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, 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 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 specific crop (commercial humidification
systems are available) and the product should be
promptly removed from the forced-air precooler upon achieving
2023 Vegetable Crop Handbook for Southeastern United States
Southeastern U.S. Vegetable Crop Handbook - 2023

Table of Contents for the Digital Edition of Southeastern U.S. Vegetable Crop Handbook - 2023
