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 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 the Table. 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 forcedair 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. 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 in order to achieve thorough cooling.

Sanitation of the hydrocooling water is critical, since it is re-circulated. Decay organisms 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 so as 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. As long as the contact between the ice and produce is maintained, cooling is fairly 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 fairly 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 leafy vegetables and celery but not for fruit of warm-season 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.

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 caus-es 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 forced-air 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, 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.