Throughout the period between harvest and consumption, temperature control has been found to be the most important factor in maintaining product quality. Fruits, vegetables and cut flowers are living, respiring tissues separated from their parent plant. Keeping products at their lowest safe temperature (0 C or 32 F for temperate crops or 10-12 C or 50-54 F for chilling sensitive crops) will increase storage life by lowering respiration rate, decreasing sensitivity to ethylene gas and reducing water loss. Reducing the rate of water loss slows the rate of shriveling and wilting, causes of serious postharvest losses.
Keeping products too cool can also be a serious problem. It is important to avoid chilling injury, since symptoms include failure to ripen (bananas and tomatoes), development of pits or sunken areas (oranges, melons and cucumbers), brown discoloration (avocados, cherimoyas, eggplant), increased susceptibility to decay (cucumbers and beans), and development of off-flavors (tomatoes) (Shewfelt, 1990).
Cooling involves heat transfer from produce to a cooling medium such as a source of refrigeration. Heat transfer processes include conduction, convection, radiation and evaporation.
If a ready supply of electricity is available, mechanical refrigeration systems provide the most reliable source of cold. Methods include room cooling, forced-air cooling and evaporative cooling. A variety of portable forced-air coolers have been designed for use by small-scale growers and handlers (Talbot and Fletcher, 1993; Rij et al, 1979; Parsons and Kasmire, 1974). However, a variety of simple methods exist for cooling produce where electricity is unavailable or too expensive. Some examples of alternative systems (from Thompson in Kader, 1992) include night air ventilation, radiant cooling, evaporative cooling, the use of ice and underground (root cellars, field clamps, caves) or high altitude storage. Ice can be manufactured using simple solar cooling systems, where flat plate solar collectors are used to generate power to make ice, which is then used to cool produce (Umar, 1998). Ice can be used either directly as package ice, to cool water for use in a hydro-cooler, or as an ice bank for a small forced air or room cooling system.
Several simple practices are useful for cooling and enhancing storage system efficiency wherever they are used, and especially in developing countries, where energy availability may be limited and any savings may be critical. Shade should be provided over harvested produce, packing areas, for buildings used for cooling and storage and for transport vehicles. Using shade wherever possible will help to reduce the temperatures of incoming produce and will reduce subsequent cooling costs. Trees are a fine source of shade and can reduce ambient temperatures around packinghouses and storage areas. Light colors on buildings will reflect light (and heat) and reduce heat load. Sometimes spending money will save money, as when purchasing lighting equipment. High pressure sodium lights produce less heat and use less energy than incandescent bulbs.
Another aspect to consider when handling fruits and vegetables is the relative humidity of the storage environment. Loss of water from produce is often associated with a loss of quality, as visual changes such as wilting or shriveling and textural changes can take place. If using mechanical refrigeration for cooling, the larger the area of the refrigerator coils, the higher the relative humidity in the cold room will remain. It pays however, to remember that water loss may not always be undesirable, for example if produce is destined for dehydration or canning.
For fresh market produce, any method of increasing the relative humidity of the storage environment (or decreasing the vapor pressure deficit (VPD) between the commodity and its environment) will slow the rate of water loss. The best method of increasing relative humidity is to reduce temperature. Another method is to add moisture to the air around the commodity as mists, sprays, or, at last resort, by wetting the store room floor. Another way is to use vapor barriers such as waxes, polyethylene liners in boxes, coated boxes or a variety of inexpensive and recyclable packaging materials. Any added packaging materials will increase the difficulty of efficient cooling, so vented liners (about 5% of the total area of the liner) are recommended. The liner vents must line up with the package vents to facilitate cooling of the produce inside. Vented liners will decrease VPD without seriously interfering with oxygen, carbon dioxide and ethylene movement.
Room cooling is a relatively low cost but very slow method of cooling when electricity for mechanical refrigeration is available. When using room cooling, produce is simply loaded into a cold room, and cold air is allowed to circulate among the cartons, sacks, bins or bulk load. This cooling method is best suited to less perishable commodities such as potatoes, onions, apples, sweetpotatoes and citrus fruits, since more highly perishable crops will deteriorate too much before being adequately cooled. Room cooling may be all you need if you handle chilling sensitive crops that need to be cooled from early morning harvest temperatures to storage temperatures of 10 to 13 ºC (50-55 ºF). The design and operation of cold rooms are fairly simple and no special equipment is required.
It is important to leave adequate space between stacks of boxes inside the refrigerated room in order for produce to cool more quickly. About 1 inch (2.5 cm) is sufficient to allow cold air to circulate around individual boxes. Produce in vented boxes will cool much faster than produce packed in un-vented containers. In many small-scale cold rooms, produce has been loaded into the room so tightly that cooling cannot take place at all, and despite the high cost of running the refrigeration system, the produce temperature never decreases to recommended levels.
Stacks of produce inside the cold room should be narrow, about one pallet width in depth (two or three cartons). Fans should be installed to move the cold air throughout the room. Air circulating through the room passes over surfaces and through any open space, so cooling from the outside to the center of the stacks is mostly by conduction. You'll want to monitor the temperature of the produce within the packages at various locations in the room to determine that the produce is being cooled as desired. Rearrange the stacks and measure the rate of cooling until you find the right pattern for your cold room.
(See also the USDA Portacooler on page 189)
Forced-air cooling pulls or pushes air through the storage containers themselves, greatly speeding the cooling rate of any type of produce. Many types of forced-air coolers can be designed to move cold moist air over the commodities. The example provided below is a fixed unit, where a fan is housed inside the wall of a cold room.
Cold wall forced-air cooler:
Source: Rij, R. et al. 1979. Handling Precooling and Temperature Management of Cut Flower Crops for Truck Transportation. USDA Science and Education Administration, AAT-W-5, UC Leaflet 21058.
Illustrated below is another view of a cold wall forced air cooler. The canvas sheets must be well sealed, and pallet opening blocked for the tunnel style cooler to function properly.
Source: Gast, K.L.B. and Flores , R. 1991. Precooling produce. Kansas State University Cooperative Extension, Manhattan , Kansas .
A portable forced-air cooler can be constructed using a canvas or polyethylene sheet. The sheet is rolled over the top and down the back of the boxes to the floor, sealing off the unit and forcing air to be pulled through the vents (vent area should be at least 5% of the surface area of the carton) of the cartons stacked against the cooler. This unit is designed to be used inside a refrigerated storage room. The fan unit is shown detached to illustrate how the air should flow within the cooler. For best results and minimum cost of operation, the warm exhaust air from the fan should be directed toward the return air inside the cold room.
A portable forced-air cooler:
Source: Parsons, R.A. and Kasmire, R.F. 1974. Forced-air unit to rapidly cool small lots of packaged produce. University of California Cooperative Extension, OSA #272.
The illustrations below show two types of forced-air coolers used for cooling cut flowers. Each is equipped with a fan to pull air from the cold room through the boxed produce.
Source: Rij, R. et al. 1979. Handling, Precooling and Temperature Management of Cut Flower Crops for Truck Transportation. USDA Science and Education Administration, UC Leaflet 21058.
The illustrations below show the recommended pattern of vents for cartons used to hold produce that is to be forced-air cooled. Vents should make up 5% of the total surface area, and should be located 5 to 7.5 cm (2 to 3 inches) away from the corners. A few large vents (1.3 cm =0.5 inch wide or more) are better than many small vents.
Sources: Thompson, J.F. 2002. Cooling horticultural commodities. pp.97-112. In: Kader, A.A. (ed). Postharvest Technology of Horticultural Crops. Univ. of California , Div. of Agriculture and Natural Resources, Publication 3311.
Mitchell, F.G. et al. 1972. Commercial cooling of fruits and vegetables. California Agricultural Experiment Station Extension Service, Manual 43.
Hydro-cooling provides fast, uniform cooling for some commodities. The commodity as well its packaging materials must be tolerant of wetting, chlorine (used to sanitize the hydro-cooling water) and water beating damage (Mitchell in Kader, 1992).
The simplest version of a hydro-cooler is a tank of cold water in which produce is immersed. The type shown below showers a batch of produce with icy water as the produce moves along a conveyor. A batch-type hydro-cooler can be constructed to hold entire pallet-loads of produce (Thompson in Kader, 2002). Conveyors can be added to help control the time produce stays in contact with the cold water.
These packinghouses are made from natural materials that can be moistened with water. Wetting the walls and roof first thing in the morning creates conditions for evaporative cooling of a packinghouse that is made from straw.
The packinghouse illustrated below is made with walls of wire mesh that hold charcoal. By moistening the charcoal with water each morning, the structure will be evaporatively cooled during the day.
Source: FAO. 1986. Improvement of Post-Harvest Fresh Fruits and Vegetables Handling- A Manual. Bangkok : UNFAO Regional Office for Asia and the Pacific.
Evaporative coolers can be constructed to cool the air in an entire storage structure or just a few containers of produce. These coolers are best suited to lower humidity regions, since the degree of cooling is limited to 1 to 2 C (2 to 4 F) above the wet-bulb temperature. A cooling pad of wood fiber or straw is moistened and air is pulled through the pad using a small fan. In the example provided here, O. 5 gallon of water per minute is dripped onto an 8 square foot pad, providing enough moist air to cool up to 18 crates of produce in 1 to 2 hours. Water is collected in a tray at the base of the unit and re-circulated.
An evaporative cooler can be combined with a forced air cooler for small lots of produce. Air is cooled by passing through the wet pad before it passes through the packages and around the produce. The air can be cooled to within a few degrees of the wet bulb temperature of ambient air.
Evaporative forced-air cooler:
Source: Thompson, J. F. and Kasmire, R.F. 1981. An evaporative cooler for vegetable crops. California Agriculture , March-April: 20-21.
Source: Mitchell in Kader, 1992. Postharvest Technology of Horticultural Crops. University of California , Division of Agriculture and Natural Resources, Publication 3311. 296 pp.
The evaporative cooler shown below is equipped with a vortex wind machine. Chicken wire was used to construct two thin boxes on opposite sides of the cooler that hold wet chunks of charcoal or straw. Water is dripped onto the charcoal or straw, and the wind turns the turbine, sucking moist, cool air through the load of produce inside the cooler. When using this cooler, temperatures are reduced to 3 to 5 °C (6 to 10 °F) below ambient air temperature, while relative humidity is about 85 %.
Source: Redulla , C.A. et al. 1984. Temperature and relative humidity in two types of evaporative coolers. Postharvest Research Notes, 1(1): 25-28.
Evaporative coolers can be constructed from simple materials, such as burlap and bamboo. The "drip cooler" shown here operates solely through the process of evaporation, without the use of a fan. Cooling will be enhanced if the unit is kept shaded and used in a well ventilated area.
Source: Redulla , C.A. et al. 1984. Keeping perishables without refrigeration: use of a drip cooler. Appropriate Postharvest Technology 1(2): 13-15.
Two simple evaporative coolers have been developed and used in the Philippines for cooling and storage of vegetables (such as tomatoes, sweet peppers and mustard greens). Type a, Illustrated below, is standing in a galvanized iron (GI) pan of water, and has another pan of water on top. The sides and top are covered with jute sacks kept wet by dipping their top and bottom edges into the pans of water. In type b, the inner side walls are constructed from plain GI sheet with fine holes (spaced at 5 x 5cm) while the outer walls are made of fine mesh (0.32 cm) wire. The 1.5 cm space between the inner and outer walls is filled with rice hulls, kept wet by contact with a cloth that is dipping into the pan of water placed on top of the cooler.
Produce stored in these coolers has a longer shelf life than produce kept at ambient conditions. Tomatoes and peppers lost less weight and ripened more slowly, and could be kept for as long as they typically can be stored under refrigeration (about 3 weeks). Decay can be a problem, be can be controlled by washing in chlorinated water prior to cooling. Mustard greens lost much less weight and showed little wilting for up to 5 days.
Source: Acedo, A.L. 1997. Storage life of vegetables in simple evaporative coolers. Tropical Science 37: 169-175.
Acedo, A. 1997. Ripening and disease control during evaporative cooling storage of tomatoes. Tropical Science 37: 209-213.
An evaporative cooler located in the peak of a storage structure can cool an entire room of stored produce such as sweetpotatoes or other chilling sensitive crops. The vents for outside air should be located at the base of the building so that cool air is circulated throughout the room before it can exit.
Source: Thompson, J.F. and Scheuerman, R.W. 1993. Curing and Storing California Sweetpotatoes. Merced County Cooperative Extension, Merced , California 95340
The low cost cooling chamber illustrated below is constructed from bricks. The cavity between the walls is filled with sand and the bricks and sand are kept saturated with water. Fruits and vegetables are loaded inside, and the entire chamber is covered with a rush mat, which is also kept moist. Since a relatively large amount of materials are required to construct this cold storage chamber, it may be useful only when handling high value products.
During the hot summer months in India , this chamber is reported to maintain an inside temperature between 15 and 18 °C (59 and 65 °F) and a relative humidity of about 95%.
Improved Zero-Energy Cool Chamber:
Source: Roy S.K. 1989. Postharvest technology of vegetable crops in India . Indian Horticulture. Jan-June: 7678.
Storage structures can be cooled using night air if the difference in day and night temperature is relatively large (Thompson in Kader, 2002). The storage facility should be well insulated and vents should be located at ground level. Vents can be opened at night, and fans can be used to pull cool air through the storeroom. The structure will best maintain cool temperatures during the heat of the day if it is well insulated and vents are closed early in the morning.