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Section 6: Temperature and relative humidity control


Room cooling
Forced-air cooling
Hydro-cooling
Evaporative cooling
Night air ventilation
Chilling injury
Use of ice
Alternative methods of cooling
Increasing relative humidity

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 the parent plant. Keeping products at their lowest safe temperature (0 C or 32 F for temperate crops or 1012 C or 50-54 F for chilling sensitive crops) will increase storage life by lowering respiration rate, decreasing sensitivity to ethylene and reducing water loss. Reducing the rate of water loss slows the rate of shriveling and wilting, causes of serious postharvest losses. 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).

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.

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.

Several simple practices are useful for cooling and enhancing storage system efficiency wherever they are used, and especially in developing countries, where energy 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. 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 shrivelling 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

Room cooling is a relatively low cost but slow method of cooling when electricity for mechanical refrigeration is available. The greater the refrigerator's evaporator coil area, the less moisture will be lost from the product as it cools.

It is important to leave adequate space between the stacks of boxes inside the refrigerated room in order for produce to cool more quickly. Stacks should be narrow, about one pallet width in depth. 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. (See Mitchell in Kader, 1992 for more information).

Source: Kasmire, R.F. 1977. California Tomatorama. Fresh Market Tomato Advisory Board Information Bulletin No. 17.

Low cost cold rooms can be constructed using concrete for floors and polyurethane foam as an insulator. Building the storeroom in the shape of a cube will reduce the surface area per unit volume of storage space, thereby reducing construction and refrigeration costs. All joints should be calked and the door should have a rubber seal.

Source: Tugwell, B.L. No date. Coolroom construction for the fruit and vegetable grower. Department of Agriculture and Fisheries, South Australia. Special Bulletin 11.75.

Forced-air cooling

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.

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.

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. 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 illustration below shows 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.

Source: Mitchell, F.G. et al. 1972. Commercial cooling of fruits and vegetables. California Agricultural Experiment Station Extension Service, Manual 43.

Hydro-cooling

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 palletloads of produce (Thompson in Kader, 1992). Conveyors can be added to help control the time produce stays in contact with the cold water.

Hydro-cooling:

Batch-type hydro-cooler:

Source: Kasmire, R.F. 1977. California Tomatorama. Fresh Market Tomato Advisory Board Information Bulletin No. 17.

Evaporative cooling

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.

Straw packinghouse:

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.

Straw packinghouse:

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 recirculated.

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.

Drip cooler:

Source: Redulla, C.A. et al. 1984. Keeping perishables without refrigeration: use of a drip cooler. Appropriate Postharvest Technology 1(2): 13-15.

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.

Night air ventilation

Storage structures can be cooled using night air if the difference in day and night temperature is relatively large (Thompson in Kader, 1992). 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.

Vents open:

Vents closed:

Chilling injury

Fruit and vegetable crops often are susceptible to chilling injury when cooled below 13 to 16 C (55 to 60 F). Chilling injury reduces the quality of the product and shortens shelf life. The table below provides some examples of the symptoms of chilling injury in a variety of crops. Symptoms often appear only after the commodity is returned to warmer temperatures, as when marketed.

Fruits and vegetables susceptible to chilling injury when stored at moderately low but nonfreezing temperatures

Commodity

Approximate lowest safe temperature

Character of injury when stored between 0°C and safe temperature1

°C

°F

Apples-certain cultivars

22-3

36-38

Internal browning, brown core, soggy breakdown, soft scald

Asparagus

0-2

32-36

Dull, gray-green, and limp tips

Avocados

24.5-13

40-55

Grayish-brown discoloration of flesh

Bananas, green or ripe

²11.5-13

53-56

Dull color when ripened

Beans (lima)

1-4.5

34-40

Rusty brown specks, spots, or areas

Beans (snap)

²7

45

Pitting and russeting

Cranberries

2

36

Rubbery texture, red flesh

Cucumbers

7

45

Pitting, water-soaked spots, decay

Eggplants

7

45

Surface scald, alternaria rot, blackening of seeds

Guavas

²4.5

40

Pulp injury, decay

Grapefruit

210

50

Scald, pitting, watery breakdown

Jicama

13-18

55-65

Surface decay, discoloration

Lemons

²11-13

52-55

Pitting, membranous staining, red blotch

Limes

7-9

45-48

Pitting, turning tan with time

Mangos

²10-13

50-55

Grayish scaldlike discoloration of skin, uneven ripening

Melons


Cantaloups

22-5

36-41

Pitting, surface decay


Honey Dew

7-10

45-50

Reddish-tan discoloration, pitting, surface decay, failure to ripen


Casaba

7-10

45-50

Same as above but no discoloration


Crenshaw and Persian

7-10

45-50

Same as above but no discoloration


Watermelons

4.5

40

Pitting, objectionable flavor

Okra

7

45

Discoloration, water-soaked areas, pitting, decay

Olives, fresh

7

45

Internal browning

Oranges, California and Arizona

²3

38

Pitting, brown stain

Papayas

7

45

Pitting, failure to ripen, off flavor, decay

Peppers, sweet

7

45

Sheet pitting, alternaria rot on pods and calyxes, darkening of seed

Pineapples

27-10

45-50

Dull green when ripened

Pomegranates

4.5

40

Pitting, external and internal browning

Potatoes

3

38

Mahogany browning (Chippewa and Sebago), sweetening²

Pumpkins and hardshell squashes

10

50

Decay, especially alternaria rot

Sweetpotatoes

13

55

Decay, pitting, internal discoloration; hardcore when cooked

Tamarillos

3-4

37-40

Surface pitting, discoloration

Tomatoes


Ripe

27-10

45-50

Watersoaking and softening, decay


Mature-green

13

55

Poor color when ripe, alternaria rot

1 Symptoms often apparent only after removal to warm temperatures, as In marketing.

Source: Harderburg, R.E., A. E. Watada, and C-Y. Wang 1986. The Commercial Storage of Fruits Vegetables. and Florist and Nursery Stocks. USDA, Agricultural Handbook No. 66.

Use of ice

Ice can be used as a bunker source of refrigeration (used by passing air through a bank of ice and then through the commodity) or as top ice (laid directly in contact with the product). Ice can cool a commodity only if it melts, so good ventilation is necessary for effective cooling.

Longisection - a gasoline or diesel engine MUST be mounted outside

Back of Room - an electric fan motor is commonly mounted inside the cold room, fan capacity (CFM= cubic feet/min) should at least equal the empty volume of-the room (i.e. 12x8x8=768 cu ft., so, 768 CFM in minimal. more is better. for this room)

Front elevation

Top view - vanes over the sub-ceiling greatly improve air distribution and hence cooling

Source: Grierson, W. 1987. Postharvest Handling Manual Commercialization of Alternative Handling Crops Project. The Belize Agribusiness Company./Chenomics International/USAID.

Crushed or flaked ice for package icing can be applied directly or as a slurry in water. The use of ice to cool produce provides a high relative humidity environment around the product. Package ice can be used only with water tolerant, non-chilling sensitive products (such as: carrots, sweet corn, cantaloupes, escarole, lettuce, spinach, radishes, broccoli, green onions). and with water tolerant packages (waxed fiberboard, plastic or wood).

Top ice is used for certain products during transport to help maintain a high relative humidity. Top ice can be used only with water tolerant, non-chilling sensitive products (such as: carrots, sweet corn, cantaloupes, escarole, lettuce, spinach, radishes, broccoli, green onions), and with water tolerant packages (waxed fiberboard, or wood).

Top-ice on loads should be applied in rows rather than a solid mass. It is important not to block air circulation inside the transport vehicle.

Should be Top-iced:

Can be Top-iced:

beets with tops

artichokes, globe

broccoli

beet greens

carrots with tops

beets topped

corn sweet

brussels sprouts

endive

cantaloupes

escarole

carrots, topped

green onions

celeriac

parsley

chard

radishes with tops

kohlrabi

turnips with tops

leeks

watercress

mustard greens

radish greens

parsnips

spinach

radishes

turnip greens

rutabagas

turnips


Sources: Thompson, J.F. 1992. Storage Systems, pp. 69 - 78. In: Kader, A.A. (ed). Postharvest Technology of Horticultural Crops. Univ. of California, Div. of Agriculture and Natural Resources, Publication 3311.

McGregor, B.M. 1989. Tropical Products Transport Handbook. USDA, Office of Transportation, Agricultural Handbook Number 668.

Alternative methods of cooling

Radiant Cooling

Radiant cooling can be used to lower the air temperature in a storage structure if a solar collector is connected to the ventilation system of the building. By using the solar collector at night, heat will be lost to the environment. Temperatures inside the structure of 4 C (about 8 F) less than night temperature can be achieved.

Use of Well Water

Well water is often much cooler than air temperature in most regions of the world. The water temperature of a deep well tends to be in the same range as the average air temperature of the same locality. Well water can be used for hydro-cooling and as a spray or mist to maintain high relative humidity in the storage environment.

High Altitude Storage

As a rule of thumb, air temperatures decrease by 10 C (18 F) for every one kilometer increase in altitude. If handlers have an option to pack and/or store commodities at higher altitude, costs could be reduced. Cooling and storage facilities operated at high altitude would require less energy than those at sea level for the same results.

Sources: Thompson, J.F. 1992. Storage Systems, pp. 69 - 78. In: Kader, A.A. (ed). Postharvest Technology of Horticultural Crops. Univ. of California, Div. of Agriculture and Natural Resources, Publication 3311.

Increasing relative humidity

Refrigerated air tends to be lower in relative humidity than is beneficial for storage of most horticultural crops. The simplest method of increasing relative humidity of the storage air is to wet the floor of the room or mist the storage containers with cold water and allow the water to evaporate.

For a more permanent system of high relative humidity in the storage environment, moisture can be added to the refrigerated air. A fan draws air past the refrigerator's evaporator coils (R) then past wet moss or straw (M). The moist air is then pulled into the store-room through a perforated wall (P).

Wet moss as a moisture source inside a refrigerated storeroom:

Source: Lopez, E.G. 1983. Conservación de la Producción Agrícola. Barcelona Editorial Aedos. 188 pp.

Using a polyethylene liner in a fiberboard carton can help protect produce and reduce water loss in commodities such as cherries, nectarines, kiwifruits, bananas and herbs. Water vapor given off by the product is contained within the inner, increasing the RH around the product. The liner can also reduce abrasion damage that results from fruit rubbing against the inside of the box.


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