Precooling Fruits and Vegetables in Georgia

Precooling Fruits and Vegetables in GeorgiaChangying “Charlie” Li

Extension Agriculture Engineer

Fruits and vegetables begin to deteriorate after they are harvested and separated from their growing

environment. The rate of deterioration defines how long they will be acceptable for consumption. This is known as “shelf life.” To preserve the quality of fruits and vegetables and maximize profits for growers, it is critical to control the temperature of fresh produce and minimize the amount of time that products are exposed to detrimental temperatures.

Both temperature and relative humidity are important during the postharvest handling of fruits and vegeta-bles. Heat, in particular, decreases produce quality and reduces market shelf life. Heat damage can come from two interrelated sources: the field’s temperature at har-vest and the produce’s natural respiration. High field temperatures raise product temperatures; therefore, it is important to cool produce as rapidly as possible to avoid tissue damage. Some products that are sensitive to temperature abuse can experience excessive weight loss when field temperatures are too high. Some grow-ers harvest their products at night to avoid excessive daytime heat. The second source of heat comes from natural respiration. Fruits and vegetables are still alive after they are harvested and they react with oxygen to form carbon dioxide, water and heat. Although this “heat of respiration” varies with different fruits and vegetables, in general as product temperature in-creases, respiration and heat generation also increase, shortening the shelf life. Heat generation may be expressed as British thermal units (Btu). Typical rates of heat respiration for different fruits and vegetables at various temperatures can be found in Table 1.

Relative humidity also affects the quality and shelf life of fruits and vegetables. Moisture loss is increased by low relative humidity and is a major cause of deterio-ration. Fruits and vegetables contain 80 to 85 percent water. The relative humidity (RH) of the intercellular spaces of fruits and vegetables is approximately 99 percent. If the air surrounding the product has humid-ity less than 99 percent, moisture will move out of the plant tissue into the air. Keeping the humidity high in the storage environment is the best method to reduce moisture loss. Waxing, trimming and packing produce in plastic bags can also reduce moisture loss. Recom-mended storage temperatures, relative humidity, stor-age life, freezing points and specific heat guidelines for various fruits and vegetables can be found in Table 2.

Several precooling methods can be used to reduce field heat and heat of respiration. Current practices include room cooling, forced-air cooling, hydrocooling, package-icing and vacuum cooling. In this publication, precooling is defined as a cooling process that quickly removes heat from products after harvest and before storage or shipping. The terms “precooling” and “cool-ing” are used interchangeably.

Precooling Methods

Room coolingRoom cooling is a common and simple precooling method that exposes produce to cold air in a refriger-ated room. Room cooling is usually used for products that have a relatively long storage life, such as sweet potatoes, apples and pears. These products are cooled

Precooling Fruits and Vegetables UGA Cooperative Extension Circular 10042

and stored in the same room. In general, a simple and effective arrangement is to discharge cold air into a cooling room horizontally just below the ceiling. The air sweeps the ceiling and returns to the cool-ing coils after circulating through the produce on the floor. There should be enough refrigerated air volume to provide adequate cooling. The air velocity should be kept between 200 and 400 feet per minute around and between cooling containers. When cooling is complete, air velocity should be reduced to the lowest level that will keep produce cool – usually 10 to 20 feet per minute.

One benefit of room cooling is that both the cooling and storage can be done in the same room and the pro-duce does not need to be re-handled. In addition, room cooling requires a lower refrigeration load than other, faster cooling methods, as explained later.

However, room cooling has several major disadvan-tages that may limit its use. First, at 20 to 100 hours to cool the product to the seven-eighths cooling tempera-ture (as explained later), room cooling is too slow for most commodities, particularly with containers that have minimal open air spaces. Second, it requires a relatively large empty floor space between stacked containers to achieve an optimal cooling effect. Third, it may cause serious water loss for fresh produce due to high air velocities (although air velocity in room cooling should be lower than in forced-air cooling). Fourth, it is difficult to maintain control of the cooling process. Produce in newer, more-closed containers (or in containers tightly stacked on pallets) is particularly hard to cool through room cooling. Because of these limitations, the produce industry is increasing the use of faster cooling methods to protect more perishable produce and to facilitate shipping soon after harvest. Forced-air coolingForced-air cooling is the most widely used precool-ing method in commercial practice. It is particularly popular among small operations because of its ability to handle a wide variety of products. It can rapidly air-cool produce by creating an air pressure difference on opposite faces of stacks of vented containers (Figure 2). This pressure difference forces air through the stacks and carries heat away.

Forced-air cooling has several advantages over room cooling. For instance, forced-air cooling is much faster

than room cooling because the cold air generally cools the produce by flowing around the individual fruits or vegetables in the containers. Forced-air cooling usu-ally cools fresh produce in one to ten hours, which is one-tenth the time needed for room cooling. Second, adjusting the volume of air can control the cool-ing speed. Rapid cooling can be accomplished with adequate refrigeration and a large volume of airflow per unit of produce. Third, an existing room cooling system can be converted to forced-air, which could reduce capital costs if enough refrigeration capacity is available from the existing room cooling system.

One drawback of forced-air cooling is that it can cause water loss from the fresh produce due to air movement unless humidity is kept near 100 percent. To reduce water loss, fresh produce should be cooled as quickly as possible after harvest using high airflow rates. For a forced-air system to work effectively, at least 4 percent of the carton area should be vented to allow airflow. Vents should be vertical slots at least ½-inch wide that extend to within 1 1/2 inches of the top and bottom of container.

Various stacking arrangements can be made for a forced-air cooler. In tunnel-type forced-air cooling, the product is stacked in two rows far enough apart to accommodate the fans with the tarp covering the gap between the rows (both on top of the rows and at the end away from the fan) as shown in Figures 1 and 2. When the product is adequately cooled, the fan should be turned off and the tarp should be rolled up. Other forced-air cooling arrangements include cold wall, serpentine cooling and evaporative forced-air cooling.

Figure 1. A typical commercial tunnel-type forced-air cool-ing system in Georgia (Courtesy of Lewis Taylor Farm).

UGA Cooperative Extension Circular 1004 Precooling Fruits and Vegetables3

Figure 2. Schematic diagram of airflow in forced-air cooling. Proper placement of containers and use of baffles blocks air return everywhere except through side vents in containers. Thus, air is forced to pass through containers and around produce to return to exhaust fans. As air is exhausted from the center chamber, a slight pressure drop occurs across the produce.

Hydrocooling Hydrocooling is one of the fastest precooling methods. Fruits and vegetables can be cooled rapidly by bring-ing them in contact with cold moving water (Figure 3). One main advantage of hydrocooling is that it does not remove water from the produce and may even revive slightly wilted produce. For efficient hydrocooling, water should come in contact with as much of the sur-face of each fruit or vegetable as possible. Water also must be kept as cold as possible without endangering produce. In commercial practices, water temperature is usually kept around 31°F except for chilling sensitive commodities.

Conveyor hydrocoolers are the most common. Pro-duce in bulk or in containers is carried on a conveyor through a shower of water. To avoid “channeling” (water pouring through larger openings where there is less resistance), it is necessary to either use a heavy shower over a shallow depth of produce or proportion the shower and the drainage from the bottom of con-tainers so that the containers fill partly or entirely with water. Drainage must be sufficient to keep the water in the containers moving and to remove all water before containers leave the hydrocooler.

To achieve optimal cooling and save energy, hydro-coolers should be insulated. Tests have showed that less than half of the refrigeration used in most con-veyor-type hydrocoolers was lost due to insufficient insulation.

Despite advantages such as a fast cooling rate and no water loss in fresh produce, hydrocooling does have disadvantages. First, both the product to be cooled and the containers must withstand wetness and chemicals such as chlorine. Second, due to limited capacity, some arriving produce may have to wait in a warmer environment if the cooler has reached its maximum capacity (this limitation can be mitigated by placing the hycrocooling facility inside a cold room). Third, re-handling cooled products is usually needed for either immediate shipping or transferring the produce to a cold storage room. Although there are some food safety concerns related to hydrocooling, properly us-ing active chlorine or ozone to disinfect the water used in the process can reduce the potential risk of spread-ing any contamination.

Figure 3. A commercial hydrocooler in Georgia (Courtesy of Lewis Taylor Farm).

Package-icingPacking crushed ice in containers with produce is one of the oldest and fastest cooling methods, and is particularly useful for cooling field-packed vegetables such as broccoli. It offers the advantage of fast cooling when the product directly contacts the ice, although the cooling rate could be significantly reduced when the ice melts. Another advantage is that the excess ice on the top of the product provides cooling during and after transportation.


It should be noted that there are several limitations to package-icing. First, the product must be tolerant of the wet condition at 32°F for a prolonged time. Sec-ond, the container should also be able to withstand wet conditions. Third, since the typical weight of the ice for initial cooling is equivalent to 30 percent of the product weight, this can increase the freight load significantly. Finally, the water from the melted ice could be a potential source of contamination (chlorine is usually added to the ice to address this issue).

Figure 4. A slush ice type of package-icing cooler in Georgia (Courtesy of Lewis Taylor Farm).

Figure 5. A “clamshell” type of package-icing cooler used for broccoli in Georgia (Courtesy of Lewis Taylor Farm).

Vacuum coolingVacuum cooling cools fresh produce based on the principle of evaporation cooling: The moisture evapo-rates and takes heat away from the fresh produce when

the atmospheric pressure is reduced below the boiling temperature of water. Leafy vegetables with a large surface area to mass ratio (such as iceberg lettuce) are well suited for this cooling method and can be cooled on a large scale by putting them in air-tight chambers and pumping out air and water vapor using steam-jet pumps. This method can cool packed produce quickly and uniformly in large loads (usually in 20 minutes to two hours), but container walls or other barriers that slow down evaporation can seriously inhibit cooling.

Like any other cooling method, vacuum cooling also has its limitations. One major disadvantage is that it can create weight loss from the product due to evapo-rated water. It is estimated that weight loss can be as high as 1 percent of the product weight for every 11°F, which is observable for some fresh produce. One method to overcome this drawback is to add water to the surface of the product using a spray system during the vacuum cooling process. However, it should be noted that the water used must be disinfected to avoid any food safety concerns.

Vacuum-cooling equipment is expensive and requires skilled operators. To be economically feasible, there must be a large daily and annual output of cooled pro-duce. It is best if the vacuum cooler is either located close to a long-season production area or is portable so it can be moved to locations where there is such production.

Figure 6. Vacuum cooler (Courtesy of Paul Sumner)

Cooling Method Selection CriteriaThe five cooling methods described in this publica-tion have their advantages and disadvantages. Some of the methods may not be suitable for certain fruits and vegetables due to physiological constraints. For in-


stance, some fruits and vegetables (such as berries and mushrooms) are prone to diseases under wet condi-tions. Table 3 lists recommended cooling methods for typical fresh products. In addition to the compatibility issue, product temperature requirements, cooling sys-tem costs and refrigeration capacity are several major factors that growers need to consider when planning to build a precooling facility.

The product temperature requirement is the most important issue to consider when building a precool-ing facility. Temperature requirements for various fruits and vegetables are shown in Table 2. If a grower handles multiple products with different optimal stor-age temperatures, it is usually difficult to use just one cooler. For instance, one large vegetable shipper in Georgia has four different precooling systems in his packing shed. It is particularly important that chill-sensitive commodities not be stored below the critical threshold temperatures or damage may result.

Cost of the cooling system, including capital, energy, labor and other equipment, is another important factor to consider. Liquid ice coolers are the most expensive, followed by vacuum coolers, forced-air coolers, hy-drocoolers and room coolers. However, other consid-erations should be factored in as well. For instance, a vacuum cooler is portable and can be moved to differ-ent production areas, which increases the frequency of its usage and consequently reduces its capital costs per unit cooled. Energy costs can also differ significantly. The vacuum cooler is the most energy-intensive cool-ing method, followed by the hydrocooler, water spray vacuum cooler, package-icing cooler and forced-air cooler. Labor and other equipment costs should also be considered when comparing different cooling sys-tems, especially if special packaging (e.g., waxed box or RPC box) is required.

Refrigeration capacity estimation is important for se-lecting the right cooler. Several factors need to be con-sidered, such as the heat load (the amount of product to be cooled), the initial temperature of the product, the rate of cooling, the insulation condition of the cold room and other heat sources generated from electri-cal components in the cooling facility (motors, lights, people, etc.). A detailed example of how to calculate refrigeration load is provided in the following section.

Refrigeration Load CalculationTable 2 lists optimal temperatures for fruits and veg-etables. In most commercial practices, however, prod-ucts are rarely cooled to the optimal temperature due to the relatively high costs and longer precooling time. Instead, for most commercial practices, products are cooled to seven-eighths cooling temperature or half-cooling temperature and then moved from the cooler to a cold room for further cooling.

The seven-eighths cooling temperature is the tempera-ture that is seven-eighths of the temperature differ-ence between the product and the coolant. The seven-eighths cooling time is the time required to cool the product to the seven-eighths cooling temperature. Sim-ilarly, the half-cooling time is the time required to cool the product to reach the half-cooling temperature. For example, if cabbage is harvested at 90° F and cooled in a forced-air cooler with a cooling air temperature of 30° F, the seven-eighths cooling temperature is 37.5° F and the half-cooling temperature is 60° F.

As a rule of thumb, the seven-eighths cooling time is usually three times the half-cooling time. The seven-eighths cooling time is generally used in commercial precooling practices because it is constant for a given product cooled by a specific cooling method no matter what temperatures the product and coolant are.

Example To properly design a cooling system and select the appropriate cooling equipment, it is important to know the refrigeration load of the product to be cooled. Be-low is an example of how to calculate the refrigeration load for Chinese cabbage using the forced-air cooling method (the most widely used precooling method). In this example, we assume that there are two batches of Chinese cabbage loaded in the cooling facility dur-ing the afternoon (between 1:00 p.m. and 5:00 p.m.). The first batch of cabbage is loaded at 1:00 p.m. with an initial temperature of 86° F. The second batch of cabbage is loaded at 3:00 p.m. with an initial tempera-ture of 90° F. Both batches weigh 8,000 pounds. We assume that the seven-eighths cooling time for Chi-nese cabbage is three hours. The cooling air tempera-ture is 31° F. The specific heat of the Chinese cabbage is 0.96 Btu/lb/°F. The refrigeration capacity of the coolant is 12,000 Btuh/ton.


The refrigeration load of the forced-air cooling system can be calculated by the following equation:

Where:

RT = the refrigeration capacity (tons).

∆T = the temperature drop (°F)/hour during the cool-ing period. In this case, since the seven-eighths cool-ing time for Chinese cabbage is three hours and the half-cooling time equals one third of the seven-eighths cooling time, the temperature drop ∆T within one hour is the half-cooling temperature (i.e., the initial tem-perature difference between the cold air (31° F) and the product (86° F) is (86-31)/2 = 27.5° F).

m = the weight of the product (in pounds). In this case, m = 8,000 pounds.

cp = the heat capacity of the product (Btu/lb/°F). For Chinese cabbage, cp is 0.96 Btu/lb/°F (data for other products can be found in Table 2).

k = the refrigeration capacity (Btuh/ton) of the cool-ant. In this case, k=12,000 Btuh/ton.

As calculated in Table 4, the peak refrigeration load is 23.3 tons at 3:00 p.m.

In practice, the total refrigeration load is calculated by the following equation:

RTP=RTT × (1 + Q1 + Q2)

Where:

RTP = the practical refrigeration load (tons).

RTT = the theoretical peak refrigeration load (tons).

Q1 = the coefficient to include other heat loads in the cooling facility, such as the motor, lights, people and heat infiltration from outside. It is usually set at 0.25.

Q2 = the safety coefficient to account for other un-expected heat loads such as cooling unusually warm products. It is usually set at 0.15.

In this example, the final practical refrigeration load (RTP) is 32.6 tons.

SummaryPrecooling is one of the most important procedures used to maintain the quality of fruits and vegetables after harvest and before storage or shipping. There are five major precooling methods practiced in Geor-gia: room cooling, forced-air cooling, hydrocooling, package-icing and vacuum cooling. Each method has its advantages and disadvantages. Selecting the right precooling method for the product depends on several factors, such as the suitability of the cooling method to the specific product, temperature requirement of the product, cooling rate, cost and refrigeration load.

(cp×∆T×m)kRT=

Table 4. Peak refrigeration load calculation example for Chinese cabbage in a forced-air cooling system.

TimeProduct

load (lbs.)Product temperature

(°F)Temperature drop ∆T

(°F/h)Refrigeration load for

each batch (tons)Total load

(tons)

1:00 p.m. 3:00 p.m. 1:00 p.m. 3:00 p.m. 1:00 p.m. 3:00 p.m.

1:00 p.m. 8,000 86 27.5 17.6 17.6

2:00 p.m. 58.5 13.8 8.8 8.8

3:00 p.m. 8,000 44.8 90 6.9 29.5 4.4 18.9 23.3

4:00 p.m. 37.9 60.5 14.8 9.4 9.4

5:00 p.m. 45.8 7.4 4.7 4.7

6:00 p.m. 38.4


The optimal temperatures of the fruits and vegetables listed in Table 2 provide guidelines for growers, but are rarely achieved during the precooling process in most commercial practices due to the high costs and longer cooling time required. In practice, seven-eighths cooling temperature or half-cooling tempera-ture is used. Care should be taken when cooling chill-sensitive products to avoid chilling injury.

References Boyette, M.D., et al. AG-413-8, Postharvest Cooling and Handling of Green Beans and Field Peas. North Carolina State University.

Boyette, M.D. et al. AG-414-3, Forced-air Cooling. North Carolina State University.

Boyette, M.D. et al. AG-414-4, Hydrocooling. North Carolina State University.

USDA ARS. The Commercial Storage of Fruits, Veg-etables, and Florist and Nursery Stocks. Handbook 66, 2004.

J. F. Thompson, F. G. Mitchell, and R. F. Kasmire. 2002. Cooling Horticultural Commodities. In Post-harvest Technology of Horticulture Crops. Edited by Kader, A. A. 2002. Davis, Postharvest Technology Research & Information Center, University of Califor-nia Davis. Publication 3311.

M. T. Talbot, S. A. Sargent, and J. K. Brecht. 2002. Cooling Florida Sweet Corn. University of Florida In-stitute of Food and Agricultural Sciences Cooperative Extension Service.

J. F. Thompson, F. G. Mitchell, T. R. Rumsey, R.F. Kasmire, C. H. Crisosto. 2008. Commercial Cooling of Fruits, vegetables, and flowers. Revised Edition. University of California Agriculture and Natural Re-sources. Publication 21567.

Paul E. Sumner. 1987. Commercial Cooling of Geor-gia Fruits and Vegetables. University of Georgia College of Agricultural and Environmental Sciences. Cooperative Extension Bulletin 972.


Table 1: Summary of respiration rates for fresh fruits and vegetables when stored at various temperatures.

Commodity

32°F 41°F 50°F 59°F 68°F 77°F

Btu per ton per day at indicated temperature

Apples, fall 660 1320 1980 3300 4400 nd1

Apricot 1320 nd 3520 nd 8800 nd

Beans, snap 4400 7480 12760 20240 28600 nd

Blackberry 4180 7920 13640 16500 25300 nd

Blueberry 1320 2420 6380 10560 15400 22220

Broccoli 4620 7480 17820 37400 66000 nd

Cabbage 1100 2420 3960 6160 9240 13640

Carrot (topped) 3300 4400 6820 8800 5500 nd

Cauliflower 3740 4620 7480 10120 17380 20240

Cucumber nd nd 5720 6380 6820 8140

Eggplant

American nd nd nd 15180 2 nd nd

Japanese nd nd nd 28820 2 nd nd

White egg nd nd nd 24860 2 nd nd

Fig 1320 2860 4620 nd 11000 nd

Grape, American 660 1100 1760 3520 7260 8580

Grape, muscadine 2200 3 2860 nd nd 11220 nd

Grape, table 660 1540 2860 nd 5940 nd

Honeydew melon nd 1760 3080 5280 6600 7260

Leek 3300 5500 13200 21120 24200 25300

Lettuce

Head 2640 3740 6820 8580 12320 18040

Leaf 5060 6600 8580 13860 22220 32340

Nectarine (ripe) 1100 nd 4400 nd 19140 nd

Cantaloupe 1320 2200 3300 8140 12100 14740

Okra 4620 3 8800 20020 32120 57420 75900

Onion 660 1100 1540 1540 1760 nd

Pea

Garden 8360 14080 18920 38500 59620 68860

Edible pod 8580 14080 19580 38720 60060 nd

Peach (ripe) 1100 nd 4400 nd 19140 nd

Pepper nd 1540 2640 5940 7480 nd

Persimmon 1320 nd nd nd 4840 nd

Plum (ripe) 660 nd 2200 nd 4400 nd

Prickly pear nd nd nd nd 7040 nd

Radish

Topped 3520 4400 7480 16280 28600 37840

Bunched with tops 1320 2200 3520 7040 11220 16500

Raspberry 3740 3 5060 7700 9240 27500 nd

Southern pea

Whole pods 5280 3 5500 nd nd 32560 nd

Shelled peas 6380 3 nd nd nd 27720 nd


Spinach 4620 9900 24200 39380 50600 nd

Squash, summer 5500 7040 14740 33660 36080 nd

Squash, winter nd nd 21780 2 nd nd nd

Strawberry 3520 nd 16500 nd 33000 nd

Sweet corn 9020 13860 23100 34980 57420 78980

Tomato nd nd 3300 4840 7700 9460

Turnip root 1760 2200 3520 5060 5500 nd

Watermelon nd 880 1760 nd 4620 nd1 nd = Not determined. 2 At 55° F. 3 At 36° F. NOTE: Reprinted from “The Commercial Storage of Fruits, Vegetables, and Florist and Nursery Stocks,” Agricultural Handbook 66, USDA, 2004.

Table 2: Summary of optimal handling conditions for fresh fruits and vegetables.

Common name

Storage temperature

°F

Relative humidity

%Highest freezing temperature °F

Specific heat1

Btu/lb/°FApproximate storage life

Apple 40 2 90-95 29.3 0.87 1-2 months

Apricot 31-32 90-95 30 0.88 1-3 weeks

Beans

Lima 41-43 95 31 0.73 5-7 days

Snap, wax, green 40-45 95 30.7 0.91 7-10 days

Berries

Blackberry 31-32 90-95 30.6 0.88 3-6 days

Blueberry 31-32 90-95 29.7 0.86 10-18 days

Elderberry 31-32 90-95 30.0 0.84 5-14 days

Strawberry 32 90-95 30.6 0.92 7-10 days

Broccoli 32 95-100 31.0 0.92 10-14 days

Cabbage

Chinese, Napa 32 95-100 30.4 0.96 2-3 months

common, early crop 32 98-100 30.4 0.94 3-6 weeks

late crop 32 95-100 30.4 0.94 5-6 months

Carrots

topped 32 98-100 29.5 0.91 6-8 months

bunched, immature 32 98-100 29.5 0.91 10-14 days

Cauliflower 32 95-98 30.6 0.93 3-4 weeks

Collards 32 95-100 31.1 0.90 10-14 days

Corn, sweet 32 95-98 31.0 0.79 5-8 days

Cucumber 50-54 85-90 31.1 0.97 10-14 days

Eggplant 50-54 90-95 30.6 0.94 1-2 weeks

Fig, fresh 31-32 85-90 27.6 0.82 7-10 days

Kale 32 95-100 31.1 0.89 10-14 days

Leek 32 95-100 30.7 0.88 2 months

Lettuce 32 98-100 31.7 0.96 2-3 weeks


Common name

Storage temperature

°F

Relative humidity

%Highest freezing temperature °F

Specific heat1

Btu/lb/°FApproximate storage life

Melons

Cantaloupes and other netted melons

36-41 95 29.9 0.94 2-3 weeks

Honeydew 41-50 85-90 30.1 0.94 3-4 weeks

Nectarine 31-32 90-95 30.3 0.85 2-4 weeks

Okra 45-50 90-95 28.7 0.92 7-10 days

Onions

Mature bulbs, dry 32 65-70 30.6 0.90 1-8 months

Green 32 95-100 30.4 0.91 3 weeks

Parsley 32 95-100 30 0.88 1-2 Months

Peach 31-32 90-95 30.3 0.91 2-4 weeks

Pear 29-31 90-95 29.0 0.86 2-7 months

Peas in pods; snow, snap & sugar peas

32-34 90-98 30.9 0.79 1-2 weeks

Peppers

Bell pepper 45-50 95-98 30.7 0.94 2-3 weeks

Hot peppers, chilies 41-50 85-95 30.7 nd3 2-3 weeks

Persimmon, Japanese 32 90-95 28.0 0.83 2-3 months

Plums and prunes 31-32 90-95 30.5 0.89 2-5 weeks

Pumpkin 54-59 50-70 30.5 0.92 2-3 months

Radish 32 95-100 30.7 0.96 1-2 months

Rutabaga 32 98-100 30.1 0.91 4-6 months

Spinach 32 95-100 31.5 0.94 10-14 days

Squash

Summer (soft rind), courgette

45-50 95 31.1 0.95 1-2 weeks

Winter (hard rind), calabash

54-59 50-70 30.5 0.88 2-3 months

Sweet potato, yam 55-59 85-95 29.7 0.75 4-7 months

Tomato

Mature-green 50-55 90-95 31.1 0.94 2-5 weeks

Firm-ripe 46-50 85-90 31.1 0.95 1-3 weeks

Turnip root 32 95 30.1 0.93 4-5 months

Watermelon 50-59 90 31.3 0.94 2-3 weeks1 The specific heat data were taken from “The Commercial Storage of Fruits, Vegetables, and Florist and Nursery Stocks,” Agricultural Handbook 66, USDA, 1968.

2 Some data are presented for low temperatures, which cause chilling injury for certain commodities; these temperatures are potentially injurious and should be avoided.

3 nd = Not Determined.

NOTE: Reprinted with permission from J. F. Thompson, F. G. Mitchell, and R. F. Kasmire. 2002. Cooling Horticultural Commodi-ties. In Postharvest Technology of Horticulture Crops. Edited by Kader, A. A. 2002. Davis, Postharvest Technology Research & Information Center, University of California Davis. Publication 3311.


Table 3: Recommended cooling methods for selected fresh fruits and vegetables

Commodity

Size of operation

RemarksLarge Small1

Tree fruits

Stone fruits (peaches, nectarines) FA, HC FA

Pome fruits (apples, pears) FA, R, HC R

Tropical FA, R FA

Berries FA FA

Grapes FA FA Require rapid cooling adaptable to SO2 fumigation

Leafy vegetables

Cabbage VC, FA FA

Iceberg lettuce VC FA

Kale, collards VC, R, WVC FA

Leaf lettuces, spinach, Chinese cabbage VC, FA, WVC, HC FA

Root vegetables

With tops HC, PI, FA HC, FA Carrots can be VC

Topped HC, PI HC, PI, FA

Sweet potatoes HC R

Stem and flower vegetables

Broccoli HC, FA, PI FA, PI

Cauliflower FA, VC FA

Green onions, leeks PI, HC, WVC PI

Pod vegetables

Beans HC, FA FA

Peas FA, PI, VC FA, PI

Bulb vegetables

Dry onions R R, FA Should be adapted to curing

Fruit-type vegetables

Cucumbers, eggplant R, FA, FA-EC FA, FA-EC Fruit-type vegetables are chill-sensitive at varying temperatures

Melons

cantaloupes HC, FA, PI FA, FA-EC

honeydew FA, R FA, FA-EC

watermelons FA, HC FA, R

Peppers R, FA, FA-EC, VC FA, FA-EC

Summer squashes, okra R, FA, FA-EC FA, FA-EC

Sweet corn HC, VC, PI HC, FA, PI

Tomatoes R, FA, FA-EC

Winter squashes R R1 small scale operation refers to coolers handling up to 1,000 lb/hr.

Key: FA = Forced-air cooling FA-EC = Forced-air evaporative cooling HC = Hydrocooling PI = Package-icing R = Room cooling WVC = Water spray vacuum coolingVC = Vacuum cooling

NOTE: Reprinted with permission from J. F. Thompson, F. G. Mitchell, and R. F. Kasmire. 2002. Cooling Horticultural Commodities. In Postharvest Technology of Horticulture Crops. Edited by Kader, A. A. 2002. Davis, Postharvest Technology Research & Information Center, University of California Davis. Publication 3311.

Circular 1004 July 2011

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Precooling Fruits and Vegetables in Georgia