Prompt and thorough postharvest cooling of fresh produce immediately after harvest is one of the most important steps in what can be a long journey from the grower to the consumer. Temperature control is one of the most important aspects of the postharvest handling of fresh produce. Many in the industry believe that cooling is synonymous with proper postharvest handling. However, without the technology and the vast cold chain infrastructure of temperature management, the modern fresh produce supply system would not be possible.

This introduction to postharvest engineering reviews the benefits of postharvest cooling, the five common fresh produce cooling methods and their practical applications, the benefits and liabilities of each, and the basic principles of heat transfer that produce cooling.

Slows ripening. Respiration and the enzymatic activities of ripening are chemical processes that may be slowed significantly by cooling. Since the rate of respiration of produce depends on the temperature, cooling fresh produce can reduce the rate of respiration and significantly increase shelf life. Harvested items that may be held for only hours in good condition at field temperatures may be kept for days when promptly cooled to their optimum storage temperature.

Slows water loss. Crisp produce with bright colors is viewed correctly by consumers as a mark of quality. Excess water loss or wilting can reduce the quality of many produce items. As water is lost from the produce cells to the surrounding air, the produce wilts and loses its bright color. Water loss is much slower at cool temperatures.

Slows or inhibits the growth of decay producing micro-organisms. The molds and bacteria that cause postharvest decay generally require warm temperatures to grow. These organisms and the spores that spread them cannot be eliminated totally but they can be controlled with good sanitation and prompt cooling.

Reduces the production and effects of ethylene. The production and effects of the ripening agent ethylene are connected with the respiration rate. The rate of respiration is less at lower temperatures, and so is the production of ethylene.

Increases marketing flexibility. Postharvest cooling and storage can provide some marketing flexibility by providing a wider marketing window. The ability to cool and store produce for a short period immediately after harvest may allow for more orderly harvesting and marketing. The length of time that a grower can prudently store fresh produce depends on the item. However, the ability to store some produce may be a distinct advantage to growers both large and small. A modest cooling and storage facility can be very useful for small growers who supply restaurants or grocery stores or who plan to organize truckload lots for efficient shipment. Frequently, harvest and shipment are delayed on holidays and weekends although the ripening of produce never takes the day off. Thus, having a refrigerated space to hold harvested produce until shipment is a distinct advantage for growers.

Cooling is also a quality factor and quality is definitely a selling point. Anything a grower or packer can do to shift selling interest from price to quality is beneficial. All other things being equal, properly cooled and handled produce will always outsell uncooled produce.

To select the best cooling method, it is important to understand the basic principles of cooling and the various factors that affect the cooling rate. Heat is a form of energy and in scientific terms, cooling is simply heat transfer. Just as water flows downhill, heat always seeks to move from warm to cool. Heat transfer can occur by several different mechanisms.

• Heat can be conducted from a warm object or region to a cooler one when there is physical contact. When a cold potato is dropped into boiling water, for example, the heat is conducted from the warm outside to the cold inside.
• Heat can radiate from a warm surface to cooler areas just as heat is radiated from the sun. The radiated heat, in the form of infrared waves, warms the air and any surface it meets. Radiated heat needs no transfer media and can be radiated through a vacuum as it is from the sun.
• Heat can also be transferred by convection. Cool air that is warmed as it moves past a hot surface is an example of convection. Convection always requires a moving fluid (such as water or air) to carry away the heat. Convection can be natural (by the natural buoyancy of the heated fluid) or forced (by pumps or fans). Because the movement of the fluid is usually much less with natural convection, heat transfer is much slower with natural convection than with forced convection. Some of the most effective produce cooling relies on forced convection.

With the cooling of produce, heat is transferred first by conduction from inside the produce to the surface. The heat is then removed from the surface of the produce mainly by convection. A small and generally insignificant amount of heat may also be lost by radiation. Many different factors affect the rate of heat transfer (cooling rate) of different types of produce. These include:

Temperature difference. If we place a peach with a temperature of 90°F in 40°F water, it should take longer to cool down to 50°F (a difference of 40 F°, indicated as ΔT = 40) than from 70°F to 50°F (ΔT = 20). It would not take twice as long, however, because heat moves more quickly when the temperature difference is greater. The difference in temperature is the driving force behind heat transfer: the greater the ΔT, the more rapid the heat transfer. This is fortunate because the first 20 or so degree decrease in temperature is the most beneficial in terms of increased shelf life. Further, methods to reduce the temperature at the start of cooling will be beneficial in terms of increased shelf life, decreased cooling time, and reduced cost of cooling.

Electrical energy is a major variable in postharvest cooling. Any practice that reduces energy requirements will reduce the cost of cooling. Methods to lower the temperature of the produce before it goes into the cooler include harvesting during the cooler part of the day, keeping the produce away from direct sunlight, and drenching it with cool water, if wetting is appropriate.

Size and shape of the produce item. In general, the larger the item, the longer it takes to cool. If the other factors are equal, it may take a cantaloupe 12 hours to cool from 90°F to 40°F, whereas it may only take 2 hours for strawberries. This is because the heat must be conducted from further inside a large item. In addition, since the heat must leave the item through its outside surface, the ratio of surface area to the volume of a strawberry is much greater than a cantaloupe. In general, the smaller the produce item, the more quickly it will cool. Knowing that size is a major factor that will influence the proper scheduling of cooling times.

Characteristics of the product. Produce items with a high moisture content usually cool faster than drier items because water is a relatively good conductor of heat. The presence of a surface covering such as the shuck of an ear of sweet corn may radically reduce the heat transfer rate compared to that of a bare ear of corn. Shape also has an influence because rough or oddly shaped items cool faster than round or smooth items because they do not pack as closely together and have a larger surface area to volume ratio.

Packaging. The producer often has little influence on packaging. The sizes and materials of the packaging of a specific commodity are usually dictated by the market. However, small changes in the packaging can have a large influence on the cooling rates. For example, relative thick fiberboard cartons designed to protect the produce from physical damage can also act as insulation and inhibit heat removal. The size, shape, and position of the package openings can also have a large influence on cooling rates. In general, the larger the openings, the faster the cooling. However, since openings can weaken the packaging, the size, shape, and position of the openings is always a compromise between rapid cooling and package stability.

An important function of produce packaging is to protect especially delicate produce items from crushing when containers are stacked on a pallet. If not carefully designed, the addition of openings on the sides of a produce carton can substantially compromise the package’s resistance to vertical loads. The actual positioning of openings is critical. When cross-stacked on a pallet, ventilation holes on the side of the carton should ideally line up with those on the ends of the cartons so that air can pass freely through the palletized cartons. A ventilation opening area of as little as 5% of the carton surface perpendicular to the flow of air is adequate in many cases, although 10% to 15% is more common. As strong reusable plastic cartons with up to 50% open space have become more common, the issue of limited open space has become less important.

To select the most appropriate cooling method, it is important to consider the demands of the market, nature of the product, packaging, and shipping, as well as the amount of produce to be cooled at any one time. In most cases, proper cooling is no longer an option but is often required by the buyers as a condition of sale. Ownership and operating costs can vary considerably among cooling methods on an absolute as well as a per unit cooled basis. Nevertheless, the added expense of cooling must be justified by either higher selling prices or other economic benefits. The size of the operation, as translated in volume to be cooled, is often a major deciding factor. Some cooling methods by their nature are more economical for larger produce handling operations. Fortunately, there is some latitude in selecting cooling methods for some produce items. Below is a brief description of different cooling methods and applications appropriate for small as well as large volume growers. More detailed information about each method follows in this publication.

Room cooling is the most simple cooling method. This involves placing the items in a refrigerated room where heat is removed by natural convection. Room cooling is what we do when we place a warm object in a refrigerator.

Forced-air cooling is the application of forced convection. Fans are used in a refrigerated space to create an air pressure differential across containers of produce so that cool air is actively pulled past the warm produce, which removes the heat. Moving cool air can find its way through the smallest opening and, given the pressure differential, can permeate and cool through to the center of a packed carton of produce. Forced-air cooling has become the preferred method of postharvest cooling for most types of produce in the last few decades.

Hydrocooling is the process of cooling warm produce by bringing it in direct contact with chilled water. Hydrocooling has been practiced in various forms for many years and is an especially fast and effective method to cool produce for items that can tolerate wetting. There are disadvantages to hydrocooling that will be discussed later in this publication.

Ice cooling is the process of cooling warm produce by contact with ice. Ice cooling has the added benefit of not only cooling the produce, but also keeping it cold as long as the ice remains. Like hydrocooling, ice cooling has some significant disadvantages and is gradually being replaced by forced-air cooling or other methods.

Liquid icing is a relatively new technology that has been used successfully with a number of crops, particularly broccoli and asparagus that are packaged in the field. Liquid icing is a hybrid of package icing and hydrocooling. In the simplest form, a slurry of water and finely crushed ice is pumped into open containers as they move along a conveyor. The cold water cools the produce and then slowly drains out, which leaves the crushed ice in contact with the produce. Like hydrocooling and icing, liquid ice cooling has some disadvantages but can be beneficial with the correct application.

Vacuum cooling is unique because it cools through the evaporation of a small amount of water in the produce. The range of produce that can benefit from this application is very limited. However, for some produce, it is very fast and effective. This is the most energy efficient of all cooling methods.

Portable cooling. Any cooling method described here can be made portable for use by small-scale growers to cool and transport small amounts of fresh produce.

Additional Factors to Consider When Selecting a Cooling Method

When selecting a cooling method, the most important factor is the limitations imposed by the produce. Some produce will not tolerate water, others will be harmed by contact with ice, while others wilt easily in cool air. Product packaging requirements are another important factor. The best choice of a cooling method depends on if the produce is packaged in a box, bin, or bag. The package design can affect the method and rate of cooling. For example, produce cooled by water or ice must be packaged in a waterproof container that is far more expensive than a plain fiberboard carton. In addition, some methods of cooling work more quickly than others.

If the volume of produce to be cooled per season, day, or hour is very large, it may be necessary to use a faster cooling method than would be used for smaller volumes. Economic constraints are also important. Construction and operating costs vary greatly among cooling methods. The expense of cooling must be justified by higher selling prices and other economic benefits. In some cases, such as when the volume of produce is low, the more expensive cooling methods will not pay for themselves. However, as discussed later in this publication, the reality is that cooling to the proper temperature is now almost universally demanded by buyers.