Hydrocooling is the process in which warm produce is cooled directly by moving chilled water. Like forced-air cooling, hydrocooling is a form of forced convection that is an especially fast, effective method to cool produce that can tolerate wetting. Under similar conditions, the cooling rate of produce in moving water can be 10 to 20 times faster than moving through air of the same temperature.

Hydrocooling has been practiced for millennia. Ancient people who placed summer fruit or crocks of milk in cool streams or springs to prolong storage time were practicing hydrocooling. Many types of produce that cool slowly with other refrigeration methods respond well to hydrocooling. Produce that has a large volume to surface area (such as sweet corn, apples, eggplant, cantaloupes, and peaches) can be effectively hydrocooled. Unlike cooling in air, hydrocooling causes no water loss from the produce.

In most hydrocooling systems, a pump moves chilled water into contact with warm produce. The warmed water is then re-cooled and re-cycled. Large hydrocoolers may have more than 3000 gal of water in circulation. For cooling the water, most hydrocoolers have a vapor-compression refrigeration system with a heat exchanger located in the water reservoir. These large hydrocoolers are most often found in facilities that cool large amounts of different produce over an extended harvest season. Most conventional hydrocoolers are high-production units with large refrigeration systems and heavy duty components. Since these machines often cost several hundred thousand dollars and need a large amount of electrical energy, their use cannot be economically justified by a small volume grower.

For hydrocooling small amounts of produce, some hydrocoolers do not use a refrigeration system. Instead, crushed or chunk ice is used to cool the water. Typically, large blocks of ice that weigh as much as 300 lb are trucked from an ice plant, crushed, and added to a water reservoir attached to the hydrocooler. Alternatively, a grower may have an icemaker that continually makes and stores ice to be used later in the hydrocooler. The capital cost of a hydrocooler such as this is much less than one with a refrigeration system and may be preferred by growers with a limited amount of produce or a short cooling season. However, in an economic comparison, the cost of ice must be considered. A reliable source of ice must be available at a reasonable cost.

Hydrocoolers are available in many different sizes and configurations depending on the type and amounts of produce to be cooled:

Large Conventional Hydrocooler. Most commercially available hydrocoolers are the conventional type in which the produce (either in bulk bins, single cartons or crates, or palletized) passes along a conveyor under a large shower of chilled water. Warm produce is placed on the conveyor at one end and cooled produce emerges at the opposite end. The rate at which the produce advances through the hydrocooler varies, but active cooling time typically ranges from 10 to 30 minutes or more depending on the nature of the produce to be cooled.

Manufacturers of conventional hydrocoolers often describe their different models by length. For example, a 10-ft hydrocooler has an active cooling length of 10 ft, although the hydrocooler may be as long as 20 ft. The additional length is required for the input and output conveyors. The longer the active cooling area, the greater the capacity of the hydrocooler. Conventional hydrocoolers with as much as 50 ft of active cooling length and a width of 8 ft are available. However, specifying the capacity of a hydrocooler entirely by length can be misleading if the conveyor speed, conveyor width, water temperature, and flow rate are not considered. When comparing different hydrocoolers, buyers should identify how many pounds of a certain commodity the hydrocooler is able to cool per hour from one temperature to another. Such information should be available from most hydrocooler manufacturers.

Hydrocooling requires large quantities of water to be passed by the produce. Water flow rates as great as 20 gal per minute per sq ft of active cooling area are common. For example, a hydrocooler with an active cooling area 4 ft wide by 20 ft long (80 sq ft) would require the circulation of 1,600 gal per minute. As shown in Figure 3b-1, a large conventional hydrocooler has the infeed and outfeed conveyors at the ends and the large water tank above the active cooling area. Since these hydrocoolers are used in the open, a considerable portion of the cooling capacity is wasted in the air. A gross energy efficiency of less than 10% has been documented with some hydrocoolers. This means that 90% of the electrical energy to operate the hydrocooler is wasted.

Figure 3b-1. Conventional hydrocooler.

Source: NC State University.

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Figure 3b-1. Conventional hydrocooler.

Source: NC State University.

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Batch hydrocoolers are enclosures without conveyors. Palletized cartons or bulk bins of produce are loaded into the enclosure with a fork lift. The door of the enclosure is closed, and large quantities of chilled water are flooded over the top of the produce, collected at the bottom, and then re-cooled and re-cycled.

Most batch hydrocoolers can cool only one pallet of produce at a time, as shown in Figure 3b-2. Some larger batch units can cool as many as eight pallets at one time. These hydrocoolers generally have a smaller capacity than conventional hydrocoolers, may be less expensive, and are better suited to growers with a limited amount of produce that would not economically justify purchasing a larger unit.

A frequent complaint about conventional and batch hydrocoolers is that they do not cool all containers in a uniform way. The chilled water may “channel” and not be evenly distributed throughout the load, which results in undercooling of some parts. Cold water that does not make contact with the produce will not cool it. To overcome this deficiency, some batch hydrocoolers use a high-capacity fan to pull a fine mist of chilled water through the packages of produce. The forced air makes the cooling more consistent because it pulls the water past the produce more evenly than gravity flow alone. The reduced amount of cool water used with the mist system reduces the cooling rate. (See Figure 3b-3).

Figure 3b-2. Batch hydrocooler.

Source: NC State University.

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Figure 3b-2. Batch hydrocooler.

Source: NC State University.

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Figure 3b-3. Cross-sectional view of an air mist hydrocooler.

Source: NC State University.

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Figure 3b-3. Cross-sectional view of an air mist hydrocooler.

Source: NC State University.

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Immersion hydrocoolers are large, shallow, rectangular tanks that hold moving chilled water. Crates or boxes of warm produce are loaded into one end of the tank and moved by a submerged conveyor belt to the other end where they are removed. Crushed ice or a vapor-compression refrigeration system keeps the water cold, and a pump keeps the water in motion. Most produce is only slightly buoyant so it tends to remain submerged. The length of time the produce remains in the water varies with the initial conditions and desired ending temperature. Figure 3b-4 shows an immersion hydrocooler loaded with wire-bound crates of sweet corn.

Tests have shown that immersion hydrocooling can be faster and more efficient than conventional hydrocoolers when cooling tightly packed products such as sweet corn in wire-bound crates. With conventional hydrocooling, the cold water that is sprayed or flooded over the produce comes in contact with only a portion of its surface. The result is less than maximum heat transfer. Immersion hydrocooling reduces the temperature more rapidly because moving chilled water completely surrounds the exterior surface of the produce.

In the past, immersion hydrocooling was practiced by placing loose produce into tanks of chilled water. This method is seldom practiced today because it is very labor intensive and much of the cooling effect is lost when the produce is packed. In addition, packing shed workers are often reluctant to handle the wet, cold produce. The bulk "fluming" of string beans in chilled water prior to grading and packing is an example of immersion hydrocooling. Bean fluming is gradually being replaced by other hydrocooling methods.

Figure 3b-4. Immersion hydrocooler cooling of wire-bound crates of sweet corn.

Source: M. Boyette.

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Figure 3b-4. Immersion hydrocooler cooling of wire-bound crates of sweet corn.

Source: M. Boyette.

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This method of hydrocooling is used occasionally with sweet corn in wire bound crates, bulk cucumbers, and peppers. The warm produce is placed into the enclosed van, truck, or trailer and chilled water is flooded over the top by a large perforated pipe. The water percolates through the load where it adsorbs the heat. The water subsequently flows back out of the truck onto a concrete pad where it is collected, re-cooled, and re-cycled back into the truck. Water flow rates of 1,000 gal per minute or more are not unusual for this type of hydrocooler. The water may be cooled with a refrigeration system or with crushed ice. Research on truck hydrocooling has shown that the flow of water past the produce is uneven, that much of the water runs off, and that the result is poor and inefficient cooling. In addition, since the water washes the truck and the concrete collection pad, there is substantial risk of contamination with hazardous chemicals and rot organisms that pose potentially serious food safety issues.

Hydrocooling is a fast process, but it has some negative features. First, hydrocooling can be used only with produce that will tolerate wetting. This means that the products suitable for hydrocooling are limited. Second, the container that holds the produce must be able to tolerate wetting. This typically involves using expensive wire bound or wooden crates, wax coated cartons, plastic bins, or totes. Inexpensive paper cartons cannot be used. Third, many postharvest disease organisms are activated, encouraged, and spread by wetting. Thus, the hydrocooler water must be treated continually with a fungi/bactericide. This chemically treated water requires proper disposal. Fourth, hydrocooling should be used only if there is refrigerated space (room or refrigerated truck reefer) available for storing cooled produce immediately after hydrocooling to maintain the cool. Produce that is wet by hydrocooling and allowed to re-warm is likely to develop postharvest disease. Fifth and most importantly, hydrocoolers are expensive to purchase and operate. A hydrocooler that will cool 10 truckloads of produce per day can cost more than a forced-air cooling system that is built with the same capacity. In addition, hydrocoolers are energy inefficient. Tests have shown that as little as 10% to 15% of the cooling capacity is used to cool the produce. The remainder is wasted and lost to the environment. There have also been issues in the past with cross contamination of pesticides. The cold water cools the produce and also washes it. When the same water is used to hydrocool several different types of produce, pesticides labeled for one type will be washed off and deposited on another for which it is not appropriate.

To operate a hydrocooler effectively and efficiently, it is necessary to understand how water removes heat from the produce and the factors that affect the rate of cooling of various types of produce. Many factors affect the rate of heat transfer with hydrocooling including the surface area of the produce and the temperature difference between the surface of the produce and the chilled water. These factors relate directly to the time required to cool a specific type of produce. The rate of heat transfer for produce increases as the ratio of surface area to volume of the produce increases or when the difference between the produce temperature and the water temperature increases.

The surface area exposed to the chilled water varies considerably among different types of produce. For example, the surface area of 10 lb of sweet corn is considerably less than 10 lb of snap beans. Therefore, if hydrocooled in water of the same temperature, snap beans will cool much more quickly than sweet corn because they have a larger surface exposed to the cold water. The greater the difference between the water and the produce temperature, the faster the cooling.

There are many factors that affect the hydrocooling rates of various produce and hydrocoolers. Field and laboratory studies have provided useful data on hydrocooling rates for many fruits and vegetables. This type of information can be used to estimate the time required to cool the interior of a particular product to a desired temperature when the starting temperature and the water temperature are known.
Figure 3b-5 summarizes the current information about heat transfer for various fruits and vegetables. This figure shows eight ideal cooling lines for various types of produce immersed in agitated chilled water.

Key to Figure 3b-5

A — Greens

B — Beans, peas, asparagus

C — Small cucumbers, radishes, beets (< 1.5" dia.)

D — Small apples and peaches, slicing cucumbers

E — Sweet corn, apples, and peaches

F — Large apples and peaches (> 3" dia.)

G — Cantaloupes, large eggplant

These lines are estimates and are based primarily on the physical size of the produce. The horizontal axis is the cooling time in minutes and the vertical axis is the decimal temperature difference (normalized temperature). For example, if the starting temperature is 90°F and the temperature you want to reach is 40°F, then the temperature difference is 90°F - 40° = 50 °F.

Figure 3b-5. Seven ideal cooling lines for types of fresh produce immersed in moving chilled water.

Source: M. Boyette.

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Figure 3b-5. Seven ideal cooling lines for types of fresh produce immersed in moving chilled water.

Source: M. Boyette.

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Figure 3b-6. Cooling curve of sweet corn.

Source: M. Boyette.

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Figure 3b-6. Cooling curve of sweet corn.

Source: M. Boyette.

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Hydrocooling is an acceptable cooling method, although it wets the produce. The surface of warm, wet produce is an excellent site for postharvest diseases. It is essential that after being hydrocooled, produce should not be allowed to re-warm. Produce is particularly susceptible to postharvest diseases when it is stressed by too much or too little water, high rates of nitrogen, or mechanical injury such as scrapes, bruises, or abrasions. The water used for hydrocooling is re-circulated and can spread disease.

Pathogenic (disease causing) organisms enter the hydrocooling water in the active vegetative form and in the form of spores. In the past, reducing the spread of postharvest diseases required the use of chlorine as a disinfectant in hydrocooler water. The chlorine quickly kills the vegetative form of organisms, but the spores are 10 to 1,000 times more difficult to kill. Treatment with chlorine does not eliminate all pathogens and spores that remain on surfaces and will develop later when the opportunity arises with produce that is allowed to re-warm. The effectiveness of chlorine treatment depends on the level of the chemical in the water and the length of exposure. The long exposure that is common with hydrocooling is much more effective than a quick dip treatment. However, chlorination is only a surface treatment. If the pathogens have already started to develop below the surface, chlorine will not be effective. In addition, chlorine solutions can produce surface bleaching, corrode metal parts, and make working conditions uncomfortable.

The North Carolina Agricultural Chemicals Manual is the legal authority on the selection and use of postharvest chemicals in hydrocoolers. Chlorine is regulated for drinking water but not for post-harvest produce operations. Any chemical used for produce production must be registered with the EPA (Environmental Protection Agency), have an EPA registration number, and be used in accordance with label instructions.

The Produce Safety Alliance (PSA) has been responsible for coordinating the development of the current national training curriculum required by the Food Safety Modernization Act (FSMA) Produce Safety Rule. The PSA has also developed the most current list of EPA-registered chemicals for post-harvest cleaning and sanitation. There are a number of chlorine-based sanitizers that can be used for post-harvest operations (dump tank, food contact surfaces) with the caveat that the users follow the label instructions.

The wastewater from hydrocooling is usually dumped at the end of each workday or more often as necessary. This wastewater often contains high concentrations of sediment, pesticides, and other suspended matter. Hydrocooler water may be considered an industrial wastewater if the product is discharged to a municipal wastewater treatment plant or to surface waters (canals, creeks, or ponds). Land application of this material is normally permitted, but a non-discharge permit may be required. A hydrocooler owner may be required to obtain a wastewater discharge permit.