When warm produce is cooled directly by chilled water, the process is known as hydrocooling. Hydrocooling is an especially fast and effective way to cool produce. With modern technology, hydrocooling has now become a convenient and attractive method of postharvest cooling on a large scale.
Many types of produce respond well to hydrocooling. Produce items that have a large volume in relationship to their surface area (such as sweet corn, apples, cantaloupes, and peaches) and that are difficult to cool can be quickly and effectively hydrocooled. Unlike air cooling, no water is removed from the produce. In fact, slightly wilted produce may sometimes be revived by hydrocooling.
Hydrocooling is one of several postharvest cooling methods available to growers, packers, and shippers.
Hydrocooling:
Caution. Not all types of fresh produce may be successfully hydrocooled. Some are sensitive to wetting, which promotes the growth of decay organisms. For a listing of produce items that may be hydrocooled, refer to Extension Service publication AG-414-1, Introduction to Postharvest Cooling and Handling Methods. Of course, no matter what cooling method is employed, the produce should never be allowed to rewarm once it has been cooled.
Some hydrocoolers do not use a refrigeration system. Instead, crushed or chunk ice is used to cool the water. Typically, large blocks of ice weighing as much as 300 pounds are trucked from an ice plant, crushed, and added as needed to a water reservoir attached to the hydrocooler. The capital cost of a hydrocooler of this type is much less than one with an integral refrigeration system and may be preferred by growers with a limited amount of produce or a short cooling season. However, to make a valid economic comparison, the cost of the ice must be considered. For a hydrocooler of this type, a reliable source of ice must be available at a reasonable cost.
Wire-bound cartons and crates with a large percentage of open space are more suitable for hydrocooling because they allow for sufficient entry of water. Produce in 20-bushel bulk bins cools especially well because the cool water can easily percolate down through the product.
Figure 1. Typical conventional hydrocooler.
Manufacturers of conventional hydrocoolers often specify their different models by length. For example, a "10-foot" hydrocooler has an active cooling length of 10 feet, even though it may be as much as 20 feet long. The additional length is used for input and output conveyors. The longer the active cooling area, the greater the capacity of the hydrocooler. Conventional hydrocoolers with as much as 50 feet of active cooling length and a width of 8 feet have been constructed. However, specifying the capacity of a hydrocooler entirely by length can be misleading if conveyor speed, conveyor width, water temperature, and flow rate are not taken into consideration. Therefore, when comparing different hydrocoolers, buyers should find out how many pounds or bushels of a certain commodity the hydrocooler will cool per hour from one temperature to another. Most hydrocooler manufacturers can readily supply this type of data. Hydrocooling requires large quantities of water to be passed by the produce. Water flow rates as great as 20 gallons per minute per square foot of active cooling area are common. For example, a hydrocooler with an active cooling area 4 feet wide by 20 feet long (80 square feet) would require the circulation of 1,600 gallons per minute.
Most conventional hydrocoolers are high-production units with large refrigeration systems and heavy-duty components. Because of their relatively high cost, they must be operated for considerable periods each year to be economically justified. With the relatively small acreages of produce common in North Carolina, these hydrocoolers would need to be used by more than one grower (or with more than one crop) or by a co-op of growers and packers in order to be cost effective. More information on the economics of hydrocooling is given in a later section.
Most batch hydrocoolers can cool only one pallet of produce at a time, as shown in Figure 2. However, some larger batch units are occasionally built that can cool as many as eight pallets at once. These hydrocoolers generally have a smaller capacity than conventional hydrocoolers and therefore may be less expensive. They are better suited to growers with a limited amount of produce that could not economically justify a larger unit.
Figure 2. Single pallet batch cooler.
A frequent complaint about both conventional and batch hydrocoolers is that they do not cool all containers uniformly. The chilled water may not be evenly distributed throughout the load, resulting in undercooling of some parts. To overcome this deficiency, some batch hydrocoolers use a high-capacity fan to pull a fine mist of chilled water through the produce packages. The forced air has the effect of making the cooling more consistent because it pulls the water past the produce more evenly than would occur by gravity flow alone. This design, known as "hydro-air-cooling," has been successfully used with items that are particularly difficult to cool. Figure 3 shows a typical hydro-air-cooling unit.
Figure 3. Cut-away view of a hydro-air-cooling hydrocooler.
Figure 4. Immersion hydrocooler.
Test have shown that the most rapid hydrocooling is obtained by immersing produce in tanks of agitated chilled water. Immersion hydrocooling is nearly twice as rapid as conventional hydrocooling methods. With conventional hydrocooling, the cold water that is sprayed or flooded over the produce contacts 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.
Immersion hydrocooling was once practiced by placing loose produce into tanks of chilled water. This method is seldom practiced today because it is 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 5. Truck hydrocooling operation in Eastern North Carolina.
To cool the water, blocks of ice from a commercial ice plant are crushed and added to the tank. Approximately one 300-pound block of ice is required per 100 gallons to cool the water initially. As the water warms during the cooling process, ice is added periodically.
Truck hydrocoolers can be built by the grower at a central location on his or her farm for a small fraction of the cost of a commercial hydrocooler. A truck hydrocooling system capable of cooling several truckloads of fresh produce per day can be built from locally available parts.
Until recently, it was assumed that truck hydrocooling removed 30 to 40 degrees of field heat from sweet corn cooled in this manner. However, recent tests conducted at several locations in eastern North Carolina have not supported this common belief. Temperature sensors positioned throughout a trailer load of sweet corn hydrocooled for 1 hour and 45 minutes showed an average decline in temperature of only 15 F. This poor cooling rate apparently occurs because most of the water flows through the spaces between the crates, decreasing the total heat transfer.
The basic heat transfer equation governing convection is:Q = h x A x (Ts - Tw)
where:
Q = the rate at which heat is transferred from a batch of produce, in units of Btu/hr
h = the heat transfer coefficient, in units of Btu/(hr ft^2 F)
A = the total exposed surface area of the produce, in square feet, over which the heat transfer occurs
Tw = the temperature of the water (F)
Ts = the temperature of the produce surface (F)
From the equation, it may be seen that the rate of heat transfer, Q, is directly proportional to the magnitude of the heat transfer coefficient, the amount of surface area, and the temperature difference between the surface of the produce and the water.
The surface area that is exposed to the chilled water varies considerably among different types of produce. For example, the surface area of 10 pounds of sweet corn is considerably less than that of 10 pounds of snap beans. Therefore, if hydrocooled in water of the same temperature, snap beans can be expected to cool much faster than sweet corn. In addition, the greater the difference between the water and the produce temperature, the faster the cooling.
So far, the discussion has centered on heat transfer from the standpoint of the surface temperature of produce. When cooling produce, the rate of cooling of the interior of the produce is of primary importance. The theoretical basis just reviewed will promote a better understanding of the factors that affect the hydrocooling rates of various produce and hydrocoolers.
To calculate the cooling rates for different types of produce and other types of hydrocoolers, refer to the instructions in the box on the next page.
Researchers have developed a graph summarizing current information about heat transfer for various fruits and vegetables. Presented in Figure 6, this graph shows a family of eight ideal cooling lines for various types of produce immersed in agitated chilled water. These lines are approximations only 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 (DTD) at the temperature you want to reach. To calculate the hydrocooling time for a specific type of produce, use this equation for DTD:T - W DTD = ----- P - Wwhere:T = the target temperature (F)
W = the temperature of the water (F)
P = the starting temperature of the produce (F)
Follow the steps in the example below to calculate the hydrocooling time for sweet corn.
Example: Sweet corn, with a center cob temperature of 85 F, is to be hydrocooled by immersion in 35 F water. How long will it take to reduce the center cob temperature to 55 F?
Figure 6 must first be consulted to locate the cooling line (E) that applies to sweet corn. DTD is then calculated by using the above equation:
55 - 35 DTD = ------- = 0.4 85 - 35By finding the place on Figure 6 where curve E intersects the DTD = 0.4 line and projecting downward to the X-axis, you will see that the time required to cool the sweet corn from 85 F to 55 F is approximately 28 minutes. Suppose the packer was considering cooling the corn not to 55 F but to 42 F. In this case the DTD would be:42 - 35 DTD = ------- = 0.14 85 - 35Again, by finding the place on Figure 6 where curve E intersects the DTD = 0.14 line and projecting downward to the X-axis, the time required to cool the sweet corn from 85 F to 42 F is found to be approximately 56 minutes.In the example above, it was assumed that the individual ears of corn were completely immersed in agitated, chilled water. Recall that immersion is the fastest possible hydrocooling method. When the sweet corn is totally immersed, all outside surfaces are covered with cold water. If the sweet corn were hydrocooled in a conventional hydrocooler or closely packed in a wire-bound crate with other ears of corn, the cooling time would be considerably longer. Data published by the manufacturers of conventional hydrocoolers suggest that cooling times for their equipment be estimated at 40 to 80 percent longer than for immersion hydrocooling. The cooling time for sweet corn on a conventional hydrocooler can be estimated by multiplying the immersion hydrocooler rate by a conversion factor of 1.5:
28 minutes x 1.5 = 42 minutesFigure 6 should be used as a rough guide only. Consult a hydrocooler manufacturer for more precise hydrocooling rates.(immersion (conversion (conventional hydrocooling factor) hydrocooling rate) rate)
Key
Figure 6. Time-temperature response of various fruits and vegetables immersed in agitated chilled water.
Figure 7 shows the cooling curve for sweet corn with a starting temperature of 85 F and a cooling water temperature of 35 F. Note that the cooling proceeds rapidly at first but then slows as the produce temperature approaches that of the chilled water. It takes 28 minutes to cool the corn 30 degrees (from 85 F to 55 F), but it takes twice that long to cool it another 13 degrees (from 55 F to 42 F). The graph illustrates that hydrocooling is much more efficient at removing the first 30 degrees or so of heat from the produce. Attempts to reduce the heat further by hydrocooling may result in decreased productivity and increased cooling costs. Alternate cooling methods, such as top icing, may be more practical if additional cooling is desired.
Figure 7. Example of cooling rate for sweet corn.
Several factors affect the amount of chlorine available in the hydrocooling water over time. Chlorine used in hydrocooling water is quite volatile and will disperse into the air at a rate that increases with the temperature of the water. The warmer the water, the faster the chlorine will leave the solution. Furthermore, chlorine tends to attach to soil particles. Thus, dirty produce uses up the available chlorine much faster than relatively clean produce.
In addition to the factors discussed above, the pH of the water has a significant effect on the availability of chlorine. Even if a normally sufficient amount of chlorine is added to the hydrocooling water, the chlorine may not be available in a usable form if that water is acid (below a pH of 6.5) or basic (above a pH of 7.5). For the chlorine to be optimally available, the pH of the hydrocooling water should be nearly neutral (pH of 7.0). Over seven times more chlorine is needed to disinfect at a pH of 8.5 than at a pH of 7.0.
Because the pH of well water in North Carolina varies from moderately acid to moderately basic, it is a good idea to check the water with a pH meter or test papers. Even if your water has a nearly neutral pH, the addition of hypochlorites will cause the water to become basic. Therefore, adding a small amount of common acids like lemon juice or vinegar may be necessary to correct the pH. Inexpensive test papers for checking both the chlorine level and pH may be obtained from most swimming pool and chemical supply houses.
Pathogenic (disease-causing) organisms may enter the hydrocooling water both in the active vegetative form and in the form of spores. The chlorine will quickly kill the vegetative form, but the spores are 10 to 1,000 times more difficult to kill. Therefore, chlorine treatment does not usually eliminate all pathogens and sterilize the surfaces of the produce. Many spores may remain on the surface to develop later if the opportunity arises (that is, if the produce is allowed to rewarm). The effectiveness of the chlorine treatment depends on the length of exposure. Fortunately, 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 be ineffective. Also, chlorine solutions can produce surface bleaching.
The wastewater from hydrocooling is usually dumped at the end of each workday or more often if 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 nondischarge permit may be required. A hydrocooler owner may be required to obtain a wastewater discharge permit.
Anyone using or planning to use a hydrocooler should check with the local office of the North Carolina Department of Environment, Health, and Natural Resources to determine whether a permit is required. The illegal disposal of hydrocooler wastewater may result in a substantial fine.
During hydrocooling, much of the energy loss occurs because the falling water gains heat from the air. Energy loss also results from the lack of insulation on refrigerated surfaces. Additional losses occur if a hydrocooler is operated at less than full capacity, if it is operated intermittently, or if more water than necessary is used.
Immersion hydrocoolers are the most energy- efficient type of hydrocooler because the water-to-air contact is minimized. A well-designed immersion hydrocooler may have an energy efficiency as great as 70 percent.
Energy use in hydrocoolers may be substantially reduced by:
Annual Fixed Costs. These are expenses that will be incurred whether the hydrocooler is used or not. They include expenses for depreciation, insurance, interest on money borrowed to buy the equipment, and taxes. Typically, annual fixed costs for hydrocoolers are substantial because of the high cost of the refrigeration unit and all associated equipment. A standard commercial-size conveyor hydrocooler capable of cooling 10,000 pounds of produce per hour may cost more than $125,000. More modest units or farm-built systems can reduce the investment costs, but costs are likely to remain substantial. Average annual fixed costs per unit of product hydrocooled decrease as the amount of produce hydrocooled increases; that is, as more product is cooled, the average annual fixed cost per unit declines.
Annual Variable Costs. These expenses vary directly with the amount of produce hydrocooled. Variable costs include wages paid to laborers to load and unload the hydrocooler unit, operate and maintain the hydrocooler, and make repairs. They also include costs of maintenance supplies, water, and electrical power. Block ice used as a source of cooling is another variable cost. While total variable costs increase as the volume of product cooled increases, the variable cost per unit of produce hydrocooled remains approximately constant.
The total cost per unit of product hydrocooled is the sum of both the annual fixed costs and the annual variable costs. When the total cost is divided by the number of units hydrocooled, the result is the average cost per unit of produce hydrocooled.
Intangible benefits, although harder to define, are just as important. These benefits include added harvesting and marketing flexibility, reduced losses in transit, and the pride that comes with shipping a quality product. In times when there is more produce than the market can accept, produce that has been properly hydrocooled will hold a clear market advantage over produce that has not.
For hydrocooling to be economically viable, the average cost per unit of produce cooled must be less than the sum of the benefits expected. Given the high initial costs and operating expenses, usually only large-scale producers and handlers (those producing on several hundred acres or more) find hydrocooling to be economically viable. However, growers and handlers with moderate acreage have explored several ways to reduce initial investment and variable costs. One option is to lease hydrocooling equipment. Another option is to increase the volume of produce hydrocooled by increasing the diversity of crops planted or staggering planting periods to expand the length of the harvest season. These options increase the amount of produce hydrocooled, thus reducing the average annual fixed cost and the total cost of cooling. Finally, a group of growers and shippers may jointly purchase or lease portable hydrocooling equipment.
AG-414-1, Introduction to Proper Postharvest Cooling and Handling Methods
AG-414-2, Design of Room Cooling Facilities: Structural and Energy Requirements
AG-414-3, Forced-Air Cooling
AG-414-5, Crushed and Liquid Ice Cooling
AG-414-6, Chlorination and Postharvest Disease Control
Copies may be obtained from your county Cooperative Extension Center or from Agricultural Communications, Campus Box 7603, North Carolina State University, Raleigh, NC 27695-7603. For assistance with postharvest cooling, contact your county Extension Service agent.
M. D. Boyette, Extension Agricultural Engineering Specialist
E. A. Estes, Extension Marketing Specialist
A. R. Rubin, Extension Agricultural Engineering Specialist
Sponsored by the Energy Division, North Carolina Department of Economic and Community Development, with State Energy Conservation Program funds, in cooperation with North Carolina State University. However, any opinions, findings conclusions or recommendations expressed herein are those of the authors and do not necessarily reflect the views of the Energy Division, North Carolina Department of Economic and Community Development.
Published by
NORTH CAROLINA COOPERATIVE EXTENSION SERVICE
10/92--2M--TAH--220544 AG-414-4