In a competitive market, produce buyers demand the highest quality products. For many fresh fruits and vegetables, rapid and thorough cooling immediately after harvest is essential to preserving quality.
Ice has been used since early times to remove heat rapidly from food and to extend its shelf life. It is the preferred cooling method for many types of extremely perishable produce items. Ice not only removes heat rapidly when first applied to produce but, unlike other cooling methods, continues to absorb heat as it melts. Crushed and liquid ice cooling methods can be used effectively by growers with both small and large operations.
This publication is intended to help growers, packers, and shippers of fresh produce make informed decisions concerning the application of crushed and liquid ice cooling. Included are discussions of icemaking equipment and ways to purchase ice, types of produce that may be suitably iced, various produce-icing methods, how to calculate the amount of ice required to cool a given amount of produce, and the economic considerations of cooling with ice.
Asparagus Cauliflower Broccoli Green onions Cantaloupes Leafy greens Carrots Sweet corn
When faced with an uncertain supply of expensive block ice, some moderate- and large-scale growers have opted to install their own ice-making equipment. The decision to purchase ice-making equipment should be made only after thoughtful consideration of both the benefits and costs. (See the later section on economic considerations.) On-farm ice makers are often situated over or near an insulated enclosure or bunker that has sufficient storage capacity to hold one or two days' production of ice. Chipped ice stored for more than a day or so will freeze together into a solid mass. Therefore, both the ice maker and the ice bunker must be correctly sized to maximize utilization and minimize cost. A number of equipment manufacturers are now marketing ice makers and ice storage bunkers as modular units designed specifically for on-farm postharvest icing operations.
Ice crushers can be extremely dangerous when improperly used. Anyone using a crusher must be properly instructed in its safe use, and all guards and safety devices must be in place. All electrical equipment, including ice crushers, should be equipped with an approved electrical ground.
Individual Package IcingThe simplest icing method is to add a measured amount of crushed ice manually to the top of each carton filled with produce. This method is sufficient in many instances but can result in uneven cooling because the ice generally remains where it was placed until it has melted. Because each carton must be opened, iced, and reclosed, the process is slow and labor intensive. Individual package icing has been automated to some extent by ice-dispensing devices and the use of package conveyors and roller benches. Individual package icing is usually not considered for high-volume production.
Liquid IcingSometimes known as slush icing, liquid icing is the preferred method when large amounts of produce must be iced in a relatively short period of time. The required equipment consists of an ice crusher, a slurry tank with mixer, a pump, and delivery hoses.
In the simplest form of liquid icing, a mixture of water and finely crushed ice is pumped into open containers traveling along a conveyor under an injection nozzle, as shown in Figure 1. This method is a significant improvement over hand icing because the water carries the ice into the voids throughout the package, thus bringing it into greater contact with the produce.
Figure 1. Liquid icing single cartons of produce.
Liquid icing may be considered a hybrid of package icing and hydrocooling. The chilled water in the slurry has a pronounced cooling effect on the produce. In the liquid icing of broccoli, it has been estimated that as much as 40 percent of the total product cooling is accomplished by the water alone. The balance of the cooling is accomplished by the ice as it melts inside the carton.
If the produce has been packed and palletized in the field, the water and ice mixture can be alternatively pumped from a hose into the hand openings of each container, as shown in Figure 2. This method is fast and effective, and it does not require that the cartons be opened or removed from the pallet. With the proper equipment, two workers can liquid ice a pallet of 30 cartons in about 5 minutes.
Figure 2. Injecting liquid ice into palletized broccoli cartons through the hand openings.
An even faster and more automatic method for liquid icing of palleted produce has been developed, as shown in Figure 3. A pallet of filled cartons is placed inside a metal enclosure that is rapidly filled with a slurry of water and ice. The water and ice penetrate all the voids in the cartons. After the enclosure is entirely filled, the excess slurry is drained away, leaving the ice inside the cartons. An automatic pallet-icing operation can be controlled by the lift truck operator alone.
Figure 3. Automated pallet icing.
The equilibrium temperature of a mixture of melting ice and water is 32 F. Users of liquid icing have occasionally added salt (sodium chloride) to the slurry in an attempt to lower its temperature and effect faster cooling. Adding 40 pounds of salt per hundred gallons of slurry will lower the temperature to approximately 27 F. However, this 5 F reduction in temperature will not reduce cooling time significantly. Furthermore, the brine solution may cause the produce to lose more water (wilt more) than is acceptable. Therefore, many produce buyers strongly discourage this practice. The brine solution may also contribute to the corrosion of the icing equipment.
Liquid icing is an excellent cooling method, although it does wet the produce. The surface of warm, wet produce provides an excellent site for postharvest diseases to develop. Therefore, it is essential that, once iced, produce not be allowed to rewarm. Produce that has been stressed by too much or too little water, high rates of nitrogen, or mechanical injury (scrapes, bruises, or abrasions) is particularly susceptible to postharvest diseases. To help reduce the spread of postharvest diseases, I recommends the use of chlorine as a disinfectant at a rate of 55 to 70 parts per million. Chlorine to be used in hydrocooling water can be purchased in the form of a sodium hypochlor-ite solution (for example, Clorox) or as dry, powdered calcium hypochlorite.
Top IcingTop icing is the placing of a 2- to 4-inch layer of crushed ice over the top of pallets of precooled produce. Trucks loaded with precooled fresh produce, such as hydrocooled sweet corn, are frequently top iced just before shipment. Top icing can add considerably to the shipping weight of the load. Top icing a 40-foot trailer can require 8,000 pounds of ice. It has been previously supposed that as the top ice melts, the chilled water would trickle down through the load to continue the cooling process. Tests have shown, however, that top icing affects only the uppermost layers of produce. Furthermore, the chilled water from the melting ice often passes through spaces between the cartons and pallets with little cooling effect. In general, top icing provides little additional cooling and is useful only in preventing an increase in temperature.
As an example of how to calculate the amount of ice needed to cool a given amount of produce, consider the icing of broccoli. Broccoli is an extremely perishable crop and is one of the most commonly iced fresh vegetables. Most buyers require broccoli to arrive with ice inside the container as proof of sufficient and continuous cooling. Broccoli is frequently packed in cartons containing approximately 20 pounds of product. Cooling 20 pounds of broccoli from 85 F to 45 F requires the removal of approximately 800 Btu of heat. Assuming that during shipping and handling another 1,000 Btu would enter the container from the outside, the total heat load per carton would be 1,800 Btu. Therefore, the minimum amount of ice required to provide proper cooling would be:
1,800 Btu/carton -------------------- = 12.5 pounds of ice / carton 144 Btu/pound of iceIn actual practice, approximately 16 pounds of ice is required for each 20-pound carton of broccoli. The additional ice ensures proper cooling under extraordinary conditions. If the liquid icing method is used, another 10 pounds of ice per carton may be required to cool and maintain the water in the slurry at a low temperature. In this example, therefore, a total of approximately 26 pounds of ice would be required to properly cool each 20-pound carton of broccoli.
A second question is whether the loss is directly related to lack of cooling or is due to other reasons, such as oversupply, improper harvesting, carelessness in grading, or poor marketing. Icing equipment can do little to control losses not related to cooling.
A third question is whether the initial investment costs and system operating costs of an icing system are less than the benefits expected from icing the produce. That is, does the sum of the market value of the produce saved or the price premium received for precooled product exceed the annualized cost of cooling? A final question is whether the volume of product handled matches the type of precooling system considered. For example, a high-volume grower or handler may discover that use of a hydrocooler is more cost effective than package icing.
Beyond economic considerations and the physical characteristics of the produce to be cooled, marketing traditions also influence the type of precooling method used. While either hydrocooling or package icing are recommended practices for cooling sweet corn, broccoli, and leafy greens, most wholesale buyers expect loads to arrive with ice visible inside the shipping carton. Similarly, while it is permissible to package ice or top ice cantaloupes, buyers seem to prefer hydrocooled cantaloupes over iced loads. (However, icing is preferred to no cooling.) When alternative precooling methods are possible, economic considerations and buyer traditions should both be considered.
For many small- to moderate-volume growers and for producers of short-season fruit and vegetable crops, icing provides a low-cost precooling option. Initial investment and operating costs can be obtained by requesting price quotes from the companies that supply icing equipment. General information about precooling options, including package icing, can also be obtained through a computer program called "Precool Advisor." This computer program provides help in examining the economic and engineering advisability of installing and operating produce cooling equipment. For information about this software, contact your county Cooperative Extension Center.
Update: 28/02/ 2002