Prevention Of Losses

The goal of postharvest handling is to minimize the loss of product quality between harvest and either processing or consumption. The first line of defense is prevention of physical injury. Environmental conditions during storage and handling represent the next line of defense. Addition of chemical compounds can prevent losses. Finally, sophisticated packaging techniques are being developed and used that employ one or more of the preceding lines of defense.

Physical protection of products from injury is used to minimize mechanical damage. Reduction of mechanical injury can be achieved by decreasing the number and height of falls of individual items or containers during handling. When falls are unavoidable, damage due to impact can be minimized by cushioning, which can be achieved by foam padding, liquid foam, water, or even product that will be discarded. Impact damage of one fruit on another can be reduced by decreasing dumping operations and using spacer bars in conveyor lines to minimize fruit-to-fruit contact. Vibration damage tends to increase with an increase in the size of bulk handling and the distance from field (or orchard) to packing facility. Packing the product in wholesale or retail packages close to the field, reducing or eliminating tractor-drawn vehicles from field to packinghouse, and use of paved roads for transport will reduce vibration damage.

Likewise, microbial decay and insect damage can be reduced by physical protection. Since mechanical damage frequently provides a route for invasion, any means of mechanical protection will help decrease microbial and insect damage. Once endogenous protective barriers such as rind or peel are penetrated, chances for future losses are increased. A greater incidence of pests and disease in the field will be reflected in greater problems during postharvest handling. By leaving these problems in the field, similar problems during handling and storage will be reduced. Physical removal of diseased or infected items during sorting or grading to prevent the spread of pathogens reduces subsequent infections. To prevent cross-contamination from human pathogens, all areas in which fresh fruits and vegetables are handled should be kept free from insects, birds, their droppings, and any raw animal products. The relatively short handling periods of fruits and vegetables are such that insect infestation is not usually a major problem. With grain products, however, storage times are long, and insects, and the microbes they deposit during their visits, pose a more serious threat. Physical barriers are an important part of an insect-protection plan.

In addition, screening products from light affects quality. Light enhances chlorophyll breakdown and thus speeds yellowing of green vegetables. Light also enhances chlorophyll synthesis in nongreen vegetables such as potatoes. This greening is a quality defect as it is an indicator of light-catalyzed synthesis of toxic alkaloids such as sola-nine. Light enhances ^-carotene synthesis in tomatoes, but the effect on color quality is much more significant preharvest than postharvest.

Manipulation of environmental conditions is also an important tool available to postharvest handlers. In general, lowering the temperature while maintaining high relative humidity increases the shelf life of a product by reducing the rates of respiration and transpiration. Composition of gaseous atmosphere can be either modified or controlled in the storage room, container, or consumer package to slow ripening and senescence.

Proper temperature control is the most important tool in preventing postharvest losses. As the temperature is lowered, rates of respiration and transpiration decrease. The growth of microorganisms is also slowed by lower temperatures. For most fresh products, storage at temperatures as close to freezing as possible will extend shelf life. Freezing should be avoided as inadvertent freezing and thawing of fresh items leads to breakage of cell membranes and loss of desirable texture. Quick cooling after harvest to remove field heat is imperative in items like strawberries and green vegetables that respire rapidly and perish quickly. Hydrocooling and icing are used for products that can withstand water, but water is an excellent vehicle for spreading microorganisms. Forced-air cooling is another effective method, while vacuum-cooling is used for high-value items with a large surface area like lettuce. Slower cooling such as room cooling is permissible for products being stored for a longer time such as apples, but the final temperature should be as close to optimal as possible. When calculating refrigeration requirements, it must be remembered that respiring plant material evolves heat, known as the heat of respiration.

Prevention of chilling injury can be achieved by storing susceptible commodities at temperatures above the critical storage temperature, which ranges from 4°C for snap beans to 15°C for bananas. A complete list of optimal stor age temperatures is available (7). In commercial practice a compromise temperature between 5 and 10°C is frequently used to store most fresh items. At this temperature it is assumed that damage to chilling-susceptible product will be minimal while the decrease in shelf life to nonsus-ceptible items will not be economically significant. Ice is usually added to green vegetables to lower the temperature and increase relative humidity (RH) without changing room temperature. The success of these strategies depends on a rapid turnaround of fresh product to minimize losses.

Maintenance of a high RH lowers transpiration of heavily transpiring products. Just as each commodity has an optimal storage temperature, it also has an optimal RH. If the RH is too high, microbial growth is enhanced. If it is too low, shriveling or wilting can result. Rapid changes in temperature of a product can lead to condensation on the surface and increased susceptibility to microbial decay.

Food additives are effective agents for the protection of plant products, but they are coming under greater scrutiny as consumers become more wary of chemicals. Many currently used compounds are under regulatory review and the approval of new compounds is unlikely. Microbial inhibitors help prevent the growth of spoilage microorganisms. Fumigants have been used to disinfest products from insects. External waxes are applied to porous fruit such as citrus fruits and cucumbers to slow water loss. These waxes also enhance appearance by providing gloss.

Although not a chemical as such, food irradiation induces chemical changes similar to conventional processing and is considered a food additive by the FDA. Low-dose irradiation has been approved in many countries for the inhibition of sprouting in potatoes and onions, insect dis-infestation, and shelf-life extension. In some crops like strawberries, irradiation is effective in extending shelf life, but in others damage is induced at doses lower than effective for extension. Irradiation appears to be a safer technique for disinfestation than chemical fumigation, but questions of consumer acceptance of irradiated product have limited willingness of the food industry to adopt it as a widespread technique.

Shelf life of fresh products may also be extended by modification of the composition of atmospheric gases. Respiration and other metabolic processes are slowed with a decrease in oxygen and an increase in carbon dioxide. In some crops such as apples, pears, and onions, long-term storage is enhanced by controlling the atmosphere. Other crops such as lettuce and most root crops are susceptible to C02. Optimal storage atmospheres for crops have been published (7). Controlled-atmosphere storage usually occurs in large storage rooms where the gaseous atmosphere is monitored and changed to maintain the desired composition. In modified-atmosphere storage the initial gaseous composition is established but changes as respiration leads to decreased 02 and increased C02 in the container. Atmosphere modification is very effective at maintaining texture and appearance but can lead to the development of off-flavors. Modified atmosphere packaging (MAP) is the major technique used to preserve fresh-cut products. MAP is most effective when used in conjunction with temperature reduction. In fresh-cut vegetable products like lettuce, the consumer has shown a willingness to sacrifice some losses in fresh flavor for added convenience. It is not clear that the consumer is willing to make the same sacrifice for fresh-cut fruits.

Advances in film technology have introduced greater sophistication in packaging of fresh products. Packaging protects the product by confining it, preventing contamination, shielding from mechanical damage and pests, permitting atmosphere modification, and providing instructions for optimal handling. The type of container used and its function can change as the item moves through handling and distribution. In general, the fewer handling steps and product transfers, the less opportunity for damage.

Plastics are being widely used in the packaging of fresh fruits and vegetables. They are employed at the pallet level for containerization and prevention of moisture loss around as well as used for MAP within shipping cartons, retail packages, or even individual items. Barrier films have different transmission properties to permit or exclude specific gases such as water vapor, C02, 02 and C2H4, depending on specific requirements of the individual items. These films permit in-package atmosphere modification, extending the advantages of the technology to the supermarket shelves. Determination of the best initial composition of gases has been limited by the variation of the individual items in response to differing atmospheres. While accumulation of C02 and C2H4 could be detrimental to product quality, absorbers of these compounds can be included in sachets enclosed in the package or imbedded in the packaging material itself (8).

A logical extension of MAP is shrinking of the film tightly around the individual produce item. Although generally considered a type of modified atmosphere, individual plastic films are really more analogous to externally applied waxes, which change the diffusion properties of the item with the external atmosphere. These films slow transpiration while modifying the internal gaseous composition. Shelf life of some products like lemons is dramatically extended while other products develop off-odors and off-flavors due to altered metabolism. Edible films such as sucrose polyesters and proteins permit moisture control and changes in respiration of whole and cut products either as a retail item or within a retail package (9).

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