Preservation Methods

Meat is preserved to extend product shelf life. This is accomplished by applying techniques to delay or prevent microbial, enzymatic, chemical, and/or physical changes from occurring that would render a product unacceptable. Historically, meat was preserved for future consumption by curing, smoking, and drying. Chilling, freezing, thermal processing, dehydration, and irradiation are preservation methods currently employed by the meat industry, van Laack (18) presented an overview of spoilage mechanisms and techniques for preservation of muscle foods.

Refrigeration and freezing are the most common methods used to prolong the shelf life of meat. Refrigerated meats are stored between -2 and 5°C (28-41°F). Chilling meat typically begins immediately after slaughter. Car casses are placed into coolers with forced-air movement to facilitate carcass cooling, while poultry and fish are chilled by immersion in ice water. Following fabrication, refrigerated shelf life of meats is dependent on the initial microbial load, temperature, and humidity conditions during storage, packaging, species, and type of product stored. Although the freezing point of muscle is approximately — 2°C (28°F), it is not until -30°C (-22°F) that nearly 100% of the water in meat is frozen. The quality of frozen meat depends on freezing rate conditions and subsequent frozen storage, length of frozen storage, packaging, lighting, and type of product frozen. The rate of freezing impacts the size of ice crystals formed within a product. If meat is rapidly frozen, many small ice crystals form resulting in minimal muscle fiber shrinkage and distortion, and less drip loss upon thawing. Improperly packaged meat is susceptible to freezer burn, which is caused by a loss of moisture on the surface of meat. Freezer-burned meat will have a dry, discolored surface that, when cooked, will be tough and taste bland or rancid.

Canned meat products may be pasteurized or commercially sterile. Pasteurized products are heated to 58 to 75°C (136-167°F), which results in the destruction of all pathogenic bacteria; however, spores and some thermoresistant spoilage organisms may survive. Canned, pasteurized products must be stored under refrigeration. Pasteurized canned hams and picnics are examples of this type of product. Commercially sterile products generally are heated to an internal temperature of at least 107°C (225°F) to render the product free of microorganisms capable of growing at nonrefrigerated conditions (above 10°C/50°F). Canned, commercially sterile products may contain spores of thermophilic bacteria that do not germinate below 43°C (109°F). Meats are canned using a steel tank, called a retort, in which metal crates or baskets containing filled, sealed cans are placed for cooking and subsequent cooling. The amount of heat, time, and temperature required for a given degree of sterility depends on the nature of the product, pH, ingredients such as salt and nitrite, shape and size of the can, and the type of retort used. Footitt and Lewis (19) and Pearson and Gillett (6) provide considerable information on canning meat products.

The preservative effects of dehydration are due to reduction of water activity (aw). The lean portion of freshly cut meat has an aw of 0.99. When meat is dehydrated, the aw is reduced to a level that inhibits microbial growth, allowing products to be shelf stable without refrigeration. According to Leistner and Rodel (20), factors most important in influencing aw in processed meat products are the addition or removal of water, the addition of salts, and the amount of fat. Nieto and Toledo (21) demonstrated that both soluble and insoluble components affected aw in processed fish products. The fat content of meat products has an indirect effect on aw due to its low water-binding properties. In general, aw increases with increasing fat content in a meat product because fat has little effect in depressing aw (22).

Meat is dehydrated by using hot air drying or freeze drying. Jerky is commonly dried using hot air drying. Freeze drying involves the removal of water from meat by sublimation where water, in the form of ice, is directly transformed into water vapor without going through a liquid phase. In a conventional freeze drying process, meat is frozen, then dried under vacuum with pressures of 1.0 to 1.5 mm of mercury while it is in a frozen state to a residual moisture content of less than 2%. Because freeze-dried meat products are susceptible to enzymatic changes, rancidity development, nonenzymatic browning, and protein denaturation, oxygen and moisture impermeable packaging is necessary.

Irradiation involves treating a food item with energy from electrons or y-rays to prevent foodborne illnesses, spoilage, and insect infestations. Food does not become radioactive when treated with irradiation. This process is not a substitute for good sanitary practices, and it will not make a "dirty" food clean. Food irradiation makes meat and food of good quality safer, plus increases the length of time food can be stored before it becomes spoiled. Organizations such as the American Medical Association and World Health Organization have endorsed the safety of irradiation for food. The Food and Drug Administration (FDA) has approved irradiation of wheat and wheat flour to control insects; of white potatoes to control sprouting; of spices to kill insects and control bacteria; of pork to control trichinosis; of fruits, vegetables, and grains to control insects and growth and ripening; and of uncooked poultry to control bacteria, particularly Salmonella. In 1997, the FDA approved irradiation for fresh or frozen red meats. A list of materials approved by the FDA that can be used to package foods before irradiation is found in 21CFR 179.45. A scientific status summary on irradiation, particularly muscle foods, was prepared by Olson (23).

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