Gamma irradiation can be used to sterilize products and to decrease the number of microorganisms in foods. Ultraviolet light can be used to disinfect surfaces. Microwaves do not kill microorganisms directly, but by the heat they generate.
■ What is the purpose of irradiating fresh meat?
■ How does ultraviolet light kill microorganisms?
■ Why could sterilization by gamma irradiation be carried out even after packaging?
Figure 5.8 The Electromagnetic Spectrum Visible wavelengths include the colors of the rainbow.
Preventing or slowing the growth of microorganisms extends the shelf life of products such as food, soaps, medicines, deodorants, cosmetics, and contact lens solutions. Their ingredients often include preservative chemicals added to prevent or slow
124 Chapter 5 Control of Microbial Growth the growth of microbes that are inevitably introduced from the environment. Other common methods of decreasing the growth rate of microbes include low-temperature storage such as refrigeration or freezing, and reducing available water. These methods are particularly important in preserving foods. ■ food spoilage, p. 811 ■ factors influencing the growth of microorganisms in foods, p. 802
Some of the germicidal chemicals previously described can be used to preserve non-food items. For example, shampoo may contain formaldehyde, mouthwash may contain a quaternary ammonium compound, contact lens solutions may contain thimerosal, and leather belts may be treated with one or more phenol derivatives. Food preservatives, however, must be non-toxic for repeated safe ingestion.
Benzoic, sorbic, and propionic acids are weak organic acids that are sometimes added to foods such as bread, cheese, and juice to prevent microbial growth. In the form that predominates at a low pH, these weak acids alter cell membrane functions and interfere with energy transformation. The low pH at which they are most effective is itself sufficient to prevent the growth of most bacteria, so that these preservatives are primarily added to acidic foods to prevent the growth of fungi. These organic acids also occur naturally in some foods such as cranberries and Swiss cheese.
Another preservative, nitrate, and its reduced form, nitrite, serve a dual purpose in processed meats. From a microbiological viewpoint, their most important function is to inhibit the germination of endospores and subsequent growth of Clostridium botulinum. Without the addition of low levels of nitrate or nitrite to cured meats such as bologna, ham, bacon, and smoked fish, C. botulinum may grow and produce deadly botulinum toxin. At higher concentrations than are required for preservation, nitrate and nitrite react with myoglobin in the meat to form a stable pigment that gives a desirable pink color associated with fresh meat. Nitrates and nitrites also pose a potential hazard, however, because they can be converted to nitrosamines during the frying of meats in hot oil or by the metabolic activities of intestinal bacteria. Nitrosoamines have been shown to be potent carcinogens, which has caused concern regarding the use of nitrate and nitrite as preservatives.
Growth of microorganisms is temperature-dependent. At low temperatures above freezing, many enzymatic reactions are very slow or nonexistent. Thus, low temperature storage is extremely useful in preservation. Psychrotrophic and some psy-chrophilic organisms, however, can grow at normal refrigeration temperatures. ■ psychrophiles, p. 86
Commercially, some fruits and vegetables such as apples and potatoes are held in cold storage for many months. Products are stored at temperatures near 10°C, in the dark, and at appropriate humidity and oxygen concentration. Prior to refrigeration, foods are sometimes irradiated in UV light to reduce the number of spoilage organisms.
Freezing is also an important means of preserving foods and other products. Freezing essentially stops all microbial growth. The formation of ice crystals can irreversibly damage microbial cells, killing up to 50% of the microorganisms. The remaining organisms, however, can grow and spoil foods once they are thawed.
For many years, salting and drying have been used to preserve food. Both processes decrease the water activity (aw) of a food below the limits required for growth of most microorganisms. The high-solute environment causes plasmolysis, which damages cells. ■ aw, p. 803 ■ plasmolysis, p. 89
Sugar and salt draw water out of cells and essentially dry them, thereby decreasing the available water in the food, which prevents the growth of microorganisms. High concentrations of sugars or salts are added to many foods as preservatives. For example, fruit is made into jams and jellies by adding sugar, and fish and meats are cured by soaking them in salty water, or brine. Some caution should be exercised when using salt as a preservative, however, because the food-poisoning bacterium Staphylococcus aureus can grow under quite high salt conditions. ■ Staphylococcus aureus, p. 812
Removing water, or dessicating, food is often supplemented by other methods, such as salting or adding high concentrations of sugar or small amounts of chemical preservatives. For example, meat jerkies usually have added salt and sometimes sugar.
Lyophilization (freeze-drying) is widely used for preserving foods such as coffee, milk, meats, and vegetables. In the process of freeze-drying, the food is first frozen and then dried in a vacuum. When water is added to the lyophilized material, it reconstitutes. The quality of the reconstituted product is often much better than that of products treated with ordinary freezing or drying methods. The light weight and stability without refrigeration of freeze-dried foods make them popular with hikers.
Although drying stops microbial growth, it does not reliably kill bacteria and fungi in or on foods. For example, numerous cases of salmonellosis have been traced to dried eggs. Eggshells and even egg yolks may be heavily contaminated with Salmonella species from the gastrointestinal tract of the hen. To prevent the transmission of such pathogens, some states have laws requiring dried eggs to be pasteurized before they are sold.
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