Food spoilage encompasses any undesirable changes in food. Spoilage microorganisms and their microbial metabolites that cause repugnant tastes and odors, although aesthetically disagreeable, generally are not harmful. This is not surprising when their growth requirements are considered. Most human pathogens grow best at temperatures near 37°C, whereas most foods are usually stored at temperatures well below the normal body temperature. Similarly, the nutrients available in fruits, vegetables, and other foods are generally not suitable for the optimum growth of human pathogens. As a result, the non-pathogens can easily outgrow the pathogens when competing for the same nutrients. Spoiled foods are considered unsafe to eat, however, because high numbers of spoilage organisms indicate that foodborne pathogens may be present as well.
A wide range of bacteria is important in food spoilage. Pseudomonas species can metabolize a wide variety of compounds, and they can grow on and spoil many different kinds of foods, including meats and vegetables. Psychrophilic pseudomonads can multiply at refrigeration temperatures and are notorious for spoiling meats as well as other foods. Members of the genus Erwinia can produce enzymes that degrade pectin, and so they commonly cause soft rot of fruits and vegetables. Acetobacter species can transform ethanol to acetic acid, the principal acid of vinegar. Although this property is very beneficial to commercial producers of vinegar, it presents a great problem to wine producers. Milk products are sometimes spoiled by members of the genus Alcaligenes that form a glycocalyx, causing strings of slime, or "ropiness," in raw milk. The lactic acid bacteria, including species ofStreptococcus, Leuconostoc, and Lactobacillus, all produce lactic acid. Anyone who has unexpectedly consumed sour milk knows that this can be disagreeable. Members of the genera Bacillus and Clostridium are particularly troublesome because their heat-resistant endospores survive cooking and, in some cases, canning. Bacillus coagu-lans and B. stearothermophilus spoil some canned foods. ■ glycocalyx, p. 63
A wide variety of fungi, including species of Rhizopus, Alternaria, Penicillium, Aspergillus, and Botrytis, spoil foods. Since fungi grow readily in acidic as well as low-moisture environments, fruits and breads are more likely to be spoiled by fungi than by bacteria. Aspergillus flavus infects peanuts and other grains, producing aflatoxin, a potent carcinogen monitored by the Food and Drug Administration.
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