The microorganisms that are principally involved in food deterioration are bacteria, molds, and yeasts. There are thousands of genera and species of microorganisms. Several hundred are associated in one way or another with food products. Not all cause food spoilage, and many types are used in preserving foods, such as the lactic-acid-producing organisms of cheese, sauerkraut, and some types of sausage. Other microorganisms are used for alcohol production as in wine or beer-making, or for flavor production in other foods. However, except where these microorganisms are especially cultivated by selective inoculation or by controlled conditions to flavor their growth over that of less desirable types, microorganism multiplication on or in foods is a major cause of food deterioration. The microorganisms will attack virtually all food constituents. Some will ferment sugars and hydrolyze starches and cellulose. Others will hydrolyze fats and produce rancidity. Still others will digest proteins and produce putrid and ammonia-like odors. Some will form acid and make food sour. Others will produce gas and make food foamy. Some will form pigments, and a few will produce toxins and give rise to foodborne illnesses. When food is contaminated under natural conditions, several types of organisms will be present together. Such mixed organisms contribute to a complex of simultaneous or sequential changes which may include acid, gas, putrefaction, and discoloration.

a. Bacteria. Bacteria are unicellular microorganisms of many forms, although three principal shapes of the individual cells predominate. These are the spherical shape represented by several forms of cocci, the rod shape of the bacilli, and spiral forms possessed by the spirilla. Some bacteria produce spores which are remarkably resistant to heat, chemicals, and other adverse conditions. Bacterial spores are far more resistant than yeast or mold spores, and more resistant to most processing conditions than natural food enzymes. All bacteria associated with foods are small. Most are of the order of one to a few microns in cell length and somewhat smaller than this in diameter. (A micron is one-thousandth of a millimeter (0.001 mm) or about 0.00004 inch.) All bacteria can penetrate the smallest of openings, and many can pass through the natural pores of an egg shell once the natural bloom of the shell is worn or washed away.

b. Molds. Molds are larger than bacteria and yeast and more complex in structure. They grow by a network of hair-like fibers called mycelia and send up fruiting bodies that produce mold spores referred to as conidia. The blackness of bread mold and the blue-colored veins of blue cheese are due to the conidia, while beneath the fruiting heads, the hair-like mycelia anchor the mold to the food. The mycelia are a micron or so in thickness and, like bacteria, can penetrate the smallest opening; or in the case of weakened skin or shell can digest the skin and make their own route of penetration.

c. Yeasts. Yeasts are somewhat larger than bacteria, of the order of 20 microns in individual cell length and about half this size in diameter. However, yeasts are smaller than molds. Most yeasts are spherical or ellipsoidal in shape. Most yeast cultures are cream, tan, or gray. However, some are yellow, pink, red, green, or brown. Yeasts are associated with nearly all types of food products. Foods such as fresh vegetables, meat, poultry, and cheese often contain yeasts, but in these foods, bacteria outgrow the yeasts. When bacterial inhibitors are added, yeasts can dominate. Some yeasts are found in foods such as honey, molasses, sugar, and fruit. Salt-tolerant yeasts grow as films on brine food and on salted food and ham.

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