Scientific Basis And Implications

The majority of the pathogens that contaminate food products are natural inhabitants of the environment, soil, plants, and animals. Their survival and growth in foods is affected by a wide range of factors, which have been categorized as intrinsic and extrinsic. Application of combined or synergistic effects of these intrinsic and extrinsic factors in food preservation is the basis of barrier or "hurdle" technology. Pathogen growth and survival are also affected by the relationships among the varied types of microorganisms that make up the complex microbial flora. Depending on environmental conditions, these microorganisms may grow either competitively or cooperatively.

Intrinsic Factors Affecting Growth and Survival

A number of factors intrinsic to foods affect microbial growth and survival. These include pH, moisture content, oxidation-reduction potential, nutrient content, antimicrobial constituents, and biological structure. The intrinsic factors are thought to have evolved as defense mechanisms against foreign microorganisms that can invade and multiply in plant or animal tissues, and they collectively represent nature's way of protecting and preserving the tissues. For example, most fruits—whose biological function is protection of the vital reproductive body or seed—have pH values below those tolerated by many spoilage organisms. Although the pH of living animals favors the growth of microorganisms, other intrinsic properties of animal tissue may control microbial growth and survival. By assessing the various intrinsic factors in individual foods, one can predict what general types of microorganism may be present and adjust handling and processing procedures to ensure a high-quality, safe product.

Acidity It is well established that most microorganisms survive and grow well within a pH range of 6.5-7.0. However, the pH range for microorganisms growing on food is quite wide (pH 4.0-9.5). Although few microorganisms

TABLE 9.1. Minimum pH Values for Selected Foodborne Bacteria"

Aeromonas hydrophila A licyclobacillus acidocaldarius Bacillus cereus

Clostridium botulinum, Group I C. botulinum, Group II C. perfringens Escherichia coli 0157:H7 Gluconobacter spp. Lactobacillus brevis L. plantarum Lactococcus lactis Listeria monocytogenes Plesiomonas shigelloides Pseudomonas fragi Salmonella spp. Shewanella putrefaciens Shigella flexneri S. sonnei

Staphylococcus aureus Vibrio parahaemolyticus Yersinia enterocolitica

(primarily yeast and molds) grow below pH 4.0, several are capable of survival at low pH (Table 9.1). Growth and survival at low pH values are dependent on the type of microorganism and on other factors such as temperature, acid type, salt level, food composition, and the presence of preservatives (e.g., potassium sorbate or sodium benzoate). Microorganisms are generally more susceptible to pH change during early or logarithmic growth phases, in which rapid growth occurs, than during stationary or resting growth phases.

As shown in Tables 9.2 and 9.3, the acidity of food products is highly varied. Thus growth and survival of microorganisms also vary among food systems. In general, less acidic products, including meats, seafood, and vegetables, are more susceptible to bacterial spoilage as well as to pathogenic growth. High-acid products, such as fruits, fruit juices, soft drinks, vinegar, and wines, possess pH values below those at which bacteria usually grow. Therefore, it is common for these products to undergo yeast and mold spoilage and not bacterial spoilage (one exception being the spoilage of certain fruit juices by lactic acid bacteria and other acid-tolerant bacteria).

The pH of some foods is inherent, but in others the pH may be affected by the action of certain microorganisms. This effect, known as biological acidity, is seen in lactic acid-fermented products such as cheese, cultured dairy products, sauerkraut, and pickles. Some foods resist changes in pH caused by

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