Factors Influencing the Growth of Microorganisms in Foods

Select microorganisms have a competitive advantage in those foods with characteristics that enable them to multiply most rapidly. Understanding the factors that influence microbial growth is essential to maintaining food quality, whether utilizing microorganisms to produce fermented foods, or suppressing organisms to prolong the shelf life of perishable foods.

The conditions naturally present in the food, such as moisture, acidity, and nutrients, are called intrinsic factors. Environmental conditions, such as the temperature and atmosphere of storage, are called extrinsic factors. All of these factors combine to determine which microorganisms can grow in a particular food product and the rate of that growth. For example, bacteria are well suited for growth in the environment found in fresh meat and other moist, pH-neutral, nutrient-rich foods. Although other microorganisms, such as yeasts and molds, can also grow under these conditions, the more rapid increase of bacteria overwhelms these competitors. Different types of bacteria also

Figure 32.2 Examples of Spoiled Food (a) Apples infected by the fungus Venturia inaequalis, cause of apple scab. (b) Orange affected by a Pénicillium mold. (c) Ear of com and a lemon found after several weeks in the author's refrigerator.

compete with one another. On moist, pH-neutral foods, members of the genus Pseudomonas tend to flourish. Slightly acidic foods inhibit Pseudomonas species, however, and the slower-growing lactic acid bacteria become dominant. When conditions such as lack of moisture or high acidity restrict the growth of bacteria, fungi predominate despite their relatively slow growth.

Intrinsic Factors

The multiplication of microorganisms in a food is greatly influenced by the inherent characteristics of that food. In general, microorganisms multiply most rapidly in moist, nutritionally rich, pH-neutral foods.

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