Control of Microbial Growth in Foods

In virtually every food, there is a need to control the growth of microorganisms to ensure both safety and quality. Even in fermented foods, conditions must be established during the fermentation process to enhance the growth of the starter culture, but more importantly to retard the growth of contaminating flora. In foods, the shelf life of a product is defined by the maximum time that a product can be stored under the prescribed conditions before either its quality and/or its safety falls below a set limit. Shelf life can vary from days to years; it has been suggested that the shelf life of properly processed canned foods is virtually indefinte.

There are two basic factors that control the growth of microorganisms in foods, the extrinsic and the intrinsic factors. The extrinsic factors include the environment surrounding the food, (e.g., the oxygen tension), whereas the intrinsic factors are the properties of the food itself (e.g., the pH or the water activity). Microorganisms as a collective population are capable of growth over a wide range of both intrinsic and extrinsic factors. There are, for example, microorganisms that can grow at refrigerated temperatures (4°C) and others that can grow at extremely high temperatures exceeding 100°C.

Although a given microbial population can be controlled through the judicious selection of intrinsic and extrinsic factors, there is virtually no food system that can be formulated that will totally exclude the growth of all microorganisms. The microbial flora will change depending on the extrinsic and intrinsic factors. For example, at low water activity the molds may predominate. They would normally be excluded by the more rapidly growing bacterial flora except that the growth of the bacterial flora is usually suppressed at water activities of less than 0.8. Similarly, at low pH, certain bacteria, including Lactobacillus, may predominate to the exclusion of other organisms. Some microorganisms can alter their environment or the intrinsic properties of the food to exclude the growth of other competitive microflora. This is the basis for fermentation processes as described in a previous section.

The shelf life of a product is also determined by the initial microbial load in the product, therefore one of the most fundamental steps in food product manufacture is processing. Processing can be accomplished using a variety of technologies, including heating, drying, and freezing. Heating or thermal processing was perhaps the single greatest advance in food manufacturing by providing a significant in crease in the shelf-life and hence the widespread distribution of the product. Prior to commercial thermal processing, the shelf life of most products was limited, perhaps with the exception of fermented foods.

Thermal processing can range from mild heating through pasteurization to heating regimens known as 12-D kills. This latter term was specifically designed for thermal processing of canned foods, where the concern was to eliminate the potential for botulism. Two key terms in thermal processing are the D-value and Z-value. The D-value is the time necessary to reduce the microbial population 10-fold at a prescribed temperature. For example, the D-value for C. botulinum can be as high as 30 min at 100°C. D-values can vary several 100-fold, with the spore formers being the most thermal resistant. A 12-D process is the time and temperature necessary to reduce the population of a standard spore-forming test organism (e.g., FS617) by 12 logarithms. FS is a term for a microorganism whose growth in a canned food leads to a flat (F) and sour (S) condition.

The flat refers to the lack of bulge in the can, due to a lack of gas production. The sour reflects the low pH of the spoiled product, hence acid production. In theory, a 12-D process should reduce the likelihood of C. botulinum to less than one in 10¹² cans.

The Z-value refers to the change in temperature necessary to effect a 10-fold difference in the D-value. Heating, in addition to destroying microorganisms, causes a deterioration in the organoleptic properties (i.e., most notably the texture) and the nutrient content of foods. D-values and Z-values can be calculated for these nonmicrobial destruction processes as well. However, the Z-values do differ dramatically and therefore can be used to optimize the destruction of the microorganism with only a limited loss in texture or vitamin content. In general, as the temperature is increased, the rate of vitamin or texture loss is lower in comparison to the inactivation of microorganisms.

Therefore “high-temperature, short-time” (HTST) process such as that used in certain milk products is preferred. Not all food products can be processed using HTST regimens, and this is particularly true of particulate foods or any food that has a low thermal conductivity.