TABLE 9.5. Approximate Minimum aw Values for Growth of Microorganisms Important in Foods"

Organism(s) aw


Most spoilage bacteria 0.9

Most spoilage yeast 0.88

Most spoilage molds 0.80

Halophilic bacteria 0.75

Xerophilic molds 0.61

Osmophilic yeast 0.61

Specific organisms

Clostridium botulinum, type E 0.97

Pseudomonas spp. 0.97

Acinetobacter spp. 0.96

Escherichia coli 0.96

Enterobacter aerogenes 0.95

Bacillus subtilis 0.95

Clostridium botulinum, types A and B 0.94

Candida utilis 0.94

Vibrio parahaemolyticus 0.94

Botrytis cinerea 0.93

Rhizopus stolonifer 0.93

Mucor spinosus 0.93

Candida scottii 0.92

Trichosporon pullulons 0.91

Candida zeylanoides 0.90

Staphylococcus aureus 0.86

Alternaria citri 0.84

Pénicillium patulum 0.81

Aspergillus glaucus 0.70

Aspergillus conicus 0.70

Zygosaccharomyces rouxii 0.62

Xeromyces bisporus 0.61

yeast and molds grow over a wider aw range than bacteria, which usually require a higher water activity. For example, most spoilage bacteria will not grow below an aw of 0.91, whereas molds can grow as low as 0.81 aw. Among bacterial pathogens, S. aureus can grow as low as 0.84 ¿/w, but its toxin production may be reduced. Clostridium botulinum cannot grow below 0.94 aw.

Relationships between aw, temperature, pH, £),, and nutritional factors can influence the growth of many foodborne microorganisms. For example, at any given temperature, lowering aw reduces the ability of microorganisms to grow. The aw range that allows growth of a specific microorganism can be extended by the presence of certain nutrients or growth factors. Table 9.5 lists approximate minimum aw values for some foodborne microorganisms.

Although microorganisms do not grow in dehydrated food products, they are generally capable of surviving in them. Long-term survival of Salmonella at low aw is well documented. Species variability in survival characteristics has been shown for Salmonella during long-term (19 month) storage of chocolate and cocoa products and nonfat dry milk (Tamminga et al., 1977). L. monocytogenes has also been shown to survive the manufacture and long-term storage of nonfat dry milk (Doyle et al., 1985).

In general, microorganisms grown at suboptimal aw accumulate compatible osmoprotective solutes such as K+, glutamine, glutamate, proline, sucrose, trehalose, and polyols (i.e., glucosylglycerol) to counteract osmotic stress. Such solutes accumulate through cellular synthesis or increased transport. For example, although enteric pathogens such as E. coli and S. typhimurium exposed to adverse aw do not synthesize proline as a protective measure, they nonetheless accumulate proline by enhanced transport into the cells (Grothe et al., 1986). L. monocytogenes is able to accumulate several osmoprotectants (primarily carnitine) when grown under unfavorable osmotic conditions (Beumer et al., 1994). Many researchers (see Park et al., 1995) believe that growth of L. monocytogenes at 4°C is caused by its accumulation of glycine betaine. Salmonella oranienburg grown at suboptimal water activity levels has demonstrated an elevation in respiratory activity in the presence of proline (Townsend and Wilkinson, 1992).

Overall, the effect of reduced water activity on the nutrition of microorganisms appears to be of a general nature, because cell metabolism depends on reactions in an aqueous environment. Microorganisms that can grow under extreme water activity conditions do so by virtue of their ability to concentrate salts, polyols, and amino acids to internal levels sufficient not only to prevent water loss but to allow the microorganism to extract water from its environment.

Nutrient content To grow, microorganisms require, besides water, (1) an energy source, (2) a nitrogen source, (3) vitamins (especially B vitamins) and related cofactors, and (4) minerals. Energy sources for microorganisms include simple sugars, alcohols, and amino acids. Very few microorganisms are able to metabolize polysaccharides such as starch, cellulose, and glycogen (which must first be degraded to simple sugars), and few can utilize fats. The primary nitrogen source for food microorganisms is amino acids; some species can also hydro-lyze and use more complex nitrogen sources such as peptides and proteins.

B vitamins are found in most foods at levels adequate to support the growth of microorganisms (such as gram-positive bacteria) that cannot synthesize these vitamins. Gram-negative bacteria and yeast can synthesize B vitamins and as a result can grow in and on foods low in B vitamins. Fruits tend to fall into this category, which (along with their usually low pH and positive Eh) rnay help explain why fruits are generally spoiled by molds rather than bacteria.

Structure The coverings of many foods help prevent the entry of microorganisms and subsequent food damage and spoilage. For instance, the skin of fish and meats tends to dry out faster than the flesh it covers, retarding spoilage. Fruits and vegetables are also usually covered by skins and spoil faster when these are damaged or broken than when they are intact.

Extrinsic Factors Affecting Growth and Survival

Extrinsic factors are those factors associated with the storage environment that can affect both a food and the associated microorganisms. These include heat treatment, storage temperature, relative humidity of the environment, presence and concentration of gases, and presence and activity of other microorganisms.

Heat treatment Food products may be subjected to a variety of treatments that eliminate or reduce the potential for pathogenic microorganisms. The most common approach is heat treatment, including pasteurization, sterilization, and cooking. Microorganisms vary in their heat resistance, with the most heat stable being termed thermodurics. With exception of the spore formers (e.g., Clostridium and Bacillus), most microbial pathogens can be destroyed by high-temperature heating. However, certain bacterial toxins (e.g., the S. aureus enterotoxin) as well as some viruses (e.g., hepatitis A) are relatively heat stable (Cliver, 1994).

Selection of appropriate process parameters of temperature and heating time for a particular food is based on the properties of the most heat-resistant pathogen associated with the product, heat penetration and transfer characteristics, and compositional parameters. In commercial sterilization, appropriate heat treatment is applied to achieve a 12-log reduction of test spores of higher heat resistance than C. botulinum. Pasteurization is a milder heat treatment applied to destroy pathogens likely to be associated with a specified food system. For example, the heat treatment involved with milk pasteurization is based on the destruction of Coxiella burnetii (the causative agent of Q fever), and the pasteurization requirements for egg products are designed to destroy Salmonella.

Storage temperature Because microorganisms grow over a wide temperature range, it is important to select proper food storage temperatures to help control their growth. The general effect of temperature on microbial activity is shown in Figure 9.1. The lowest temperature at which microorganisms are known to grow is -34°C (—29°F), and the highest is slightly over 100°C (212°F). Microorganisms are generally categorized into three groups based on their growth temperature requirements. The largest category are the meso-philes, which grow well between 20°C (68°F) and 45°C (113°F). Those that grow well between 55°C (131°F) and 65°C (149°F) are called thermophiles. The foodborne thermophilic bacteria of most importance belong to the genera Bacillus and Clostridium. Finally, some mesophiles, termed psychrotrophs, are

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