Water activity microbial growth death and survival

The optimum aw for growth of the majority of microorganisms is in the range 0.99-0.98. Every microorganism has limiting aw values below which it will not grow, form spores, or produce toxic metabolites (Beuchat, 1987). Considering aw in relation to microbial stability, the minimum aw values that permit microbial growth for different types of microorganisms are of great concern. Table 8.1 presents the aw values of various foods and their associated microbial spoilage, showing also the classification of microorganisms into osmosensitive and osmotolerant. Extensive tables with minimum aw values for growth and toxin production of several pathogenic and spoilage microorganisms have been reported by many authors (Corry, 1973, Beuchat, 1983, 1987, and Gould, 1989). Table 8.2 shows the minimal aw for growth of selected microorganisms at their optimal conditions of pH, nutrient availability and temperature. Several findings obtained from literature data and Table 8.2 can be summarized as follows:

• aw limits for growth differ between microorganisms. In general, common spoilage bacteria are inhibited at an aw about 0.97; clostridial pathogen at aw 0.94; and most Bacillus species at aw 0.93. Staphylococcus aureus is the most aw-tolerant pathogen, and can grow in aerobiosis at aw 0.86 and in anaerobiosis at aw 0.91. Many yeasts and moulds are able to proliferate at an aw below 0.86, while some osmophilic yeasts and xerophilic moulds are capable of slow growth just above 0.6. So, to preserve a food by using only a reduction in aw as stress factor, its aw should be at least lowered to 0.6. Fully dehydrated foods, for instance, have aw about 0.3 in order to control not only microbial growth but other physico-chemical and biochemical reactions deleterious to colour, texture, flavour and nutritive value of foods.

• Minimum aw for growth is always equal or lower than minimum aw for toxin production.

• Minimum aw for growth depends on the solute used to control aw. Gould (1989) recognized that in some instances solute effects may depend on the ability of the solute to permeate the cell membrane, as in the case of glycerol, which readily permeates the membrane of many bacteria and therefore has a lower inhibitory water activity. Chirife (1994) discussed in detail the 'specific solute effect' for S. aureus. He concluded that the inhibitory effects of solutes most often present in low aw preserved foods, such as NaCl and sucrose, are

Table 8.1 Water activity values of selected foods and associated microbial spoilage

Range of aw Microorganisms inhibited Examples of foods

Table 8.1 Water activity values of selected foods and associated microbial spoilage

Range of aw Microorganisms inhibited Examples of foods

1.00-0.95

Some yeasts, Gram-negative rods, bacterial spores

Fresh foods; foods containing 40% sucrose or 7% salt (canned foods, processed cheese, several sausages, bread, etc.)

0.95-0.91

Most cocci, lactobacilli, vegetative cells of bacilli, some moulds

Foods containing 50% sucrose or 12% salt (mayonnaise, bacon, some hard cheeses, raw ham, low-calorie jams, etc.)

0.94

Growth and toxin production by all types of Clostridium botulinum

0.91-0.86

Most yeasts

Foods containing 65% sucrose or 15% salt (dry ham, fruit jams, fruit juice concentrates, some hard cheeses, etc.)

Staphylococcus aureus

0.86-0.80

Most moulds

Foods containing 15-20% water (fruit cake, high moisture prunes, sweetened condensed milk, etc.)

0.80-0.75

Most halophilic bacteria

Foods with 26% salt or very high sugar content (salted fish, molasses, prunes, fondants, etc.)

0.80

Production of micotoxins

0.75-0.65

Xerophilic moulds

Foods containing less than 10% water (dates, figs, nuts, rolled oats, etc.)

0.68

Practical limit for fungi

0.65-0.60

Osmophilic yeasts

Confectionery products, dried fruits containing 15-20% water, honey

Below 0.60

No microbial growth

Dried milk, instant coffee, dried egg, spices, crackers, flour, cereals, etc.

Adapted from Troller (1978).

Adapted from Troller (1978).

primarily related to their aw lowering capacity. But for other solutes such as ethanol, propylene glycol, butylene glycol and various polyethylene glycols, antibacterial effects (attributed mainly to the effects of these molecules on membrane enzymes responsible for peptidoglycan synthesis) are important.

When a microorganism is transferred to a new environment, there are two possible outcomes, survival or death. Microbial survival or death will be based on the ability of the microorganism to adapt in the new environment. The basis

Table 8.2 Minimal water activity for growth of foodborne bacterial pathogens under optimum pH and temperature conditions

Infectious pathogens Campylobacter jejuni Aeromonas hydrophila Shigella spp Salmonella spp Yersinia enterocolitica Escherichia coli Listeria monocytogenes Vibrio parahaemolyticus

Toxinogenic spore-forming pathogens Clostridium perfringens

Clostridium botulinum A & proteolytic B strains Clostridium botulinum E & non-proteolytic strains B and F Clostridium botulinum G Bacillus cereus

Toxinogenic pathogens Staphylococcus aureus (anaerobic) Staphylococcus aureus (aerobic) Staphylococcus aureus (aerobic) Staphylococcus aureus (aerobic) Staphylococcus aureus (aerobic)

Moulds and yeasts Aspergillus flavus Aspergillus parasiticus Botrytis cinerea Byssoclamys nivea Aspergillus ochraceus Penicillium citrinum Penicillium cyclopium Penicillium patulum Eurotium spp Monascus bisporus Saccharomyces cerevisiae

Zygosaccharomyces bisporus Zygosaccharomyces rouxii Torulopsis candida

TP: toxin production.

0.93-0.95 0.90-0.92 0.94 (glycerol) 0.95 (NaCl) 0.96 (sucrose)

0.93 (xylitol) 0.95 (erythritol) 0.89 (glycerol)

0.80

0.81

0.61

0.89 (glucose)

0.90 (sucrose)

0.70

0.65

0.65

for survival and death of microorganisms as influenced by aw is complex. Several intrinsic and extrinsic factors may affect this relation but differ within food types and processes and among types of microorganisms involved. Temperature, oxygen, chemical and other physical treatments are some extrinsic factors that influence microbial spoilage of foods and also the aw-microorganism response.

The effects of temperature on survival of microorganisms are widely documented, the heat resistance of vegetative cells and spores as influenced by aw probably being the most extensively studied area in terms of microbial inactivation (Lenovich, 1987). In general, vegetative cells and spores are more resistant as aw of the heating menstrum is reduced. But the type of solute used to adjust aw to the same value may result in significant differences in the heat resistance of a given microorganism. Ionic solutes may decrease heat resistance at low levels but afford considerable protection at a high concentration. Non-ionic solutes have a variable effect. Larger molecular weight solutes, such as sucrose, exert a protective effect against heat inactivation, while glycerol causes only a little increase in heat resistance. For instance, S. aureus heated in skim milk has a D60°C value of 5.3min while in skim milk plus 57% sugar the D60»C value is of about 22min. For bacterial and fungal spores, the resistance to the lethal effects of heat may increase a thousand times or more at a low aw, usually showing a maximum in the aw range 0.2-0.5 (Mossel et al., 1991).

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