ABSTRACT
Microorganisms require the presence of available water for growth and metabolism. This available water is best measured by aw. The term Ow has been accepted as predicting inhibition of growth and metabolism by solutes or desiccation. Ow is not the moisture content. Moisture content cannot even make a rough estimate of aw. Microorganisms differ in their response to aw· A reduction in aw generally results in a decrease in growth rate (increase in generation time) (11-13), enzyme activity (11,14,15), sporulation (6,11), and toxin production (11). Generally, yeasts and molds can grow at lower Ow than bacteria. Minimum aw allowing bacterial growth is listed in Table 3.3. Table 3.4 shows the minimum Ow for growth of selected yeasts and molds. The minimum aw for growth for most bacteria is 0.90. Staphylococcus aureus, an exception, can grow at an Ow of 0.86 or above. Salt-loving (halophilic) bacteria grow at a very low aw of0.75 but, have not been reported as a spoilage problem in cosmetics or drugs ( 16,17). Osmophilic (xerophilic, "dry-loving") yeasts can grow down to an Ow of 0.75, but most yeasts require at
Water Activity and Self-Preserving Formulas
TABLE 3.3 Minimum Sw Permitting Growth of Selected Bacteria Bacterium Sw
Bacillus cereus 0.95 B. megsterium 0.95 B. stesrothermophilus 0.93 B. subtilis 0.90 Clostridium botulinum type A 0.95 C. botulinum type B 0.94 C. botulinum type E 0.97 C. perfringens 0.95 Escherichia coli 0.95 Hslobacterium halobium 0.75 Microccoccus ha/odenitrificans 0.86 M. lysodeikticus 0.93 Pseudomonas fluorescens 0.97 Salmonella spp. 0.95 Staphylococcus aureus 0.86 Source: Adapted from Ref. 26.
