Influence of Water Activity on Growth, Metabolic Activities and Survival of Yeasts and Molds.

The behavior of yeasts and molds as influenced by water activity (a) is reviewed. Fungal spoilage of foods occurs more often than bacterial spoilage at a 0.61-0.85 not because fungi grow faster at reduced a but rather because the competitive effects of the vast majority of bacteria are absent. Higher a is generally required for spore formation than for spore germination. The range of a permitting germination of spores is greatest at an optimum temperature, but optimum availability of nutrients tends to broaden the range of a and temperature at which germination and growth will occur. The minimum a levels for growth of fungi are lower than those required for mycotoxin production. It is imperative that diluents and enumeration media with reduced a be used to detect xerotolerant fungi in foods. Otherwise, vegetative cells and spores may be killed by osmotic shock or remain dormant when exposed to high a associated with diluents and media routinely used for mycological analyses.