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and Figure 3.8 demonstrates the interaction between temperature and
water activity forAspergillus flavusandPenicillium expansum.
Figure 3.9 shows the range ofawvalues associated with a number of
different food commodities. Because low water activities are associated
with three distinct types of food three terms are used to describe the
micro-organisms especially associated with these foods:

(i)halotolerant– able to grow in the presence of high concentra-

tions of salt

(ii)osmotolerant– able to grow in the presence of high concentra-

tions of unionized organic compounds such as sugars

(iii)xerotolerant– able to grow on dry foods.

These terms do not describe rigidly exclusive groups of micro-organ-
isms but are useful in the context of studies of particular food commod-
ities. Some micro-organisms actually grow better at reducedawand may
be described as halophilic, osmophilic or xerophilic, indeed the halo-
bacteria are obligately halophilic and cannot grow in the absence of high

Table 3.10 Minimum water activities at which

active growth can occur

Group of micro-organism Minimum aw

Most Gram-negative bacteria 0.97

Most Gram-positive bacteria 0.90

Most yeasts 0.88

Most filamentous fungi 0.80

Halophilic bacteria 0.75

Xerophilic fungi 0.61

Figure 3.7 Range of awvalues allowing growth of a number of species of micro-organisms

40 Factors Affecting the Growth and Survival of Micro-organisms in Foods