case, water activity would be the determinant variable. Another example
may be the curve for chymosin in Figure 8.11. Here,kis already greatly
decreased at w values where neither aw nor Deff can have changed
appreciably, and it must be assumed that concentration of a stabilizing
component has occurred. In many cases, water removal causes a significant
change in pH.
It must unfortunately beconcludedthat the relations between reaction
rate and water content of foods vary widely and that our understanding of
this is incomplete. This means that one often has to rely on experimental
determination.
8.4.3 Microbial Growth
It is well known that microorganisms cannot proliferate in dry foods and
thus cannot cause spoilage. It depends, of course, on how dry the food is,
and Figure 8.12 gives some examples of growth rate as a function of water
FIGURE8.12 Relative growth rate (arbitrary scale) of some microorganisms as a
function of water activity (aw). (a) A xerophilic mold,Xeromyces bisporus. (b) A
common mold,Aspergillus flavus. (c) A yeast,Saccharomyces cerevisiae. (d) A
bacterium,Salmonellasp.