It is generally observed, for heat inactivation of enzymes or killing of
bacteria, that at a smallerwboth the (apparent) activation enthalpyDH{
and the activation entropyDS{decrease. This implies that the temperature
dependence of the reaction becomes less; generally, the relative change in
DS{is greater than that inDH{, causing the reaction rate constant to
decrease, but this is not always so. These relations are poorly understood,
but they are of great practical importance for the stability of enzymes and
microbes during drying.
- Enzyme activity(e.g., Figure 8.10e) may change (decrease) because
the concentration of components affecting protein conformation increases,
especially if it affects the active site of the enzyme; this may involve ionic
strength, pH, and solvent quality. It is difficult to separate this effect from
that of decreased diffusivity. - Catalysts and inhibitorsalso change in concentration with water
content. Presumably, Figure 8.10d provides an example. Here water would
be an inhibitor of one or more of the reaction steps in oxidation. In such a
FIGURE8.11 First-order reaction rate constantskfor heat inactivation, plotted
against mass fraction of waterw. AP: alkaline phosphatase, in skim milk, 80 8 C. Ec:
killing ofEschericia coli, in skim milk, 63 8 C. Ch: chymosin, in whey, 80 8 C. Li:
lipoxygenase, in sucrose/calcium alginate, 72 8 C.