microbes (as well as microbial spores). Inactivation kinetics of enzymes is
often determined by the unfolding kinetics of globular proteins, hence a very
strong dependence of its rate—and of the thermal death rate of
microorganisms—on temperature. Some results are shown in Figure 4.6.
The slope of the log rate constant against 1=Tgreatly differs between
phosphatase inactivation or spore killing and Maillard reaction. (We will
discuss the curve for plasmin in Section 4.4.) Most chemical reactions, like
the Maillard one, are undesirable, whereas killing of microorganisms is
needed. By applying a high temperature for a short time, one may ensure the
latter while minimizing the former.
It may finally be noted that it is often implicitly assumed thatDH{and
DS{do not depend on temperature. This may not be true for reactions
involving changes in hydrophobic interactions; cf. Section 3.2.
FIGURE4.6 Dependence of (pseudo) first-order reaction rate constantsðkÞon
temperatureðTÞ. Approximate examples for heat inactivation of alkaline phospha-
tase and plasmin, for killing ofClostridium botulinumspores, and for the formation
of a certain small amount of Maillard products.t^00 : 1 is the time needed for the
reaction to proceed for 0.1 times the final value (not for the Maillard reaction).