the handbook of food engineering practice crc press chapter 10 ...

The critical a w limits for microbial growth and the relative rates of reactions
important to food preservation such as lipid oxidation and nonenzymatic browning can be
seen in this figure (Fig.7). The underlying reasons for this behavior has been the subject of
several studies (Taoukis et al., 1988a). Most reactions have minimal rates up to the
monolayer value. Lipid oxidation shows the peculiarity of a minimum at the monolayer
(m o ) with increased rates below and above it (Labuza, 1975; Quast et al., 1972).
The proposed theories that attempt to explain the effect of a w on food
deterioration reaction as well as ways to systematically approach and model this effect are
discussed by Labuza (1980b). The moisture content and water activity can influence the
kinetic parameters (k A , E A ), the concentrations of the reactants and in some cases even the
apparent reaction reaction order, n. Most relevant studies have modeled either k A as a
function of a w (Labuza, 1980b) related to the change of mobility of reactants due to a w
dependent changes of viscosity, or E A as a function of a w (Mizrahi, et al., 1970 a; b). The
inverse relationship of E A with a w (increase in a w decreases E A and vice versa) could be
theoretically explained by the proposed phenomenon of enthalpy-entropy compensation.
The applicability of this theory and data that support it have been discussed by Labuza
(1980a).
Additionally moisture content and a w directly affect the glass transition
temperature of the system. With increasing a w , T g decreases. As was discussed in the
previous section, transverse of T g and change into the rubbery state, has pronounced
effects, especially in texture and viscosity depended phenomena but also in reaction rates
and their temperature dependence. It has been proposed for dehydrated systems that a
critical mosture content / a w alternative to the monolayer value of the BET theory, is the
value at which the dehydrated system has a T g of 25° C ( Roos,1993). Consideration of
these critical values contribute to explain textural changes occuring at distinct a w and
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