Water Activity and Plasticization - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 18

The effect of a glass transition on reaction rates or quality changes can be
observed from experimental data. The change in diffusivity, D, above T g may be
dramatic and substantially larger than it would be according to the Arrhenius-type
temperature dependence, but the difference in the observed and the true reaction
rate constants has been found to be relatively small and the apparent change in the
rate constant occurs within a relatively narrow temperature range above T g . 5 In
practice, rates of diffusion-controlled reactions have been determined for foods above
T g and the rates have followed the Arrhenius-type temperature dependence. 72 It
should be noticed that the change in diffusion at a constant temperature as a result
of water plasticization may be even more dramatic when observed as a function of
a w or water content than as a function of temperature. This may reflect a change in
diffusivity as the concentration of reactants at low water contents is seldom a
limitation for the reaction.
If a change in a quality factor is diffusion-controlled and dependent on viscosity,
the observed rate constant may become a function of the true rate constant and
diffusivity. 73 In some studies, the observed rate constants of diffusion-controlled
reactions and quality changes in amorphous food matrices have been assumed to be
proportional to 1/

, which has led to the direct application of the WLF relationship
in the form of Eq. (1.12), where k

η

′ is the observed rate constant and k ′ s is the observed

rate constant at a reference temperature, T s .

− ( )
+−

CT T
CTT

−

k
k

′
′

1 S

log

s

= (1.12)

( )

2 S

Application of Eq. (1.12) to model rates of nonenzymatic browning at several
water contents has failed in most cases. 74,75 The use of Eq. (1.12) may be justified
when the material has a constant water content and the constants C 1 and C 2 have
been derived from experimental data. 72
Rates of diffusion-controlled reactions are likely to be affected by temperature
and water content in addition to glass transition. 73 Nelson 72 studied effects of T g on
kinetic phenomena, including crystallization of amorphous sugars and rates of chem-
ical changes in food materials. She found that deteriorative reactions occurred at
temperatures below T g , but a large increase in the rates of several reactions in the
vicinity of T g was evident. These findings were in agreement with rates of non-
enzymatic browning obtained for various amorphous foods and food models. 76
Nelson 72 concluded that the proper application of the WLF model in predicting
kinetic data involves determination of the WLF coefficients. The coefficients, C 1 and
C 2 , are dependent on the system and also on its water content. However, the rates
of chemical changes in rubbery matrices, instead of following the WLF model, often
followed the Arrhenius-type temperature dependence. 19,72

E FFECTS OF W ATER AND G LASS T RANSITION ON R EACTION R ATES

A number of studies have reported kinetic data for observed reaction rates in low-
moisture foods. These data have often been reported as a function of a w or water