Water Activity and Plasticization - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 21

Reactions in amorphous foods are complex and they may be controlled by several
factors, including T g , and the use of a single relationship, whether Arrhenius or WLF,
to model temperature dependence of food deterioration is often inadequate. 73 It
should also be noticed that the rate constant is not likely to follow the WLF-type
temperature dependence unless the reaction is fully diffusion-controlled and the
diffusion coefficient follows the WLF relationship. This is probably the situation in
milk powders, as an obvious increase in the rate constant is observed at the T g of
amorphous lactose. 83 However, Roos et al. 83 found that the amount of water produced
in the nonenzymic browning reaction may be significant and enhance the reaction
as a result of additional plasticization.
The above studies on relationships between T g and the rate of the nonenzymic
browning reaction have suggested that the reaction may become diffusion-controlled
and that the rate may be affected by T g . However, the rate constant is dependent on
a number of other factors that include temperature, water content, and structural
transformations. 74 The size of the reactants may also be an important factor that
affects rates of diffusion-controlled reactions. 74 It may be assumed that the rate of
diffusion decreases with increasing size of the diffusant. The temperature and water
content have the most important effects due to the fact that an increasing temperature
increases the true rate constant and it is also dependent on water content.
Other Reactions
Water plasticization may also have an effect on rates of vitamin destruction during
food storage. Dennison et al. 84 studied the effect of a w on thiamine and riboflavin
retention in a dehydrated food system. The product was a starch-based food model
that probably had a high T g . They found that the retention of the vitamins was high
after a storage period of 8 months at 20 and 30°C and a w < 0.65. A substantial loss
of thiamine and riboflavin was noticed at 45 and 37°C and a w > 0.24, respectively.
It may be assumed that water and thermal plasticization occurred in the freeze-dried
model and caused the observed degradation of the nutrients.
Nelson 72 observed rates of ascorbic acid degradation within freeze-dried non-
crystallizing maltodextrin matrices at various temperatures and water contents. The
reaction may occur through a number of pathways, but Nelson 72 assumed that the
reaction involved diffusion of small molecules such as oxygen in the system studied.
Nelson 72 found that the rate of ascorbic acid degradation generally increased with
increasing temperature. The reaction also occurred at temperatures below T g , prob-
ably because of the small size of the diffusing oxygen molecules. Application of the
Arrhenius model to describe the temperature dependence showed a change in the
activation energy of the reaction in the vicinity of T g for a system that had a relatively
low water content. The material with higher water contents had Arrhenius plots with
a continuous line or no data were obtained below T g . Nelson 72 found that the kinetics
of the reaction were affected by structural changes. The WLF relationship failed to
describe the temperature dependence of the reaction. Bell and Hageman 85 found that
aspartame degradation in a poly(vinylpyrrolidone) (PVP) matrix also occurred below
T g and that the rate at room temperature was more dependent on a w than on the state
of the system.