Conclusions
In contrast to the results obtained in our previous study with
a milk-like model system, the effect of aw above the glass transition
on the rate of loss of lysine by mild heat treatment or during storage
of milk powder was only significant at high aw values, and the
temperature-dependence of the reaction rate was not affected by
aw. However, the mobility of the reactants seemed to play a role in
lysine damage. Although the Maillard reaction did not stop in the
glassy state, the rate constant decreased considerably at temperatures
close to Tg. The influence of aw on Maillard reaction kinetics in
milk powder can be interpreted as a consequence of concentration
and diffusion of reactants. A dilution effect was observed at aw over
0.7, which was independent of the temperature of the system, at
least up to mild temperatures. At low aw, loss of lysine decreased
only at temperature conditions where the high viscosity affected
molecular diffusion. For this reason, as milk powder is commonly
exposed to extended periods of storage, it is critical to keep the
system at temperature and aw conditions below the glass transition
in order to minimize its nutritional damage.