Aspartame degradation as a function of "water activity"

The incorporation of aspartame into an increasing number of foods necessitates evaluation of its degradation kinetics as a function of "water activity" (aw). The kinetics of degradation were followed in model systems as a function of initial pH, temperature, and aw. An increase in aw, for each 0.1 units in the 0.3 to 0.7 range, resulted in about a 30-80% increase in degradation rate, which then decreased only slowly up to dilute solution. The presence of oil increased the degradation rate at high aw, but glucose had no effect on the rate of aspartame loss. The activation energies for loss ranged from 25 to 20 kcal/mole, decreasing as aw increased, as expected. The rates as a function of pH showed that the actual pH of the water in the condensed phase, based on the Bronsted relationship, may be very different than the initial pH. This caused a shift in the pH at which the fastest rate of degradation occurred, as aw increased.