DIFFERENTIATING BETWEEN THE EFFECTS OF WATER ACTIVITY AND GLASS-TRANSITION DEPENDENT MOBILITY ON A SOLID-STATE CHEMICAL-REACTION - ASPARTAME DEGRADATION

Recent debates have emerged on whether it is water activity (alpha(w)) or the state of the system as dictated by the glass transition temperature (T-g) that impacts the rates of chemical reactions in reduced-moisture solid systems. Previously, model systems could not evaluate the effects of water activity and glass transition independently. By using poly(vinylpyrrolidone) (PVP) of different molecular weights, the effect of water activity and glass transition on chemical reactions can be studied independently and at a constant temperature. The kinetics of aspartame degradation, via its rearrangement into diketopiperazine, was evaluated in the PVP model system. Reaction rates at constant water activity, but different T-g values, were not significantly different. However, rates at a similar distance from T-g, but different water activities, were significantly different. Thus, the rate of aspartame degradation depends upon the water activity rather than upon the state of the system.