Influence of the low-moisture state on pH and its implication for reaction kinetics

The kinetics of aspartame degradation as pH-rate profiles and patterns of degradation product formation in a semi-solid gel made with high and low moisture contents indicated that the actual pH of the aqueous phase in reduced-moisture solid systems was significantly different from the initial 'wet' pH. It is the pH of the dehydrated system rather than the pH before moisture removal that indicates the true activity of hydronium and hydroxyl ions that govern acid/base catalyzed reactions. As an example, the mechanisms of aspartame and ampicillin degradation changed as the lower moisture content changed the pH. In all cases, the pH decreased as the water activity (a_w) was lowered. Enthalpy-entropy compensation predicts that the rate constants for most aqueous phase reactions decrease with decreasing a_w. It has been observed that the rates and pseudo-order rate constants for many reactions have a maximum between a_ws of 0·6 and 0·8. In evaluating the rate constants, the concentration of hydronium or hydroxyl ions is either ignored or incorporated as an unknown constant into the pseudo-order rate constant, even when the reaction is studied at several pH values and a_ws. Conditions are observed where reaction rates decrease or increase as a function of initial pH at constant a_w. No explanation has been provided for this. The actual hydronium or hydroxyl ion concentration in the limited moisture phase over the a_w range may possibly explain the observed effect on rates. To obtain a complete picture of the influence of a_w on reaction kinetics, both the rate constant and reaction rate should be evaluated as a function of a_w, taking into account the actual reactant concentrations that include hydronium or hydroxyl ions.