Maillard reaction kinetics in milk powder: Effect of water activity at mild temperatures

The kinetics of the Maillard reaction in skim milk powder was studied at a wide range of water activities (0.31–0.98) under storage at mildly elevated temperature conditions (37, 50 and 60 °C). The reaction rate was determined by the loss of available lysine content using the o-phthaldialdehyde method. Water activity affected the rate of loss of lysine above the glass transition temperature only at high water activity values. However, the mobility of the reactants seemed to play a role in damage to lysine, as the rate constant decreased considerably at temperatures close to the glass transition. Crystallization of lactose did not affect the rate constant values. Water activity did not influence the temperature-dependence of the reaction rate, although a significant increase in activation energy was observed in the vicinity of the glass transition temperature.