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 temperaturedependence
of the reaction rate, although a significant increase in activation energy was observed in the
vicinity of the glass transition temperature.