EFFECT OF GLYCEROL ON NONENZYMATIC BROWNING IN A SOLID INTERMEDIATE MOISTURE MODEL FOOD SYSTEM

Maillard browning is one of the main chemical reactions causing deterioration and shortening shelf life of intermediate moisture food (IMF) systems. The purpose of this research was to study Maillard browning in an IMF model system containing casein, glucose and the liquid humectant glycerol. The kinetics of pigment production, glucose utilization and loss of DNP‐available lysine were studied as a function of temperature, moisture content and water activity. It was found that the factors which control reactant (glucose and available lysine) utilization also control end‐product (brown pigment) accumulation. The rate of the Maillard browning pigment production, after an initial induction period, follows zero order kinetics. The initial loss rate of both glucose and available lysine, however, follows first order kinetics. Exceedingly large nutritional (available lysine) losses occur before brown discoloration is appreciable. Slightly greater than one mole of glucose reacts per mole of lysine made unavailable. Based on this, nutritional losses may be relatively easily estimated by monitoring the loss of specific reducing sugars. The Maillard browning reaction proceeds 33 times faster at 45°C than at 2S°C, with the maximum rate occurring at 0.4–0.5 a_w. This is an a_w range considerably lower than the 0.65–0.75 a_w range usually found for maximum browning in dehydrated foods. The downward shift in the a_w maximum for browning is because glycerol being liquid has water‐like properties and increases reactant mobility and/or solubility at a_w's below which most water soluble reactions occur very slowly.