Lipid Oxidation, Secondary Reactions, and Water Activity of Foods

Abstract

12 Water acts as an antioxidant at very low levels by decreasing the catalytic activity of metal catalysts, by promoting recombination (quenching) of free radicals, and by promoting nonenzymatic browning which causes production of active antioxidants. Hydration of lipid hydroperoxides, or their concentration at lipid-water interfaces, also changes the mechanism of hydroperoxide decomposition and reduces the rate of free-radical formation. At high water activities, oxidation is accelerated by increased mobilization of components that are made nonreactive at low water activities by being trapped or "encapsulated" within a matrix of nonreactive food components. Water plasticizes this matrix and makes it permeable to various reactants and catalysts. This mechanism of oxidation acceleration at high water activities was proposed several years ago by our group on the basis of kinetic studies of lipid oxidation in cellulose matrices at different water activities. Most recently, this mechanism has been demonstrated unequivocally by experiments in which reactive fatty acids were freeze-dried from solutions containing carbohydrates. After drying, the fatty acid was present in two states, as surface lipid and entrapped lipid. The surface lipid was dispersed on cellulose surfaces or localized as small droplets and was very reactive. The remainder was entrapped