Lipid Oxidation, Secondary Reactions, and Water Activity of Foods
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 within a carbohydrate matrix and was inaccessible to atmospheric oxygen. When the surface lipid was washed away with hexane, oxidation stopped entirely. When the matrix was plasticized by increasing water activities, oxidation resumed immediately, showing water’s key role in promoting access to reactants.
In reactions between lipid peroxides and their secondary breakdown products and either amino acids or proteins, water activity had a profound impact on the course of these reactions. Among the reactions initiated by lipid oxidation which were shown to be affected significantly by water activity are: crosslinking of proteins (lysozyme), enzyme inactivation by lipid peroxidation products (lysozyme), protein scission (gelatin), and destruction of amino acids and amino acid residues (histidine, tryptophan, and lysine). Production of free radicals in proteins as a result of exposure to hydroperoxides and their decomposition products is also strongly affected by water activity levels, as is production of fluorescent pigments after oxidation.
KeywordsElectron Spin Resonance Water Activity Lipid Oxidation Methyl Linoleate Electron Spin Resonance Signal
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