In the study of the oxidation rate of methyl linoleate in protein and cellulose systems, a prooxidant effect was found at intermediate moisture contents. At lowwater content, water hydrates metals and hydrogen bonds with peroxides, and an overall decrease in the rate of lipid oxidation results. With an increase in the water content to the region with a water activity of 0.6 to 0.7, the water predominantly acts as a solvent to dissolve and mobilize previously unavailable trace metals with the result of increased oxidation rates. Use of chelating agents such as ethylenediaminetetraacetic acid and citric acid reduced oxidation significantly although some antioxidant activity was also observed for butylated hydroxyanisole. These results have important implications in the preparation of intermediate moisture foods. INTRODUCTI ON

Enzyme‐catalyzed reactions occur at a wide range of moisture levels, including those corresponding to water activities (aw) well below unity. The model system of microcrystalline cellulose, sucrose and invertase was selected to study the behavior of invertase as functions of aw, time and temperature. The results show that the Onset of reaction occurs below the anticipated mobilization for crystalline sucrose (aw 0.81) and that measurable reaction occurs at an aw as low as 0.58. The Maximum Extent of reaction is independent of moisture content at or above aw 0.75. The overall Rate of reaction for a given treatment fitted a first‐order kinetic model. Rate constants (a) obeyed Arrhenius plots (25° to 47°C) with activation energies (Ea) varying from 15.7 to 4.8 Kcal/mole for aw's from 0.58 to 0.90, respectively, (b) increased in direct proportion to enzyme concentration, and (c) increased strongly and nonlinearly with aw and moisture. A model with two rate‐limiting steps has been postulated. At high moisture content, sucrose diffusion through a resistance shell confined to the vicinity of an invertase molecule is limiting. At low moisture content, aw and temperature may affect the conformation of the enzyme itself, or the diffusion coefficient in the rate‐controlling resistance shell.

Methyl linoleate was oxidized in model systems consisting of either cellulose or casein with which the lipid was dispersed with water containing cobalt salts. The dispersion was extruded into Warburg flasks, frozen and freeze‐dried at 100 µ Hg and with platen temperatures of 80 F. The samples were then humidified over saturated salt solutions to give moisture contents from less than 1 g H2O/100 g solids up to 30 g H2O/100 g solids. The higher moisture contents were obtained by addition of glycerol to the model system during preparation and humidification at 60–75% RH. Chelating agents including EDTA and citric acid in concentrations of 1 to 10 moles per mole of cobalt ion were used in some experiments. Oxidation was followed manometrically and by peroxide analysis. At low water contents, water acts as an antioxidant through hydration of metallic catalysts. As the moisture content increases, water promotes oxidation through its solvent activity. In the region of capillary condensation, antioxidant effect of metal hydration is overshadowed by the prooxidant effect of metal solubilization. The water soluble chelating agents such as EDTA act on metals in aqueous solution and their activity is promoted by increased moisture content.