Understanding water activity change in oil with temperature

Abstract Our recent studies and several publications suggest that the water activity (aw) of oil which changes with temperature in thermal processing might be a major contributing factor towards the sharply increased thermal resistance of bacteria in oils. In this study, we developed a reliable method to measure the water activity of oil by the measurement of equilibrium relative humidity in a small headspace. Using this method, water activity of peanut oil was found to decrease exponentially with increasing temperature. A model derived from excess Gibbs free energy was fitted to the observations with an R2 = 99.6% and RMSE = 0.01 (aw). Our results suggest that the decreased water activity of oil resulting from a rise in temperature could cause desiccation of bacteria. This is a possible explanation for the protective effect of oil in thermal processing.