The ratio of the vapour pressure of water in a food to the saturated vapour pressure of water at the same temperature.
A measure of the free moisture in a product. It is determined by dividing water vapor pressure of the substance by the vapor pressure of pure water at the same temperature.
A measure of the “free” unbound water (e.g., in a processed food product) that is available to sustain the growth of microorganisms. Most bacteria are unable to grow in foods possessing a water activity below 0.90. Most yeasts and molds that cause spoilage cannot grow in foods possessing a water activity below 0.80. Sugars can be added to certain foods in order to increase Aw, as they “bind up” the (formerly) free water present.