Water activity (Aw) is the partial vapor pressure of water in a solution divided by the standard state partial vapor pressure of water. In the field of food science, the standard state is most often defined as the partial vapor pressure of pure water at the same temperature.
Water activity is important as spoilage organisms can only grow above a certain aw value. In addition in food products with layered structures (e.g. filled biscuits) controlling the Aw is important as water tends to migrate from high aw to low aw and the products will have limited shelf-life when this occurs. opaque, forming an interconnected solid mass.