water activity

Definition

Food

  • A measure of the effective water content (from the point of view of a microorganism) of a food, not necessarily related to the actual water content ( per cent by weight). It is defined as the ratio of the pressure exerted by water vapour in equilibrium with the food to the pressure exerted by water vapour in equilibrium with pure water at the same temperature as the food. (Both increase with temperature but the ratio remains roughly constant.) A food containing no water has a water activity of zero and pure water has a value of 1.0. freezing reduces water activity by turning liquid water into ice.
http://www.dictionarycentral.com/definition/water-activity.html