Water Activity

The term 'water activity' (aw) describes the (equilibrium)i amount of water available for hydration of materials. When water interacts with solutes and surfaces, it is unavailable for other hydration interactions. A water activity value of one (1) indicates pure water whereas zero (0) indicates the total absence of 'free' water molecules; addition of solutes always lowering the water activity.

Higher aw substances tend to support more microorganisms. Bacteria usually require at least 0.91, and fungi at least 0.7.