Water activity or aw was developed to account for the intensity with which water associates with various non-aqueous constituents and solids. Simply stated, it is a measure of the energy status of the water in a system. It is defined as the vapor pressure of a liquid divided by that of pure water at the same temperature; therefore, pure distilled water has a water activity of exactly one.
As the temperature increases, aw typically increases, except in some products with crystalline salt or sugar.
Higher aw substances tend to support more microorganisms. Bacteria usually require at least 0.91, and fungi at least 0.7. See fermentation.
Water migrates from areas of high aw to areas of low aw. For example, if honey (aw ≈ 0.6) is exposed to humid air (aw ≈ 0.7) the honey will absorb water from the air.
Read more about Water Activity: Formulae, Water Activity and Moisture Content, Use in Humidity Control
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