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
Famous quotes containing the words water and/or activity:
“I believe that water is the only drink for a wise man: wine is not so noble a liquor; and think of dashing the hopes of a morning with a cup of warm coffee, or of an evening with a dish of tea! Ah, how low I fall when I am tempted by them! Even music may be intoxicating. Such apparently slight causes destroyed Greece and Rome, and will destroy England and America.”
—Henry David Thoreau (1817–1862)
“Moderation is the feebleness and sloth of the soul, whereas ambition is the warmth and activity of it.”
—François, Duc De La Rochefoucauld (1613–1680)