Water activity or a_w is the relative availability of water in a substance. It is defined as the vapor pressure of water 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, a_w typically decreases, except in some salt and sugar solutions.

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

Water tends to migrate from high a_w substances to low a_w substances. For example, when honey (a_w ≈ 0.6) is exposed to humid air (a_w ≈ 0.7) the honey will absorb water from the air.

Formulae

Definition of a_w:

where p is the vapor pressure of water in the substance, and p₀ is the vapor pressure of pure water at the same temperature.

Alternate definition:

where l_w is the activity coefficient of water and x_w is the mole fraction of water in the aqueous fraction.

Relative humidity:

Estimated mold-free shelf life in days at 21° C:

Selected a_w values

External links

• Chemistry of water Activity (http://www.martin.chaplin.btinternet.co.uk/activity.html)
• Measurement of water activity (http://www.gardco.com/about_water_activity.html)