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:

<math> a_w \equiv p / p_0 <math>

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:

<math> a_w \equiv l_w x_w <math>

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

Relative humidity:

<math> \mathrm{RH} = a_w \times 100\% <math>

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

<math> \mathrm{MFSL} = 10^{7.91 - 8.1 a_w} <math>

Selected a_w values

Substance	aw
Distilled Water	1
Tap water	0.99
Milk	0.97
Juice	0.97
Saturated NaCl solution	0.75
Point at which cereal loses crunch	0.65
Typical indoor air	0.5 - 0.7
Honey	0.5 - 0.7
Dried fruit	0.5 - 0.6

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)