# Water activity

Water activity or aw 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, aw typically decreases, except in some salt and sugar solutions.

Higher aw 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 aw substances to low aw substances. For example, when honey (aw ≈ 0.6) is exposed to humid air (aw ≈ 0.7) the honey will absorb water from the air.

## Formulae

Definition of aw:

$a_w \equiv p / p_0$

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:

$a_w \equiv l_w x_w$

where lw is the activity coefficient of water and xw is the mole fraction of water in the aqueous fraction.

$\mathrm{RH} = a_w \times 100\%$

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

$\mathrm{MFSL} = 10^{7.91 - 8.1 a_w}$

## Selected aw 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