Water activity

In food science, water activity (a_w) of a food is the ratio of its vapor pressure to the vapor pressure of water at the same temperature, both taken at equilibrium. Pure water has a water activity of one. Put another way, a_w is the equilibrium relative humidity (ERH) expressed as a fraction instead of as a percentage. As temperature increases, a_w typically increases, except in some products with crystalline salt or sugar.

Water migrates from areas of high a_w to areas of low a_w. For example, if honey (a_w ≈ 0.6) is exposed to humid air (a_w ≈ 0.7), the honey absorbs water from the air. If salami (a_w ≈ 0.87) is exposed to dry air (a_w ≈ 0.5), the salami dries out, which could preserve it or spoil it. Lower a_w substances tend to support fewer microorganisms since these get desiccated by the water migration.

Water activity is not simply a function of water concentration in food. The water in food has a tendency to evaporate, but the water vapor in the surrounding air has a tendency to condense into the food. When the two tendencies are in balance— and the air and food are stable—the air's relative humidity (expressed as a fraction instead of as a percentage) is taken to be the water activity, a_w. Thus, water activity is the thermodynamic activity of water as solvent and the relative humidity of the surrounding air at equilibrium.

The definition of a_w is $${\displaystyle a_{w}\equiv {\frac {p}{p^{*}}}}$$ where p is the partial water vapor pressure in equilibrium with the solution, and p* is the (partial) vapor pressure of pure water at the same temperature.

An alternate definition can be $${\displaystyle a_{w}\equiv l_{w}x_{w}}$$ where l_w is the activity coefficient of water and x_w is the mole fraction of water in the aqueous fraction.

Relationship to relative humidity: The relative humidity (RH) of air in equilibrium with a sample is also called the Equilibrium Relative Humidity (ERH) and is usually given as a percentage. It is equal to water activity according to $${\displaystyle \mathrm {ERH} =a_{w}\times 100\%.}$$ The estimated mold-free shelf life (MFSL) in days at 21 °C depends on water activity according to $${\displaystyle \mathrm {MFSL} =10^{7.91-8.1a_{w}}}$$

Water activity is an important characteristic for food product design and food safety.^{[citation needed]}

Food designers use water activity to formulate shelf-stable food. If a product is kept below a certain water activity, then mold growth is inhibited. This results in a longer shelf life.^{[citation needed]}