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Concept
Water activity
Summary
Water activity (aw) is the partial vapor pressure of water in a solution divided by the standard state partial vapor pressure of water. In the field of food science, the standard state is most often defined as pure water at the same temperature. Using this particular definition, pure distilled water has a water activity of exactly one. Water activity is the thermodynamic activity of water as solvent and the relative humidity of the surrounding air after equilibration. As temperature increases, aw typically increases, except in some products with crystalline salt or sugar.
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 absorbs water from the air. If salami (aw ≈ 0.87) is exposed to dry air (aw ≈ 0.5), the salami dries out, which could preserve it or spoil it. Lower aw substances tend to support fewer microorganisms since these get desiccated by the water migration.
The definition of aw is
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
where lw is the activity coefficient of water and xw 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
The estimated mold-free shelf life (MFSL) in days at 21 °C depends on water activity according to
Water activity is an important characteristic for food product design and food safety.
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.
Water activity values can also help limit moisture migration within a food product made with different ingredients.
Official source
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