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Water activity or aw is the partial vapor pressure of water in a substance divided by the standard state partial vapor pressure of water. In the field of food science, the standard state is most often defined as the partial vapor pressure of pure water at the same temperature. Using this particular definition, pure distilled water has a water activity of exactly one. As temperature increases, aw typically increases, except in some products with crystalline salt or sugar. Higher aw substances tend to support more microorganisms. Bacteria usually require at least 0.91, and fungi at least 0.7. See also fermentation. 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. ==Formulae== Definition of aw: : 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 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 of air in equilibrium with a sample is called the Equilibrium Relative Humidity (ERH). : Estimated mold-free shelf life in days at 21° C: : 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「water activity」の詳細全文を読む スポンサード リンク
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