Water activity is the amount of water available for microbial growth. Water may be present but unavailable for use by microbes. How? The water may be chemically bound to other molecules, such as sugar or salt, because of their overwhelming density in the food. This is how salt and sugar are used as preservatives. Based on a scale of 0 to 1.0, with water being 1.0, foods may be exempted from acidified food regulations if they have a water activity (Aw) of .85 or less.

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