Water activity (Aw) is a measure of the energy status of water in a system. It is an important determinant of whether a micro-organism will grow or not.
The water activity of pure distilled water is defined as 1. The water activity for a system is its vapor pressure relative to pure distilled water.
A water activity above 0.95 will allow growth of many micro-organisms. Staphylococcus aureus will grow if the water activity is above 0.86. Molds may grow if the water activity is above 0.70.
A system with a low water activity will absorb water from humid air. If enough water is absorbed then micro-organisms may be able to grow. Therefore it is important to keep foods with a low water activity in a tightly sealed container or in an environment that is very dry.
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