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Water Activity. The ratio of the vapor pressure (q.v.) of the water (q.v.) or moisture (q.v.) in a food product, as any bakery product, to the vapor pressure of pure water at the same temperature (q.v.), and equivalent to one-hundredth of the relative humidity (q.v.) generated by the product in a closed system. The higher the moisture content (q.v.) in a food product, the greater is its water activity and the more likely is the product to support the growth of microorganisms (q.v.). Generally, moisture contents in products of 14% or less will not sustain mold (q.v.) growth.