Water Migration

Water migrates from areas of high AW (available water) to areas of low AW.  When two different products or product components are sealed in a form of packaging, they will exchange moisture.

For example, if you have a fruit cake with 50% moisture content containing dried fruit pieces with 30% moisture content, the assumption is water would move from the cake into the dried fruit.  If the fruit has a higher water activity than the cake the result is a soggy cake with rock hard fruit.  The reason is water moves towards equilibrium.

Water activity, not water content, produces how water will migrate within a product.  The fruit cake manufacturer can develop a recipe in which cake and fruit pieces both have the same water activity.  This will than assure no moisture surplus when the cake is stored and sold.  This creates a shelf stable product. 

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