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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