When two different products or product components are sealed up in some sort of packaging, they will exchange moisture. Whether the products are cereal flakes and dried fruit, gelcaps containing pharmaceutical ingredients, or jam-filled cookies, moisture will move between the components.
How to predict moisture migration
The water activity of each component predicts whether it will gain or lose moisture.
Moisture content doesn’t work this way. In fact, sometimes moisture moves from the lower moisture component to the higher one. Here’s one example that shows this concept at work.
Confusing moisture numbers
If you had a fruitcake with 50% moisture content containing dried fruit pieces with 30% moisture content, you might assume that water would move from the cake into the dried fruit. If the fruit has a higher water activity than the cake, however, you will end up with soggy cake and rock-hard fruit.The reason for this is that water moves toward equilibrium. This equilibrium involves the energy status, or activity, of the water, not its content by volume.
To understand this concept better, think of two tanks of water. One is almost full at 10,000 gallons. The other tank is nearly empty, containing only one gallon. Which way will water move between the tanks?
Obviously, the volumes are irrelevant. Water moves from higher pressure to lower pressure, not from full to empty. If we raise the pressure of the nearly empty tank by putting it on top of the full tank, the water will move from empty to full.
Water activity enables complete moisture control
Likewise, water activity, not water content, predicts how water will migrate within a product. The fruitcake manufacturer can develop a recipe in which cake and fruit pieces both have the same water activity. This assures no moisture surprises when this cake is stored and sold. It is a safe, palatable, and shelf-stable product.
Knowing the water activity of discrete ingredients can help determine how to process them. One solution to a moisture migration problem is to lower or raise the water activity of discrete components until they are the same. Manufacturers can also retard the diffusion process within a component by increasing its viscosity. An edible barrier, like chocolate coating on the inside of an ice cream cone, can also prevent moisture migration. Sometimes water activity differences that can’t be equalized require separate packaging.