EVERY FOOD producer needs to know what will happen to their product as it sits on the shelf. Shelf stable can mean "won't get moldy," but it covers a lot of other food qualities as well. Take, for example, a well known raisin bran manufacturer. Both the manufacturer and the consumer of the raisin bran expected crunchy flakes and chewy raisins. The consumer got a surprise when he bit into a rock hard raisin and broke a tooth. The manufacturer soon after got a surprise in the form of a lawsuit. Fortunately, not all moisture migration problems end up in a court of law. But if you produce and sell products that contain discrete ingredients, you need to know where the moisture will go as the product sits on a shelf.
A fruitcake manufacturer wants to predict conditions over time in her fruitcake. She doesn't want the cake to get soggier as the fruit pieces get dry and hard. So she measures moisture content. The cake contains 30% water, while the fruit pieces have 50% water. She knows that water wants to come to equilibrium. Therefore, she assumes that water will migrate from the wetter ingredient (the fruit) into the drier ingredient (the cake).
Unfortunately, she has her eye on the wrong ball. While she's watching the water content and feeling confident of the score, a whole different game is going on at the water activity level. If she continues without knowing the score of that game, she'll suddenly be confronted with a surprise ending-in this case, dried up cake and soggy fruit, because the cake had a higher water activity than the fruit pieces at these moisture contents.
In this case, water content is nothing more than a distraction. Pay attention to that number, and the outcome feels like sleight-of- hand. True, unless acted on by other forces, water does want to come to equilibrium. But, equilibrium occurs when the partial specific Gibbs free energy is the same everywhere in a system. Water activity is a measurement of Gibbs free energy. Water content has nothing to do with the energy of the water. To understand this concept better, think of two tanks of water. One is almost full at 10,000 gallons. The other huge tank is nearly empty, containing only one gallon. Which way will water move? Knowing the water content of the tanks is completely misleading. The volume of water is 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 almost full tank, that last gallon of water will quickly leave its spacious tank.
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. 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 you decide how to process them. One solution to a moisture migration problem is to lower or raise the water activity of discrete components until they have the same water 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 prevent moisture migration. Sometimes aw differences that can't be equalized require separate packaging.