Water activity predicts microbial growth
Water activity predicts whether or not products will mold because it measures the energy of the water in the product. Microbial proliferation is tied not to how much water is in the product, but to the energy status of that water. Higher-energy water is available to microbes, making high water activity products susceptible to mold and even bacterial growth.
Moisture content and water activity are related, but not in a direct, one to one way. Even products with a high water content, like syrups, can be safe if steps have been taken to lower their water activity. Water activity can tell you which bacteria, molds, or fungi can grow on and in your product. Reducing the water activity can rule out the growth of whole classes of microbes. At low water activities (<0.60), formulators can preclude the growth of anything at all.
Water activity is a key parameter for consistent texture
Many snack foods are famous for their texture. It might seem natural to measure texture changes with moisture content. But the rate at which a protein bar changes from soft and chewy to a rock hard lump, or a potato chip goes from crisp to soggy is determined not by water content but by water activity. Classic snack foods are defined by their water activity to water content profile, as shown below:
Water activity slows chemical reactions, such as lipid oxidation
Chemical reactions often affect snack foods. Chips, nuts, and other food that taste bitter and rancid have likely fallen prey to lipid oxidation. Food formulators aren’t powerless when confronting lipid oxidation, and water activity is one of their most useful tools. Because water activity is a measure of the energy of water in a product, it affects how quickly lipid oxidation reactions proceed.
In practice, lipid oxidation reaction rates decrease as water activity goes down. However, as water activity declines below 0.4, rates begin to increase again. This makes the general region of 0.4 an important target for snack foods and nuts, though values are specific to specific products. The best way to know how water activity affects lipid oxidation in a product is to use accelerated shelf life testing.
Use water activity to prevent moisture migration
Water activity, not moisture content, predicts how water will migrate in a multi-component snack food like a cream-filled cake or a bar containing nuts and fruit pieces. Knowing the water activity of discrete ingredients can help determine how to process them. There are several processing options. Ingredients can be processed to similar water activities. An edible barrier, like chocolate coating on the inside of an ice cream cone, can be used prevent moisture migration.The diffusion process within a component can be retarded by increasing its viscosity. And sometimes water activity differences that can’t be equalized require separate packaging. Testing water activity helps avoid problems that affect the product and end shelf life.