Water activity (aw) is one of the few important parameters that dictate the shelf life stability of food—a food safety concern. But before we get to that, let’s define first what water activity is. More often, students think water activity and moisture content are the same thing. Well, they are not. Moisture content refers to the amount of water in the food. While water activity is the amount of unbound or free water. Hence, water activity is the available water for many chemical, physical, enzymatic, and more specifically microbiological reactions.