Water activity’s usefulness as a quality and safety measurement was suggested when it became evident moisture content could not adequately account for microbial growth fluctuations. Water activity is a measure of the energy status of the water in a system. The water activity (aw) concept has served the microbiologist and food technologist for decades. It is the most commonly used criterion for safety and quality.
Microbial growth is the reason water activity began to be used in the food industry. Before the discovery that water activity controls microbial growth, food producers didn’t know why some batches molded and others didn’t. They suspected it had something to do with water, but everyone knows that high-moisture salt pork doesn’t mold, while low moisture nuts still do.
Water activity is derived from fundamental principles of thermodynamics and physical chemistry. As a thermodynamic principle there are requirement in defining water activity that must be met. These requirements are; pure water (aw = 1.0) is the standard state, the system is in equilibrium, and the temperature is defined.