03/03/2026
Water activity refers to the amount of water in a product that is available for microbial growth, not simply how much total moisture it contains. It measures how “free” the water is to support bacteria, yeast, and mould, rather than water that is chemically bound to ingredients like sugar, salt, or proteins.
Two products can have similar moisture levels but behave very differently in terms of shelf life. When available water is reduced through formulation or processing, for example by adding sugar or salt, microbial growth slows, stability improves, and safety increases.
Understanding this distinction is fundamental in product development, because controlling water activity is often more important than simply reducing moisture.