Water activity is a measure of the free moisture in a product. The water activity of a product describes its ability to facilitate the growth of microorganisms. A higher water activity leads to a greater potential for microbial contamination leading to product spoilage.

The simplest way to reduce free water (lower water activity) is with a process which drives off water, i.e. cooking, baking or dehydration. In cannabis this is known as curing. Water activity analysis is performed with a dew point moisture analyzer.