aw

aw is the symbol for water activity — the water vapour generated by moisture in a hygroscopic product. It is the ratio of the water-vapour partial pressure in the food to that above pure water at the same temperature, running from 0 for bone-dry to 1.0 for pure water. In foods it stands for the water not bound to food molecules, and this unbound fraction markedly affects chemical, microbiological and enzymic stability: most bacteria stop growing below about 0.90, and moulds below about 0.70. Controlling aw through drying, salting, or sugaring is one of the oldest and most reliable preservation strategies in food science.