Food Chemistry
The ratio of the vapour pressure of water in a food to the vapour pressure of pure water at the same temperature. It measures how much water is available for microbial growth and chemical reactions — not how much water is present.
Why honey and jam don't need refrigeration despite containing a lot of water, while fresh bread does — bacteria and mould need “available” water, not just any water.
The total amount of water in a food, expressed on a wet basis (mass of water per mass of wet sample) or dry basis (mass of water per mass of dry solids).
Combining several mild preservation factors — pH, aw, temperature, preservatives, redox potential — so that their combined effect prevents microbial growth where none alone would suffice.
A curve relating equilibrium moisture content to water activity at constant temperature, typically sigmoidal.
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