Two products can share the same moisture percentage and one lasts weeks while the other lasts days. The difference is how much of the water is free water that micro-organisms can use. Water activity measures it: the ratio of the product's vapour pressure to that of pure water, so 0.80 means a vapour pressure 80 percent of pure water's.
In jam there is a lot of water but it is bound to sugar and aw is low; in a fresh product the same water is free and feeds growth. Most foods have an aw above 0.95. The lower limit for botulinum bacteria is about 0.93.
Measurement is sensitive to temperature. aw rises with temperature and a 1 °C difference between sample and air shifts the reading by about 0.05, enough to land on either side of a limit like 0.93. Batch, sample location, preparation, measurement temperature and instrument ID are therefore recorded.
For high-aw products (fresh meat, milk, vegetables, ready meals) the cold chain sets the life; dry products often need no cooling, but packaging that lets moisture through raises aw. Condensate that forms on packaging moving from cold to warm can also raise local aw and start mould.
