Water activity measurement quantifies the availability of free water in a food product on a scale from zero to one, predicting microbial growth potential and chemical stability in a way that moisture content alone cannot.
In trade practice
Suppliers of dried fruit, nuts and intermediate-moisture products use calibrated water activity meters at production and before dispatch to confirm that lots meet specification limits. Unlike moisture content, water activity is sensitive to temperature, so measurements must be taken at a defined and recorded temperature. Buyers specifying shelf-stable products without a water activity limit, relying on moisture content alone, risk accepting product that is microbiologically unstable despite appearing dry.
In a specification
A specification for a dried or intermediate-moisture product should state the maximum water activity value, the measurement temperature and the instrument calibration standard required.