The ratio of the vapour pressure of food to the vapour pressure of distilled water under the same conditions is referred to as water activity. This is a way to measure bound water in food. Two foods may have the same moisture content, but a product with more water bound to other molecules can have a different texture. Decreased water activity also provides less available water for pathogenic and spoilage microorganisms. As such the water activity of a food product is important in food production for food safety and quality purposes. Water activity can be used as a tool to predict the shelf life of a product. It is measured from 0 – 1 with 0 being a completely dry food and 1 being water. A product like a muesli bar which is semi-solid but has a lot of bound moisture has a water activity of 0.6 – 0.9.
To test the water activity, there are devices that employ a chilled mirror dew point method. The sample is placed in a sealed unit that has a fan to circulate the air in the chamber. This circulation establishes a vapour equilibrium between the sample and the surrounding air. A mirror in the chamber provides a surface for condensation of the vapour and has an infrared sensor to detect that condensation. There is also a thermocouple attached to the mirror to determine the temperature at which condensation forms. The machine then takes these measurements and calculates water activity. The unit can be easily calibrated with a standard salt solution and gives a result in minutes.