Background:  Water activity, or a_w, is a measure of the free moisture in a product and is defined as the ratio of vapor pressure of a substance to that of pure water at a specified temperature.

The water activity of a food product describes the degree to which water is “bound” in the food, and its availability to participate in chemical/biochemical reactions and growth of microorganisms.

Hence, texture, non-enzymatic browning reactions, enzymatic activity, lipid oxidation, and other aspects of foods may be influenced by manipulation of a_w levels.

Analysis Description:  After appropriate sample preparation, a disposable sample cup is filled half way with test sample (7.5 mL for liquids), which is then placed in the water activity meter; the sample chamber lid is sealed over the sample, and vapor equilibrium begins to be achieved.  For non-volatile samples, an infrared beam focused on a tiny mirror determines the precise dew point/temperature of the sample.  That dew point/temperature is then translated into water activity.  For samples containing volatiles, a hygroscopic polymer sensor detects the equilibrium relative humidity of air in the headspace above the sample.