Fish products and Byproducts Technology

Unit 3 - Salting and Drying

3.3. Water activity
Relationship of water activity and preservation of fishery products by salting and drying
What is water activity?
Water activity is the measure of water available to support microbial growth and chemical reactions.
How the water activity is measured?
Water activity is frequently represented as the quotient of the vapour pressure exerted by a solution (P) and the vapour pressure exerted by the pure solvent, normally water, (P_o) at the same temperature.
a_w = P/P_o
More correctly, this is the relative equilibrium vapor pressure which, expressed as a percentage, is the quantity measured by ‘water activity’ meters. In such equipment, at a standard temperature, the percentage vapor pressure of the closed atmosphere surrounding the sample, after allowing the sample and atmosphere to come into equilibrium, is measured by hygrometry. The reading is termed the percentage relative humidity (% RVP) of the sample.