Water has been called the universal solvent as it is a requirement for growth, metabolism, and support of many chemical reactions occuring in food products. Free water in fruit or vegetables is the water available for chemical reactions, to support microbial growth, and to act as a transporting medium for compounds. In the bound state, water is not available to participle in these reactions as it is bound by the soluble compounds such as sugar, salt, gums, etc. ( osmotic binding ), and by the surface effect of the substrate ( matrix binding ). These water-binding effects reduce the vapour pressure of the food substrate according to Raoult's Law. Comparing this vapour pressure with that of pure water ( at the same temperature ) results in a ratio called water activity. Pure water has an water activity of 1, one molal solution of sugar - 0.98, and more molal solution of sodium chloride - NaCl solution in a closed container will develop and equilibrium relative humidity ( ERH ) in a head space of 75.5%. A relationship therefore exists between ERH and water activity since both are based on vapour pressure