The microbial and chemical stability of dry food and feed products depends on the water activity of the product. It is a measure of the availability of water in the product, to react chemically, or to support the growth and metabolism of microorganisms, such as bacteria and moulds, during spoilage. It is expressed as the ratio of the water vapour pressure of the product, or solution, to that of pure water at the same temperature. The aw of pure water is 1. 0. The aw of a saturated solution of sodium chloride (salt) at 25°C is 0. 765. Reduction of the aw in a product to low levels, i.e. Less than 0. 75, by direct removal of water by drying, or by the addition of solutes such as salt or sugar, or by some combination of these factors, can restrict the growth of many microorganisms.