Water in Dairy Products | Analysis and Measurement of Water Activity

Water activity of a system (aw) is a thermodynamic concept used to characterize the contained water. It has been considered, in spite of some limitations, as the most important parameter in food technology for the past 50 years. It can be measured by the ratio (p/p0) of the water vapor pressure in the atmosphere at equilibrium with the material to the saturated vapor pressure of pure water at the same temperature, or by the physical properties related to it (freezing point depression, mechanical/electrical properties). Although the physical/chemical mechanisms responsible for the depression of aw as compared to that of pure water are more or less identified, the expressions describing the relations between composition and aw remain mainly empirical for real foods. Water activity is only one of the several water-related factors controlling quality characteristics of food,with molecular mobility being equally important. It is, however, an essential parameter to consider in situations involving microbial activity (shelf life, production of aroma, etc.). It is also an essential tool in food technology, permitting to predict the transfer of water between two compartments of a product or between a food product and its environment during storage or a drying operation. Thus, aw controls the water content of the product, and in turn the quality characteristics such as texture of cheese, and the kinetics of physical (crystallization of lactose) or chemical/enzymatic reactions.