![]() Multiplication of water activity by 100 gives the equilibrium relative humidity ( ERH) in percent. When vapor and temperature equilibrium are obtained, the water activity of the sample is equal to the relative humidity of air surrounding the sample in a sealed measurement chamber. Relative humidity of air is defined as the ratio of the vapor pressure of air to its saturation vapor pressure. Water activity is defined as the ratio of the vapor pressure of water in a material ( p) to the vapor pressure of pure water ( p o) at the same temperature. It is this fact that allows the measurement of the vapor phase to determine the water activity of the sample. Equilibrium between the liquid and the vapor phases implies that μ is the same in both phases. For practical purposes, under most conditions in which foods are found, the fugacity is closely approximated by the vapor pressure ( f ≈ p) soĮquilibrium is obtained in a system when μ is the same everywhere in the system. When dealing with water, a subscript is designated for the substanceĪ w is activity of water, or the escaping tendency of water in system divided by the escaping tendency of pure water with no radius of curvature. The activity of a species is defined as a = f/f o. Where: μ (J mol -1) is the chemical potential of the system i.e., thermodynamic activity or energy per mole of substance μ o is the chemical potential of the pure material at the temperature T (°K) R is the gas constant (8.314 J mol -1 K -1) f is the fugacity or the escaping tendency of a substance and f o is escaping tendency of pure material (van den Berg and Bruin, 1981). ![]() ![]() These requirements are: pure water (a w = 1.0) is the standard state, the system is in equilibrium, and the temperature is defined. As a thermodynamic principle there are requirements in defining water activity that must be met. WATER ACTIVITY (aw) is derived from fundamental principles of thermodynamics and physical chemistry. ![]()
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