Psychrometric Determination of Water Activity in the High aw Range

A method is described to measure the water activity (aw) of foods in the range of 0.935 to 1.0 using a psychrometer. Samples (0.15 to 0.35 g) were equilibrated for 2 h in the sample chamber before cooling the thermocouple and recording the subsequent voltage on the instrument. The aw was read off a standard curve prepared from sodium chloride solutions of known water activity values. Results show that the method was considerably better than the graphical interpolation method and comparable to the microcrystalline method but was much quicker and required less effort.