The large number of methods and instruments reported in the literature to measure water activity in foods are reviewed. The methods are based on the colligative properties of solutions and water activity can be determined by: (a) measurement of the freezing point depression of a liquid or (b) measurement of the equilibrium relative humidity of a solid or liquid and conversion of these measurements to water activity. The methods are divided into those requiring simple laboratory apparatus and those requiring specialized apparatus. Of the methods requiring only simple laboratory apparatus, the water sorption isotherm method is reported to have the best precision and accuracy. Disadvantages of these methods are their limited range of measurement and long equilibration times necessary before measurement. The primary advantages of methods requiring specialized apparatus are that the water activity of the sample can be more rapidly determined, fewer manipulative steps are necessary and measurements can be made over a wider range than using simple equipment. The electric hygrometer, dew point hygrometer and vapor pressure manometer are reported to give good precision and accuracy. Some methods are unsuitable to test foods containing volatiles or excessive numbers of microorganisms.

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