Evaluation of Norrish's equation for correlating the water activity of highly concentrated solutions of sugars, polyols and polyethylene glycols

Abstract

Abstract: Norrish’s equation, (aw = Xw exp (-K Xs 2), where aw is water activity, Xw and Xs molar fractions of water and solute, respectively, and K is the correlating constant), has been widely used to predict aw of aqueous non-electrolyte solutions in connection with development of intermediate moisture foods, i.e. food having aw ≥ 0.85. Present work evaluated the ability of Norrish’s equation to model the water activity of solutions of sugars, polyols and some polyethylenglycols, in a wide range of concentration; i.e. from low to highly concentrated solutions. For sugar and polyols a relatively small modification of the “most accepted” literature parameters K, allowed to fit the data for the whole range of solute concentrations(range of aw 0.99 to 0.3/0.4) with high accuracy. However, a modified Norrish’s model needs to be used to model the behavior of polyethylenglycols 400 and 600 up to water activities as low 0.4/0.5.