Predicting Water Activity from 0.30 to 0.95 of a Multicomponent Food Formulation

Abstract

ABSTRACT The objective was to develop an equation to calculate the a w of a mixture of known composition at a given moisture content. A simple equation was derived by combining the Smith isotherm with the Lang‐Steinberg mass balance: log (1–a w ) = [MW–Σ(a i w i )]/Σ(b i w i ) where M is moisture content of the mixture, W is total dry weight of the mixture, a i and b i are Smith isotherm parameters for the individual ingredient, and w i is the dry weight of the ingredient. For comparison of experimental and calculated a w values, an experimental value was used to calculate M from the mixture isotherm and this M was used to calculate a w , from the model. Four binary and one ternary mixtures of two macromolecules, two solutes and one complex ingredient were included. Twenty‐one comparisons between calculated and experimental a w , over a w 0.30–0.95 resulted in a maximum error of only 1.86% and a mean error of ‐0.25%, showing excellent agreement. Only one constraint was found; when the mixture contains i solute (sugar or salt) and the a w , is below the a w of saturation for that solute, that solute is deleted from the summations in the Model.