Linearization of the Water Sorption Isotherm for Homogeneous Ingredients over a _w 0.30–0.95

Abstract

ABSTRACT Many attempts have been made to quantitate the water sorbed by a material as a mathematical function of water activity (a w ). Based upon a theory of polymolecular absorption, the Smith model states that moisture content is proportional to In (1 – a w ). Subsequent workers showed this to be accurate to 1% but only between about 0.6 and 0.9 a w . It was hypothesized that the validity of this model could be extended over a much broader a w range by applying the model to a single homogeneous ingredient. This was tested by obtaining sorption data for nine materials, including four noncrystalline macromolecules as well as five crystalline solutes, including both sugars and salts. Results showed the model to describe the sorption isotherm for these macromolecules to within 2% over the a w range 0.30–0.95. The model was shown to be valid in the case of each solute to within 2.5% between the a w of a saturated solution and 0.95. Correlation coefficients (r) for the nine materials were all greater than 0.995, showing excellent linearity for the model. Fructose showed the highest “% (error) AV ”, only 3.26.