Water sorption isotherm of solution containing hydrogels at high water activity

Abstract

Summary The water vapour sorption isotherm, in the water activity range of 0.9–1.0, was determined for hydrogels containing glucose or sodium chloride solutions, as a model of food systems. Due to their high osmotic pressure, these solutes play the major role in determining the osmotic pressure of the gel, thus its driving force for moisture uptake. As long as the gel's network pressure is relatively low, the sorption isotherm of the gel is similar to that of the solutions. At high water activity, the swollen gel develops a relatively large network pressure that not only counteracts the osmotic pressure and reduces moisture uptake, but also may induce syneresis. That is likely to take place if the network pressure exceeds the osmotic pressure of the polymeric matrix in the gel's internal solution. The onset of syneresis, which changes the gel composition, marks the end point of the sorption isotherm for the original gel.