Water sorption isotherm of solution containing hydrogels at high water activity

The water vapour sorption isotherm, in the water activity range of 0.9-1.0, was determined for hydrogels containing glucose or sodium chloride solutio ns, as a model of food systems. Due to their high osmotic pressure, these s olutes 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 pr essure is relatively low, the sorption isotherm of the gel is similar to th at of the solutions. At high water activity, the swollen gel develops a rel atively large network pressure that not only counteracts the osmotic pressu re and reduces moisture uptake, but also may induce syneresis. That is like ly to take place if the network pressure exceeds the osmotic pressure of th e polymeric matrix in the gel's internal solution. The onset of syneresis, which changes the gel composition, marks the end point of the sorption isot herm for the original gel.