Effect of pH on Water Vapor Sorption by Caseins

Water vapor sorption by whole casein and micellar casein was measured by isopiestic techniques in the water activity range of .2 to .98 as a function of pH. At high water activities, whole casein revealed minimal hydration near its isoelectric point (approximately pH 4.6). Water sorption by micellar casein increased on either side of a pH close to that of the original milk serum. Results are interpreted in terms of ion hydration, binding, and solubility equilibria. The changes in water sorption capacity of micellar casein upon acidification suggest that water activity during lactic acid fermentation in milk products is decreased significantly.