Water holding capacity and swelling of casein hydrogels

The water holding capacity of casein gels was investigated by measuring the swelling and de-swelling under a variety of conditions of temperature and salt concentration. Transglutaminase cross-linked sodium caseinate (15% w/w) gels will swell in good solvents or shrink in poor solvents until an equilibrium casein volume fraction is reached, and this is determined by the cross-link density. The results are interpreted using extended Flory–Rehner theory for weak polyelectrolytes. NaCl and CaCl2 solutions tend to shrink the gels through the decreased Donnan pressure. In contrast, high volume fraction renneted casein (48% w/w) gels tend to swell in NaCl and CaCl2. These results are consistent with theory since the equilibrium volume fractions appear to be at an intermediate value (estimated about 20%). Experiments on casein micelles, which can be considered nano-gels, show the same behavior and trends. Physically cross-linked gels such as a highly concentrated renneted casein gel and casein micelles show the same (de-)swelling behavior. Cross-linked caseinate gels (15% w/w) have a lesser tendency to swell due to the lower casein volume fraction. Physically cross-linked gels will eventually completely dissolve in a good solvent. The results presented give a clear picture of the parameters that determine the equilibrium water content of a food polymer gel.