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.