Effect of transglutaminase on structure and syneresis of neutral and acidic sodium caseinate gels

The effect of transglutaminase on nuclear magnetic resonance (NMR) relaxation behaviour, porosity, firmness and syneresis of neutral and acidic sodium caseinate gels was studied. A drastic increase in porosity was observed to increase the amount of separating water up to 15% of gel weight, but structural homogeneity at the level of confocal laser scanning microscope (CLSM) resolution did not inhibit water separation in other acidic gels. Spin–spin relaxation time was reduced from 0.32 s (peak value) of the neutral transglutaminase-containing gel to 0.14 s of the corresponding acidic gel. The reduction was attributed to reduced mobility of protein protons in exchange with water protons in the system. With sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS–PAGE) analysis, more pronounced cross-linking of protein was observed at neutral than at acidic conditions, yet the neutral gel was weak compared with the acidic gel. It was assumed that, because of transglutaminase action, electrostatic repulsion was partially removed from the surface of sodium caseinate particles, allowing, e.g., hydrophobic interactions between the particles and gel formation. The mobility of proteins in the gel network was suggested to be related to its syneresis behaviour.