Rheology of milk protein gels and protein-stabilized emulsion gels cross-linked with transglutaminase

Oscillatory shear measurements have been used to investigate the rheological properties of enzymically cross-linked milk protein gels at neutral pH with and without emulsion droplets. A Ca2+-independent transglutaminase extracted from microorganisms was used as the enzyme source. Storage and loss moduli are presented for gels formed from enzyme-treated beta-lactoglobulin solutions (13 and 14 wt% protein) and beta-lactoglobulin-stabilized emulsions (7-9 wt% protein, 32.5 wt% oil). The frequency dependence of the small-deformation elastic moduli of the enzyme-treated gels is weaker than for the equivalent heat-set beta-lactoglobulin gels (90 degrees C for 30 min), and the strain dependence of the elastic moduli of the enzyme-treated gels is of opposite sign to that of the heat-set gels at large deformations. These differences in rheological behavior are consistent with a network consisting of permanent covalent cross-links for the enzyme-induced gels and predominantly physical cross-links for the heat-set gels. Thermal processing after enzyme treatment is very effective in making a strong gel from either a beta-lactoglobulin solution or a beta-lactoglobulin-stabilized emulsion. Lecithin addition to the beta-lactoglobulin-stabilized emulsion gel before enzyme treatment was found to have a weakly positive effect on the gel strength arising from lecithin-protein complexation. When beta-lactoglobulin was replaced with sodium caseinate, the rate and extent of enzyme-induced cross-linking was found to increase substantially.