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 Ca 2+ -independent transglutaminase extracted from microorganisms was used as the enzyme source. Storage and loss moduli are presented for gels formed from enzyme-treated β-lactoglobulin solutions (13 and 14 wt % protein) and β-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 β-lactoglobulin gels (90 °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 β-lactoglobulin solution or a β-lactoglobulin-stabilized emulsion. Lecithin addition to the β-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 β-lactoglobulin was replaced with sodium caseinate, the rate and extent of enzyme-induced cross-linking was found to increase substantially.