Cross-linking of milk proteins with transglutaminase at the oil-water interface

The influence of covalent cross-linking with transglutaminase on the time-dependent surface shear viscosity of adsorbed milk protein films at the n-tetradecane-water interface has been investigated for sodium caseinate, alpha(S1)-casein, beta-casein, and beta-lactoglobulin. Proteins were adsorbed from 10(-3) wt % aqueous solutions at pH 7, and apparent surface viscosities were recorded at 40 degrees C in the presence of various enzyme concentrations. Results for casein systems showed a rapid enhancement in surface viscoelasticity due to enzymatic cross-linking with a substantially slower development of surface shear viscosity for alpha(S1)-casein than for beta-casein. While adsorbed beta-lactoglobulin showed less relative increase in surface viscosity than the caseins, the results for beta-lactoglobulin showed the presence of a substantial rate of cross-linking of the globular protein in the adsorbed state, whereas in bulk solution beta-lactoglobulin was cross-linked only after partial unfolding in the presence of dithiothreitol. A maximum in shear viscosity at relatively short times following addition of a moderately high dose of enzyme was attributed to formation of a highly cross-linked protein film followed by its brittle fracture. Enzymatic cross-linking of protein before exposure to the oil-water interface was found to produce a slower increase in surface viscosity than enzyme addition immediately after interface formation or to the aged protein film.