Cross-Linking of Milk Proteins with Transglutaminase at the Oil−Water Interface

Abstract

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, αs1-casein, β-casein, and β-lactoglobulin. Proteins were adsorbed from 10-3 wt % aqueous solutions at pH 7, and apparent surface viscosities were recorded at 40 °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 αs1-casein than for β-casein. While adsorbed β-lactoglobulin showed less relative increase in surface viscosity than the caseins, the results for β-lactoglobulin showed the presence of a substantial rate of cross-linking of the globular protein in the adsorbed state, whereas in bulk solution β-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. Keywords: Surface shear viscosity; transglutaminase; casein(ate); β-lactoglobulin; protein film; oil−water interface; surface rheology; enzymatic cross-linking