Aggregation and gelation of whey proteins induced by a specific protease from Bacillus licheniformis was revealed by turbidimetry, size exclusion chromatography, dynamic light scattering and rheology. The microstructure of the gel was examined by transmission electron microscopy. During incubation of 12% whey protein isolate solutions at 40ЊC and pH 7, the major whey proteins were partly hydrolyzed and the solution gradually became turbid due to formation of aggregates of increasing size. The viscosity of the hydrolysate simultaneously increased and eventually a gel formed. The gel had a particulate type of microstructure. We hypothesized that the aggregates forming the gel were held together by noncovalent interactions.

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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