Formation of intermolecular beta-sheet structures: a phenomenon relevant to protein film structure at oil-water interfaces of emulsions

Oil-in-water emulsions stabilized with beta-lactoglobulin (beta-lg) were made using a homogenizer or a high-speed blender. The protein was studied by Fourier transform infrared (FTIR) spectroscopy in the raw emulsion, in the bulk phase, and at the interface, as a function of pH, oil content, and homogenizing pressure. Results show that the amount of adsorbed protein varies with the available interfacial area. The protein that remains in the aqueous phase exhibit no spectral change, which suggests that homogenization causes no conformational modification or reversible ones. Strong and irreversible changes were observed in the adsorbed protein. Our findings reveal the formation of intermolecular antiparallel beta-sheets upon adsorption due to the protein self-aggregation. As deduced from transmission electronic microscopy, this surface aggregation leads to the formation of continuous and homogeneous membranes coating the globules. The structure of the adsorbed proteins is unaffected by the homogenizing pressures used in our study and slightly modified by the pH. FTIR spectroscopy allows to characterize the type of aggregates formed at the interface. An analysis of the spectra of beta-lg heat-induced gels shows that the aggregates at the interface are very close at a molecular scale to those that constitute particulate gels near the protein's isoelectric point. Since the type of aggregates is similar when the emulsion water phase is pure D(2)O and D(2)O at pD 4.4, the interface not only seems to induce aggregation, but seems to determine the type of aggregation as well. The mechanism that drives the formation of particulate aggregates (rather than fine-stranded ones) may reside in strong protein-protein interactions that are promoted by adverse oil-protein interactions.