Abstract
The mechanical stability of interfacial protein films is limited through the intermolecular interaction of the adsorbed proteins. Since the intermolecular interactions vary in dependence of the protein structure, the investigation of intentionally induced structural modifications (i.e., through high hydrostatic pressure) of interfacial active proteins is required to mechanistically understand their structure-function-relationship. The aim of this study was to link the structural analysis of pressure-treated whey protein β-lactoglobulin at the oil/water-interface with the formation of the interfacial protein film and the resulting film stability against mechanical stress. For this purpose, we treated β-lactoglobulin in water at pH 7 with hydrostatic pressures of 600 MPa for 10 min at 20 °C and analyzed the protein structure in oil/water-emulsion with Fourier-transform-infrared-spectroscopy, extrinsic fluorescence, and ζ-potential. We characterized the molecular density and interfacial film properties via Langmuir trough analysis, electron-paramagnetic-resonance-spectroscopy, interfacial shear and dilatational rheology. The structural flexibility of pressure-treated β-lactoglobulin favored its unfolding and a fast interfacial protein film formation that resulted in pronounced intermolecular interactions, including van der Waals interactions, hydrophobic associations and disulfide bonds. The high interfacial molecule density combined with many and strong intermolecular interactions stabilized the pressure-treated protein film against shear deformation and small dilatational deformation. However, against high dilatational deformation, the pressure-treated β-lactoglobulin film showed a lower stability due to a slow migration towards unoccupied interfacial area. Hence, our research contributes to the control of the mechanical protein film stability, and thus the colloidal stability of emulsions in dependence of the protein structure at the oil/water-interface.
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