Conformational state and charge determine the interfacial film formation and film stability of β-lactoglobulin

Abstract

The mechanical stability of protein-stabilized emulsions depends on physiochemical interactions within the interfacial protein film. However, intermolecular interactions vary with the protein's conformational state and charge and may in turn affect the mechanical stability through modifications in the three-staged interfacial stabilization; migration, adsorption and film formation at the oil/water-interface. Therefore, the aim of our study was to investigate the different protein conformations within the interfacial stabilization process by applying bulk water conditions and to determine the protein film stability against mechanical stress. For this purpose, we analyzed the structure and interactions of β-lactoglobulin in water and at the oil/water-interface at pH 7, pH 7NaCl (containing 100 mM NaCl) and pH 9 with membrane-osmometry, Fourier-transform-infrared-spectroscopy, extrinsic fluorescence and ζ-potential. Moreover, we characterized the conformational state and charge in context with the molecule density and interfacial film properties via Langmuir trough analysis, interfacial shear and dilatational rheology. Distinct unfolding of monomers and dimers at pH 9 resulted in the lowest interfacial molecule density but at the same time the highest film stability due to pronounced structural flexibility. In comparison, β-lactoglobulin at pH 7 was monomeric, unfolding was less pronounced, and interfacial molecule density was higher. Electrostatic shielding of β-lactoglobulin dimers at pH 7NaCl resulted in the highest density but least stable protein film that approached low molecular weight surfactant behavior due to few and weak intermolecular interactions. Our research contributes to the control of the emulsion stability against mechanical stress by varying intermolecular interactions within the interfacial protein film.