Shear and dilatational linear and nonlinear subphase controlled interfacial rheology of β-lactoglobulin fibrils and their derivatives

This work presents a linear and nonlinear interfacial rheological characterization of viscoelastic protein adsorption layers formed by β-lactoglobulin fibrils, β-lactoglobulin peptides, and native β-lactoglobulin (called monomers) at the water–oil interface at pH 2. The fibril and peptide solution presented a similar surface density, whereas β-lactoglobulin monomers lower the interfacial tension more efficiently. The interfacial tension/dilatational rheology response to drop area amplitude sweeps showed pronounced differences, as the β-lactoglobulin fibrils and monomer react nonlinear at high frequencies and area strains, an effect not observed for β-lactoglobulin peptides. Step strain experiments in combination with frequency sweeps present the material response: In the low frequency regime, β-lactoglobulin peptides and β-lactoglobulin monomers can be characterized by the behavior of irreversibly adsorbed molecules. At high frequencies, both peptides and monomers behaved like reversibly adsorbed molecules, while β-lactoglobulin fibrils showed a mixed behavior at all frequencies. The observed dilatational rheological responses can be described using two different adsorption models, the Maxwell model and a modified Lucassen and van den Temple model. In interfacial shear rheology, the pH increase led to highly nonlinear behavior. A large amplitude oscillatory shear analysis in combination with subphase pH changes showed strain stiffening occurring at the isoelectric point, which was quantified by the strain-stiffening index S.