Evidence of extraneous surfactant adsorption altering adsorbed layer properties of β-lactoglobulin
Unusual surface-shear and surface-dilation properties have been observed as a function of increasing concentration of commercial β-lactoglobulin samples. Above a threshold concentration, the time evolution of the surface-shear viscosity and surface-dilation elasticity showed a steep rise to a maximum value, followed by a gradual decrease. At higher protein concentrations the surface-dilation elasticity and surface shear viscosity never rise but rather decrease from time zero. These data were consistent with the presence of low concentrations of a low-molecular-weight surfactant in the β-lactoglobulin sample, which was adsorbed slowly and gradually disrupted the elastic properties of the protein film. This hypothesis was tested by comparing native β-lactoglobulin samples with charcoal-extracted β-lactoglobulin. Data are presented from studies of surface-tension isotherms, surface-shear and dilation measurements and foaming studies in the presence of competing Tween 20, which support our original hypothesis. The high sensitivity of surface-shear viscosity measured by damped oscillation to the presence of the contaminating surfactant species and its removal was notable. The presence of the contaminating surfactant was most clearly evident from both dilation and oscillator shear measurements in the early stages of adsorption, which emphasises the necessity to collect surface rheological data in the first hour of formation of the interface. Finally, our observations may account for conflicting reports in the literature concerning the ability of β-lactoglobulin to form multilayers at the air/water interface.