EVIDENCE OF EXTRANEOUS SURFACTANT ADSORPTION ALTERING ADSORBED LAYER PROPERTIES OF BETA-LACTOGLOBULIN

Unusual surface-shear and surface-dilation properties have been observ ed as a function of increasing concentration of commercial beta-lactog lobulin samples. Above a threshold concentration, the time evolution o f the surface-shear viscosity and surface-dilation elasticity showed a steep rise to a maximum value, followed by a gradual decrease. At hig her 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 beta-lactoglobulin sample, which wa s adsorbed slowly and gradually disrupted the elastic properties of th e protein film. This hypothesis was tested by comparing native beta-la ctoglobulin samples with charcoal-extracted beta-lactoglobulin. Data a re presented from studies of surface-tension isotherms, surface-shear and dilation measurements and foaming studies in the presence of compe ting Tween 20, which support our original hypothesis. The high sensiti vity of surface-shear viscosity measured by damped oscillation to the presence of the contaminating surfactant species and its removal was n otable. The presence of the contaminating surfactant was most clearly evident from both dilation and oscillatory shear measurements in the e arly stages of adsorption, which emphasises the necessity to collect s urface theological data in the first hour of formation of the interfac e. Finally, our observations may account for conflicting reports in th e literature concerning the ability of beta-lactoglobulin to form mult ilayers at the air/water interface.