BETA-CASEIN ADSORPTION AT THE AIR-WATER-INTERFACE

The adsorption of reductively methylated bovine β-casein to the air/water interface has been measured using the radiotracer technique. This method allows determination of both the surface excess concentration at the air/water interface and the amount of protein adsorbed on the Teflon sample container. The present results differ from previous β-casein isotherms reported in the literature due to improvements in the calibration and radiolabeling methods and accounting for protein adsorption on the container surface. The isotherm shows that increasing the bulk concentration increases the surface concentration except in the ranges 2 × 10-5 to 8 × 10-5 wt% and 2 × 10-2 to 0.1 wt%, where constant surface coverages are observed. This behavior can be interpreted as being due to molecular reorientation of adsorbed β-casein molecules. A kinetic model which adequately characterizes this adsorption behavior is presented. Below 10-4 wt% the directly measured air/water isotherm and the Teflon/water isotherm inferred from the radiotracer measurements are very similar. Adsorption experiments performed by sequential addition of protein to the bulk solution show that β-casein molecules adsorbed at the air/water and Teflon/water interfaces exchange freely with β-casein molecules in the bulk solution. © 1991.