Adsorption behavior of milk proteins on polystyrene latex. A study based on sedimentation field-flow fractionation and dynamic light scattering.

Sedimentation field-flow fractionation (SdFFF) has been used to characterize the adsorption of the proteins beta-casein (BCN) or beta-lactoglobulin (BLG) on colloidal polystyrene latices; this system was used to model hydrophobic interactions between the proteins and the surfaces of fat droplets in protein-stabilized emulsions. It was found that the SdFFF technique could determine directly the surface concentrations of BCN and BLG irreversibly adsorbed to the latex surface, provided care was taken to maintain the ionic strength of the carrier at a level which suppressed particle-wall repulsion in the separation channel. The measured surface concentrations were similar for the two proteins (about 1 mg/m2), and this was verified by quantitative amino acid analysis. These concentrations were smaller than those found in depletion studies (3 and 4 mg/m2 respectively for BCN and BLG), in which loosely associated protein may have been included in the determinations. The thickness of the adsorbed layers was determined in situ by dynamic light scattering and was found to differ significantly for the two proteins (up to 15 nm for BCN vs. 2-3 nm for BLG). The implication of these findings in terms of different surface arrangements of the two proteins is discussed.