Caseinate-Induced Competitive Displacement of Whey Protein from Interfaces

Abstract

The caseinate-induced competitive displacement of whey protein from planar air-water interfaces was investigated based on atomic force microscopy (AFM) imaging and that from the surfaces of oil droplets immersed in aqueous solution based on AFM force spectroscopy. After the addition of sodium caseinate to the sub-phase, the surface pressure of planar interfacial films of pre-adsorbed whey protein increased from 8 mN/m to up to 21 mN/m. The thicknesses of interfacial films were uniform and remained to be approximately 2 nm at relatively low surface pressures up to 18 mN/m, while they became uneven at higher surface pressures and increased to up to 7.1 nm, presumably due to the compression of interfacial whey protein networks by adsorbed caseinate. The rigidity of oil droplets coated with protein adsorbed to their surfaces was then evaluated based on the slope of approximately linear force-distance curves obtained by pressing an oil droplet against another. The adsorption of whey protein to oil droplet surfaces increased droplets’ rigidity. The subsequent addition of caseinate to the bulk solution surrounding oil droplets coated with pre-adsorbed whey protein further increased droplets’ rigidity. The present results suggest that caseinate adsorbed to an interface to which whey protein had adsorbed in advance did not completely expel pre-adsorbed whey protein molecules into the aqueous phase but caused a compaction of interfacial whey protein networks and thereby strengthened the interfacial film.