Competitive adsorption between dioleoylphosphatidylcholine and sodium caseinate on oil-water interfaces

Oil-in-water emulsions stabilized by sodium caseinate and dioleoylphosphatidylcholine (DOPC) were studied by dynamic light scattering and electrophoresis. The initial mean droplet size of the emulsions depended strongly on the concentration of casein, but was virtually independent of DOPC. However, the emulsion stability was strongly affected by the presence of DOPC; with a DOPC: casein molar ratio of <10, the emulsions were stable, but at higher ratios, the average size of emulsion droplets increased during storage. Competitive adsorption between DOPC and casein occurred during both the formation and storage of the emulsions. Of the constituents of whole casein, beta-casein interacted most with DOPC. At casein concentrations of <0.7%, beta-casein was completely removed from the oil-water interface when the emulsions were stored for 48 h, but alpha(s1)-casein remained on the surface even at the highest DOPC:casein molar ratio (49:1) used in this study. As well as changing the adsorption characteristics of the different caseins, DOPC also changed the hydrodynamic thickness of the adsorbed casein layer. Compared with emulsions stabilized by casein alone, the layer thickness of caseins in the emulsions formed with mixtures of caseinate and DOPC changed because of competitive adsorption between DOPC and casein at the oil-water interface.