Casein adsorption on the surfaces of oil-in-water emulsions modified by lecithin

The effects of incorporating an additional component, egg-yolk lecithin, on the properties of oil-in-water emulsions stabilized by casein have been studied. The impact of lecithin on the stability of the emulsions was studied using integrated light scattering and the casein-oil-lecithin interaction was studied with photon correlation spectroscopy combined with breakdown of the adsorbed protein layers by proteolysis. Lecithin was found to enhance the stability of the emulsions at low casein concentrations, below the limiting surface coverage of 1 mg m-2 of casein which is found in the absence of lecithin. Conversely, small amounts of casein also stabilized flocculating oil-lecithin emulsions. The hydrodynamic thickness of the adsorbed protein layer on the hydrophobic oil surface was modified by the presence of lecithin. When the total surface area occupied by lecithin was less than 10% (5 mg lecithin for 2 ml oil), the thickness of the adsorbed casein layer was not significantly different from that in the absence of phospholipid. At higher concentrations of lecithin, the adsorbed casein layer had a lower minimum value for the layer thickness of 6.5 nm at low casein concentration and an upper plateau value of 8 nm at saturated adsorption, compared to a low limit of 5 nm and a plateau value of 10 nm in the absence of lecithin, demonstrating that the structure of the adsorbed casein layer was changed by the presence of phospholipid.