Protein-lipid interactions at the oil-water interface.

The distribution of proteins and lipids in food emulsions and foams is determined by competitive and cooperative adsorption between the two types of emulsifiers at the fluid-fluid interfaces, and by the nature of protein-lipid interactions, both at the interface and in the bulk phase. The existence of protein-lipid interactions can have a pronounced impact on the surface rheological properties of these systems. Therefore, these results are of practical importance for food emulsion formulation, texture, and stability. In this study, the existence of protein-lipid interactions at the interface was determined by surface dynamic properties (interfacial tension and surface dilational modulus). Systematic experimental data on surface dynamic properties, as a function of time and at long-term adsorption, for protein (whey protein isolate (WPI)), lipids (monoglycerides), and protein-lipid mixed films at the oil-water interface were measured in an automated drop tensiometer. The dynamic behaviour of protein+lipid mixed films depends on the adsorption time, the lipid and the protein/lipid ratio in a rather complicated manner. The protein determined the interfacial characteristics of the mixed film as the protein at WPI>/=10(-2)% wt/wt saturated the film, no matter what the concentration of the lipid. However, there exists a competitive or cooperative adsorption of the emulsifier (WPI and monoglycerides), as the concentration of protein in the bulk phase is far lower than that for interfacial saturation.