Protein and lipid films at equilibrium at air-water interface

The surface activity of beta -casein, caseinate, and a whey protein isolate in aqueous solutions has been investigated over a range of protein concentrations (1 (.)10(-5) to 5% w/w) at pH 5 and 7. The surface pressure data were determined by the Wilhelmy plate method. Surface pressure data at low protein concentration indicate a low surface activity that rises to a plateau as the monolayer is saturated at higher protein concentrations. The protein concentration and the surface pressure at the plateau depend on the pH and the type of protein in the aqueous phase. Protein-monoglyceride interactions were investigated by spreading an insoluble monoglyceride (monopalmitin, monoolein, or monolaurin) on a film of protein previously adsorbed on the interface at equilibrium. The existence of protein-monoglyceride interactions depends on the interfacial composition and on the protein/monoglyceride ratio. The surface activity of mixed protein-monopalmitin and protein-monoolein films is determined by the lipid as the surface pressure of the mixed film is the same as the monoglyceride equilibrium spreading pressure, and the monolayer is not saturated by the protein. However, the protein determines the surface activity of mixed protein-monopalmitin and protein-monoolein films as the protein saturates the monolayer. For protein and monolaurin mixed films, protein determines the surface activity over the range of protein-monolaurin compositions due to monolaurin dissolution in the bulk aqueous phase.