Spreading of monoglycerides onto beta-casein adsorbed film. Structural and dilatational characteristics.

The effect of monoglycerides (monopalmitin and monoolein) on the structural, topographical, and dilatational characteristics of betacasein adsorbed film at the air-water interface has been analyzed by means of surface pressure (pi)-area (A) isotherms, Brewster angle microscopy (BAM), and surface dilatational rheology. The static and dynamic characteristics of the mixed films depend on the interfacial composition and the surface pressure. At surface pressures lower than that for the beta-casein collapse (at the equilibrium surface pressure of the protein, pi(e)(beta-casein)) a mixed film of beta-casein and monoglyceride may exist. At higher surface pressures the collapsed beta-casein is partially displaced from the interface by monoglycerides. However, beta-casein displacement by monoglycerides is not quantitative at the monoglyceride concentrations studied in this work. The protein displacement by a monoglyceride is higher for monopalmitin than for monoolein and for spread than for adsorbed films. The viscoelastic characteristics of the mixed films were dominated by the presence of beta-casein in the mixture. Even at the higher surface pressures (at pi > pi(e)(beta-casein)) the small amounts of beta-casein collapsed residues at the interface have a significant effect on the surface dilatational properties of the mixed films. The structural, topographical, and viscoelastic characteristics of the mixed films corroborate the fact that protein displacement for monoglycerides is higher for spread than for adsorbed mixed films.