Structural and shear characteristics of adsorbed sodium caseinate and monoglyceride mixed monolayers at the air-water interface.

The structural and shear characteristics of mixed monolayers formed by an adsorbed Na-caseinate film and a spread monoglyceride (monopalmitin or monoolein) on the previously adsorbed protein film have been analyzed. Measurements of the surface pressure (pi)-area (A) isotherm and surface shear viscosity (eta(s)) were obtained at 20 degrees C and at pH 7 in a modified Wilhelmy-type film balance. The structural and shear characteristics of the mixed films depend on the surface pressure and on the composition of the mixed film. At surface pressures lower than the equilibrium surface pressure of Na-caseinate (at pipi(e)(CS) have important repercussions on the shear characteristics of the mixed films.

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.

The Effect of pH on Monoglyceride-Caseinate Mixed Monolayers at the Air-Water Interface.

The surface pressure (pi) area (A) isotherms and Brewster angle microscopy (BAM) of monoglyceride-caseinate mixed films spread on buffered water at pH 5 and 7 and at 20 degrees C were determined as a function of the mass fraction (X) of monoglyceride (monopalmitin or monoolein) in the mixture. The structural characteristics, miscibility, and morphology of monoglyceride-caseinate mixed films are very dependent on surface pressure and monolayer composition. The monolayer structure was more expanded as the pH and the monoglyceride concentration in the mixture were increased. From the concentration and surface pressure dependence on excess area, free energy, and collapse pressure, it was deduced that, at a macroscopic level, monoglyceride (either monopalmitin or monoolein) and caseinate form a practically immiscible monolayer at the air-water interface. The BAM images and the evolution with the surface pressure of the relative reflectivity of BAM images give complementary information on the interactions and structural characteristics of monoglyceride-caseinate mixed monolayers, which at a microscopic level corroborated in part the conclusions derived from the pi-A isotherm at a macroscopic level. Over the overall range of existence of the mixed film the monolayer presents some heterogeneity due to the fact that domains of monoglyceride (especially of monopalmitin) and spots of collapsed caseinate residues are present during the monolayer compression-expansion cycle, giving relative intensity peaks with high relative film thickness. At higher pi, after the caseinate collapse, characteristic squeezing-out phenomenon was observed. At the monoglyceride monolayer collapse the mixed film is practically dominated by the presence of monoglyceride. The prevalence of monoglyceride in the interface increases with the amount of monoglyceride in the mixture and at higher pi. However, some degree of interactions exists between monoglyceride and caseinate in the mixed film and these interactions are more pronounced as the monolayer is compressed at the highest surface pressures. Copyright 2001 Academic Press.