Spreading of Monoglycerides onto β-Casein Adsorbed Film. Structural and Dilatational Characteristics

Abstract

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.

Flow-Induced Molecular Segregation in β-Casein−Monoglyceride Mixed Films Spread at the Air−Water Interface

“…The π− A isotherms for monoolein, β-casein, and a β-casein−monoolein mixed monolayer at a mass fraction of monoolein in the mixture of X MO = 0.5 (as an example) are shown in Figure A. As expected, monoolein monolayers had a liquid-expanded structure under all experimental conditions. The collapse of monoolein monolayers occurred at a surface pressure close to the equilibrium surface pressure (π e ≅ 45.7 mN/m).…”

Interfacial Properties of Diglycerol Esters and Caseinate Mixed Films at the Air−Water Interface

“…Measurements were performed with a Sigma 701 digital tensiometer (KSV, Finland), on the basis of the Wihelmy method, with a roughened platinum plate, as described elsewhere. 20 Unlike the case of lipids, true equilibrium adsorption does not seem to be possible with proteins, even after 2 or 3 days. 20 Therefore, we considered the surface pressure measured after 24 h as the pseudoequilibrium value.…”

“…20 Unlike the case of lipids, true equilibrium adsorption does not seem to be possible with proteins, even after 2 or 3 days. 20 Therefore, we considered the surface pressure measured after 24 h as the pseudoequilibrium value. Other experiments were done to study caseinate-lipid interactions at the air-water interface.…”

“…The same protein concentration dependence on surface pressure was observed for milk and vegetable proteins. 32 Caseinate showed significant differences from -casein, 20 the most surface-active component of caseinate. In fact, (1) caseinate showed significant surface activity at concentrations in the bulk phase of 1‚10 -5 %.…”

“…That is, at the plateau, the surface activity of the proteins forming caseinate gave higher surface activity than -casein alone, behavior similar to that observed with spread films. 20,33 The highest protein concentration at which the plateau is attained is the adsorption efficiency (AE). At this protein concentration, the monolayer is saturated by protein that is irreversibly adsorbed.…”