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 (π)–area (A  ) isotherm and surface shear viscosity (ηsηs) were obtained at 20 °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 π<πeCS), both Na-caseinate and monoglyceride coexist at the interface, with a structural polymorphism or a liquid expanded structure due to the presence of monopalmitin or monoolein in the mixture, respectively. At higher surface pressures, collapsed Na-caseinate residues may be displaced from the interface by monoglyceride molecules. For a Na-caseinate–monopalmitin mixed film the ηsηs value varies greatly with the surface pressure (or surface density) of the mixed monolayer at the interface. In general, the greater the surface pressure, the greater are the values of ηsηs. However, the values of ηsηs for a Na-caseinate–monoolein mixed monolayer are very low and practically do not depend on the surface pressure. The collapsed Na-caseinate residues displaced from the interface by monoglyceride molecules at π>πeCS have important repercussions on the shear characteristics of the mixed films.

Surface shear viscosity of () spread caseinate and adsorbed (○, ●, ▵, ▴) caseinate, (◊) monopalmitin, (▿) monoolein and (▵) β-casein films.Figure optionsDownload as PowerPoint slideFigure optionsDownload as PowerPoint slide