Interfacial characteristics of spread films of hen egg yolk phosvitin at the air–water interface: Interrelation with its charge and aggregation state

Phosvitin is an egg yolk protein constituted by 50% of serines which are all phosphorylated. Because of this peculiarity, phosvitin has a high charge density and is known to be one of the most natural cation-binding proteins. In addition, phosvitin naturally forms aggregates through phosphocalcic bridges in egg yolk. This high charge density, doubled by this capacity to aggregate, limits the adsorption of the protein at oil–water or air–water interfaces. In this work, we investigated the impact of both the charge, by sodium chloride (NaCl) addition, and the aggregation, by calcium chloride addition, on the interfacial properties of phosvitin. The air–water interfacial activity of the phosvitin was studied with a Wilhelmy-type film balance and a Brewster angle microscope. In the presence of increasing NaCl concentrations, phosvitin presents improved interfacial properties and the film formed was more dense. The better adsorption of the phosvitin is attributed to the charge screening due to NaCl which could permit a higher spreading of the protein at the interface. Concerning the aggregation effect, calcium–phosvitin aggregates favour the contact between the protein and the interface and act on the interfacial film morphology inducing the formation of a thicker and a more rigid interfacial phosvitin film.