Comparison of the dilational behaviour of adsorbed milk proteins at the air-water and oil-water interfaces

Abstract

The interfacial dilational properties of two milk proteins, β-casein and β-lactoglobulin, have been compared at the air-water and paraffin oil-water interfaces. The measurements were performed as a function of bulk protein concentration using a modified Langmuir trough technique at a frequency of 0.1 Hz. It appears that the dilational properties of the two proteins are essentially the same whether the upper phase is paraffin oil or air. This is attributed to rather limited penetration of the oil phase by the protein molecules. The interfacial properties have been shown to be dependent upon the bulk concentration of the protein. This, together with the history of the surface, presumably determines the interfacial concentration and conformation of the protein molecules. At low bulk concentrations both the modulus and the loss tangent are similar for the two proteins. It is conjectured that both proteins have a similar, reasonably extended, structure in the interface. At higher bulk concentrations the behaviour of the two proteins diverges. The interfacial dilational modulus for β-lactoglobulin is around 62 mN m −1 for bulk concentrations ⩾ 3× 10 −2 g l −1, and the viscosity of the surface is rather low. It is postulated that this protein forms a cohesive elastic network within the interface. The interfacial dilational modulus for β-casein at similar concentrations is around 13 mN m −1. It is thought that some intermolecular cohesion is present in the interface, but that this is much weaker than for the globular protein. High concentrations of β-casein impart significant dilational viscosity to the interface, due either to diffusional relaxation or to rearrangement of the adsorbed primary layer and multilayers of protein.