Effect of β-casein on surface activity of Quillaja bark saponin at fluid/fluid interfaces

Effect of a model bovine milk protein, β-casein, on surface activity of Quillaja bark saponin (QBS) from Sigma was studied at three fluid/fluid interfaces: air/water, tetradecane/water and olive oil/water. In all cases, the protein concentration was fixed at 10−6 mol L−1, and QBS concentration was varied between 5·10−7 and 1·10−3 mol L−1. Dynamic interfacial tension on the timescale 5 s–3600 s was measured using a drop shape analysis technique. For the air/water system, they were complemented with short-term (50 ms–5 s) measurements using a maximum bubble pressure technique. The dynamic results together with the extrapolated equilibrium surface pressures are discussed from the point of view of a complexation between β-casein and QBS, with the surface activity of the complex changing with its stoichiometry. At low biosurfactant/protein ratios, the interfacial tension at all three interfaces passes through a maximum, corresponding to a transient decrease of both foam and emulsion formation ability. In addition, the effect of QBS on deterioration of β-casein's surface activity upon ageing at room temperature is discussed.

Figure optionsDownload as PowerPoint slide