Competitive adsorption of l-α-lysophosphatidylcholine/β-lactoglobulin mixtures at the interfaces of foams and foam lamellae

The stability of foams formed with the protein β-lactoglobulin as a function of increasing concentration of the lipid analogue l-α-lysophosphatidylcholine were investigated using a microconductivity technique. The drainage, surface diffusion and thickness properties of thin liquid films (foam lamallae) were also studied using optical microscopy including epi-illumination, fluorescence recovery after photobleaching and film interferometry techniques. In addition, the surfactant binding properties of the protein were examined. The addition of small quantities of l-α-lysophosphatidylcholine to β-lactoglobulin (molar ratio, R < 7:1) increased the foam stability, whereas a slightly higher concentration of surfactant in the mixture (R = 10) caused foam destabilisation. The explanation of these observations is based on changes in the composition and structure of the adsorbed interfacial layers of the thin films caused by competitive displacement of the protein by the surfactant.