THE INFLUENCE OF SURFACE-COMPOSITION AND MOLECULAR-DIFFUSION ON THE STABILITY OF FOAMS FORMED FROM PROTEIN SURFACTANT MIXTURES

A conductimetric study of foams formed from mixtures of the protein, β-lactoglobulin (10.9 μM), and the nonionic surfactant, Tween 20, revealed that their stability was reduced at concentrations of Tween 20 in the range 20-90 μM. The interaction of Tween 20 with β-lactoglobulin was investigated by fluorimetry and a dissociation constant of 0.11 μM was calculated for the complex. Surface tension studies confirmed the presence of interaction between the two components and provided evidence for the progressive displacement of β-lactoglobulin from the air/water interface with increasing Tween 20 concentration. Experiments using air-suspended microscopic thin liquid films revealed transitions in the drainage characteristics and thickness of the film at Tween 20 concentrations below that which resulted in destabilization of the foam. However, measurements of surface mobility of fluorescent-labeled β-lactoglobulin by a photobleaching method identified that a transition to a mobile system occurred at a Tween 20 concentration which correlated with the onset of instability in the disperse phase. The results would indicate that maintenance of the viscoelastic properties of the surface is of paramount importance in determining the stability of interfaces comprising mixtures of protein and surfactant. © 1990.