DESTABILIZATION OF ALPHA-LACTALBUMIN FOAMS BY COMPETITIVE ADSORPTION OF THE SURFACTANT TWEEN-20

The destabilization of foams formed from alpha-lactalbumin (0.5 mg ml-1; 35.7-mu-M) as a function of Tween concentration has been investigated using a conductimetric method. Very low levels of Tween 20 corresponding to surfactant-to-protein molar ratios (R) in the range 0.05-0.15 significantly destabilized the foams. The decrease in foam stability was accompanied by a decrease in the equilibrium thickness and a change in the drainage behaviour of air suspended thin liquid films of the solutions. Minimum foam stability was observed at R = 0.15 and correlated with a sharp decrease in the surface tension. At higher R values, the stability of the foams gradually increased. This transition coincided with the appearance of surface diffusion in the adsorbed protein fraction. At R = 0.4 the stability of the mixed component foam exceeded that of the protein alone even though there was insufficient Tween to support a foam in the absence of the protein. It is concluded that the two components act synergistically and this behaviour is contrasted with that of other proteins in the presence of Tween 20.