SURFACE SHEAR VISCOSITY AND PROTEIN SURFACTANT INTERACTIONS IN MIXED PROTEIN FILMS ADSORBED AT THE OIL-WATER INTERFACE

The surface shear viscosity of protein films adsorbed from pure or mixed beta-lactoglobulin + cationic gelatin aqueous phases have been studied at the n-hexadecane-water interface (5 mmol/dm(3) bis-tris-propane buffer, pH 7.0, 2 x 10(-3) wt% protein). The effect of the introduction of nonionic surfactant Tween 20 (polyoxyethylene (20) sorbitan monolaurate) and anionic surfactant SLES 2EO (sodium lauryl ether sulphate) into the subphase below the 1-day-old pure or mixed protein films has been investigated. The addition of Tween 20 to individual beta-lactoglobulin or gelatin films brings the surface viscosity rapidly down to zero, but not so for the mixed beta-lactoglobulin + gelatin film. The addition of SLES 2EO to the gelatin film produces a molecular rearrangement and a gradual decrease of the surface viscosity. The effect of oil-soluble surfactant C(12)E(2) (diethylene glycol monododecyl ether) on the surface shear viscosity of adsorbed beta-lactoglobulin has been studied. A small amount of C(12)E(2) induces a large increase in surface viscosity. Synergistic adsorption effects are proposed between the oppositely charged beta-lactoglobulin and gelatin molecules and between the oil-soluble surfactant C(12)E(2) and the beta-lactoglobulin molecules.