The role of additives for the behaviour of thin emulsion films stabilized by proteins

Abstract

Experimental results obtained with thin aqueous films of emulsion type stabilized by bovine serum albumin (BSA) and β-casein are presented. The film behaviour is time dependent. The contact angle increases with ageing and exhibits pronounced hysteresis. With BSA one observes slow reversible aggregation on the surface (but not in the bulk) and the protein lumps are gradually squashed by the capillary pressure as the film thins. The findings can be explained by slow surface denaturation, accompanied by developing attraction and partial entanglement of the BSA molecules. These processes are promoted by oleic acid dissolved in the oil phase. Electrostatic interactions were found to be important: without salt the films remain thick, whereas in the presence of 0.15 M NaCl one obtains Newton black films whose contact angle depends upon the molecular charge. A marked difference in the surface mobility is observed with foam and emulsion films stabilized by BSA. Lenses, containing protein aggregates and liquid, when surrounded by an area which has reached the black film stage, remain entrapped in foam films but are slowly squeezed out in emulsion films. Hydrophobization of the protein molecules may be responsible for this behaviour. With β-casein, ageing effects in films are observed only at the isoelectric point. This protein strongly aggregates in the bulk, but the lumps are readily flattened on the film interfaces. Addition of Ca 2+ ions leads to a decrease in film thickness, depending on the concentration.