Competitive adsorption of protein and surfactants in highly concentrated emulsions: effect on coalescence mechanisms

The effect of the addition of low-molecular-weight surfactants on the coalescence-stability of emulsions stabilized by the protein β-lactoglobulin was studied in highly-concentrated emulsions. Although Tween-20 and Span 80 are both able to displace the protein from the droplet interface, only with Span 80 does this lead to coalescence by spontaneous rupture of thin films between the emulsion droplets. This suggests that this mechanism of coalescence is only enhanced if the adsorbed protein layer is displaced by a surfactant that is by itself not effective in stabilizing O/W-emulsions. Flow-induced coalescence, another important mechanism for coalescence in highly-concentrated protein-stabilized emulsions, was reduced after the addition of SDS and Tween-20 at concentrations where they are able to displace the adsorbed protein. Span 80 and a sucrose monoester of fatty acids did not reduce flow-induced coalescence at the concentrations applied, which is probably due to their moderate ability to displace adsorbed protein. The above observations give new insight into the function of added surfactant in protein-stabilized emulsions.