Protein/surfactant interfacial interactions part 1. Flocculation of emulsions containing mixed protein + surfactant

Abstract

The effect of interactions of positively charged gelatin and anionic surfactant sodium lauryl ether sulphate (SLES 2EO) with negatively charged β-lactoglobulin in solutions and emulsions has been studied. Precipitation was observed in solution for a gelatin + SLES 2EO mixture with and without β-lactoglobulin. The precipitation concentrations of gelatin and SLES 2EO were found to be strongly dependent on the bulk pH conditions and whether β-lactoglobulin was present. The introduction of SLES 2EO to a gelatin-stabilized emulsion induces destabilization followed by restabilization. The optimum surfactant concentration for the maximum degree of flocculation was found to be much higher than that for maximum precipitation of the corresponding mixture in bulk aqueous solution. Different extents of flocculation were observed when a certain amount of gelatin or gelatin + SLES 2EO was added to a 0.4 wt.%, β-lactoglobulin-stabilized emulsion. Higher gelatin and SLES 2EO concentrations can cause the flocculated emulsion to become restabilized. The character of the interfacial protein surfactant interactions are therefore found to be different in nature from those in the bulk aqueous phase. A bridging mechanism and a cross-linking mechanism were separately suggested to explain the types of flocculation induced by two different kinds of interactions. The effects on the flocculation of changing the temperature, incorporating oil-soluble surfactant, and adjusting the procedure, have been investigated. The present results are discussed in the light of other experimental results obtained on the same systems by complementary techniques.