Emulsifying behaviour of protein in the presence of polysaccharide under conditions of thermodynamic incompatibility

Abstract

The influence of the non-ionic polysaccharide dextran on the emulsifying properties of the proteins 11S globulin Vicia faba and bovine serum albumin (BSA) has been investigated at pH 8.0, ionic strength 0.1 mol dm^–3. The average droplet size in hydrocarbon oil-in-water emulsions (0.5 wt.% protein, 10 vol.% oil) was found to depend on the polysaccharide : protein molar ratio and the molecular weight of the dextran (4 × 10⁴ or 5 × 10⁵ u). Enhancement in emulsifying capacity is favoured by a mixed biopolymer system (11S globulin + dextran) with a positive cross second virial coefficient, implying thermodynamic incompatibility at high biopolymer concentrations. Loss of emulsifying capacity of 11S globulin in the presence of high concentrations of high-molecular-weight dextran is due to incipient phase separation during emulsification. Smaller droplets in the fresh emulsion confer improved stability with respect to creaming and coalescence, but this may be reversed by deleterious effects of flocculation induced by the added polysaccharide. Whether the droplet flocculation arises from a depletion mechanism or a bridging mechanism depends on whether the protein–polysaccharide interaction is net repulsive or net attractive. Weak adsorption of the polysaccharide on to the surface of protein-coated emulsion droplets is consistent with bridging flocculation in the BSA + dextran system.