Emulsifying properties of covalent protein—dextran hybrids

Abstract

The emulsifying behaviour of covalent Maillard complexes of the non-ionic polysaccharide dextran with three proteins (11S globulin Vicia faba, bovine serum albumin, β-casein) has been investigated as a function of the molecular weight of the dextran (4 · 10 4 or 5 · 10 5 Da) and the polysaccharide/protein molar ratio R. Protein—polysaccharide hybrids are made by dry-heating mixtures of protein and dextran at 60°C for (usually) 3 weeks. The droplet-size distribution and serum layer thickness are monitored as a function of storage time for oil-in-water emulsions (10 vol.% n-tetradecane 0.5 wt.% protein. pH 8, ionic strength 0.1 M) containing various proportions of polysaccharide, present either as a Maillard complex or in a simple admixture with the protein. The results show that covalent binding of dextran to the globular proteins (bovine serum albumin and 11S globulin) can lead to a substantial improvement in the emulsifying behaviour, whereas complexation with the disordered protein β-casein has a negative effect. The globulin—dextran hybrids generally give much finer emulsions than the 11S globulin alone which is a rather poor emulsifying agent. For each protein, the emulsifying capacity and emulsion stability are sensitive to dextran molecular weight and R, and it appears that for each protein—polysaccharide combination there is a critical value of R which confers the optimum stabilization. Reaction of protein with dextran for longer than is necessary (especially that of high molecular weight) leads to loss of functionality, as does the presence of unadsorbed (unreacted) polysaccharide. The primary single reason for the improved emulsifying properties of globular proteins on complexing with polysaccharide is the enhanced steric stabilization provided by the bulky hydrophilic polysaccharide moiety. As the measured surface shear viscosity of the adsorbed globulin—dextran hybrid is only slightly different from that of the adsorbed native protein alone, it appears that the protein—protein interactions in the adsorbed layer are relatively unaffected by the complexation.