Emulsion stabilization by ionic and covalent complexes of beta-lactoglobulin with polysaccharides

The influence of protein-polysaccharide interactions on the surface and emulsifying properties of beta-lactoglobulin at pH 7 has been investigated for systems containing the non-ionic polysaccharide dextran, and the anionic polysaccharides dextran sulphate and propylene glycol alginate (PGA). Tension measurements at the n-hexadecane-water interface are indicative of complex formation with dextran sulphate or PGA, but not with dextran. Emulsions made with simple 1:1 mixtures of protein + polysaccharide (0.5 wt% protein, 10 wt% n-hexadecane) give poorer stability than with protein alone under similar conditions. The presence of unadsorbed polysaccharide gives rapid serum separation due to depletion flocculation. Of the three polysaccharides, PGA gives the largest droplets in the fresh emulsion, but the best stability with respect to coalescence; this is consistent with the formation of a weak protein-PGA complex at the oil-water interface. An attempt has been made to produce strong complexes of beta-lactoglobulin with each of the polysaccharides by dry-heating the mixtures at 60 degrees C for up to 3 weeks. This treatment yields a beta-lactoglobulin-dextran hybrid which is believed to contain a covalent Maillard-type linkage between the two biopolymers, and which gives emulsions with excellent stability with respect to creaming, coalescence and serum separation. A beta-lactoglobulin-dextran sulphate composite produced by the same dry-heating treatment shows little evidence for Maillard-type browning and has poor emulsifying properties. Dry-heating of a mixture of beta-lactoglobulin and PGA gives a complex that confers emulsion stability which is at least as good as that for the beta-lactoglobulin-dextran hybrid. These results show that protein-polysaccharide emulsifiers with good solubility and excellent emulsion stabilizing properties at pH 7 can be made with food-grade ingredients without the use of chemicals.