Competitive Adsorption of Mixed Anionic Polysaccharides at the Surfaces of Protein-Coated Lipid Droplets

Charged polysaccharides can improve the stability of protein-coated lipid droplets by forming a protective coating around them. Potentially, the interfacial characteristics of these coatings can be controlled by assembling them from mixed polysaccharides with different molecular characteristics. The purpose of this study was to examine the competitive adsorption of two anionic polysaccharides (carrageenan and pectin) to beta-lactoglobulin coated-lipid droplets. Carrageenan has a higher charge density than pectin, and carrageenan has a linear backbone whereas pectin has a linear backbone with branches. Emulsions (Phi = 1 wt % oil, d(43) = 0.40 +/- 0.03 mu m) were mixed with polysaccharide solutions (0 or 0.04 wt%) atpH 7, then the pH was decreased to promote polysaccharide adsorption. The adsorption of the polysaccharide molecules to the droplet surfaces occurred at a higher pH for carrageenan (pH approximate to 5.85) than for pectin (pH approximate to 5.45). When polysaccharide mixtures were added at pH 7, the carrageenan molecules preferentially adsorbed to the droplet surfaces when the pH was reduced. At pH 3.5, carrageenan coated droplets had a higher negative charge (xi = - 38.5 +/- 3.1 mV) than pectin-coated droplets (xi = -17.9 +/- 2.0 mV). Carrageenan was much more effective at displacing pectin from the surfaces of pectin-coated droplets, than pectin was at displacing carrageenan from carrageenan-coated droplets. The stability of pectin-coated droplets was better than carrageenan-coated droplets, which was attributed to steric hindrance effects. These results have important implications for the design of delivery systems based on polysaccharide/protein-coated droplets.