Chemical investigation of the structural basis of the emulsifying activity of gum arabic

Gum arabic, an exudate from Acacia trees, has a unique combination of excellent emulsifying properties and low solution viscosity. These properties make gum arabic very useful in several industries but especially in the food industry where it is used as a flavor encapsulator and stabilizer of citrus oil emulsion concentrates in soft drinks. Gum arabic is a mixture of principally polysaccharides and proteoglycans, the latter being arabinogalactan proteins (AGPs). Gum arabic also contains trace levels of lipids. Our hypothesis is that these lipids are attached to the gum arabic AGPs as glycosylphosphatidylinositol (GPI) lipids, as found in rose and other AGPs, and that these lipids make important contributions to the emulsifying activity of gum arabic. To test this hypothesis, chemical treatments expected to cleave GPI lipid anchors have been applied to gum arabic, and the resulting effects on structure and emulsifying activity have been examined. Treatment of gum arabic with nitrous acid resulted in diminished emulsifying activity, loss of some glucosamine and nitrogen, but very little effect on the principal carbohydrate composition. Treatment with 50% aqueous HF at 0 °C resulted in diminished emulsion properties but also significant loss of arabinosyl residues. The approximately 1–3% subfraction of gum arabic components that adsorb at the surface of oil droplets has higher abundance of GPI linker components, much higher relative lipid content, much higher nitrogen content, and somewhat higher emulsifying action than the whole gum. These results are consistent with roles of both lipid and protein in the emulsifying activity of the gum.