Submitted to: Food Hydrocolloids
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/1/2006
Publication Date: 6/1/2007
Citation: Yadav, M.P., Igartuburu, M.J., Yan, Y., Nothnagel, E.A. 2007. Chemical investigation of the structural bases of the emulsifying activity of gumarabic. Food Hydrocolloids 21,p.297-308. Interpretive Summary: Gum arabic, a natural exudate from Acacia trees, forms a very concentrated solution that is very low in viscosity. This property makes it useful in several industries but especially in the food and beverage industries as an emulsifier, encapsulating agent, clouding agent etc. In particular, it is used as an emulsifier (a stabilizing agent for oil/water mixtures) for citrus oil emulsions in several soft drinks. The properties of gum arabic that make it such a good emulsifying agent are not well understood, and the variable quality in the current supply makes the identification of alternative emulsifiers desirable. To better understand how gum arabic functions as an emulsifier, we tested the hypothesis that when gum arabic is mixed in a water-in-oil dispersion, its intact lipid groups readily localize to the water-oil interface at the surface of the oil droplets and its large water loving carbohydrate portion position itself in the water phase keeping the oil droplets dispersed. The emulsifying capacity of gum arabic is diminished by the chemical cleavage of the lipid containing part. We found that only a very small population of gum arabic, which adsorbs to the oil droplets, has relatively higher contents of lipids as compared to the whole gum. The results which identify the structural basis of the emulsifying activity of gum arabic can be useful to find its natural or domestically prepared alternative, since its commercial supplies are variable due to change in climate and politics etc. These finding are of great importance to the food, pharmaceutical and cosmetic industries, which can use this information to potentially modify the gum arabic and make it a better emulsifier.
Technical Abstract: Gum arabic, an exudate from Acacia trees, has a unique combination of excellent emulsifying properties and low solution viscosity despite its molecular mass of about 384 kDa. These properties make gum arabic very useful in several industries but specially in the food industry where it is used as a flavor encapsulator and stabilizer of citrus oil emulsion concentrates in soft drinks. It 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 carbohydrate composition and emulsifying activity have been examined. Treatment of gum arabic with nitrous acid resulted in diminished emulsifying activity without affecting the principal carbohydrate composition. Treatment with 50% aqueous HF at zero degree resulted in diminished emulsion properties but also detectable effects on glycosyl composition. The approximately 1% subfraction of gum arabic components that adsorb at the surface of oil droplets has higher abundance of GPI linker components and much higher relative lipid content that the whole gum.