Characterisation and molecular association of Nigerian and Sudanese Acacia gum exudates

Authors: GASHUA, I - Glyndwr University; WILLIAMS, P - Glyndwr University; Yadav, Madhav; BALDWIN, T - Faculty Of Science And Engineering, University Of Wolverhampton

Submitted to: Food Hydrocolloids
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/31/2015
Publication Date: 6/9/2016
Citation: Gashua, I.B., Williams, P.A., Yadav, M.P., Baldwin, T.C. 2016. Characterisation and molecular association of Nigerian and Sudanese Acacia gum exudates. Food Hydrocolloids. 51:405-413.

Interpretive Summary: Gum arabic, a naturally occurring polymer, is widely used in the food industry as a stabilizer, encapsulator and emulsifier of citrus oil or other beverage flavors. The protein portion of gum Arabic is thought to be mostly responsible for its emulsifying activities. The gum arabic isolated from different acacia trees contain different percentages of protein. This finding has initiated us to study gum arabic from two different origins (Nigeria and Sudan). Interestingly the Nigerian samples were found to contain significantly more protein than the Sudanese samples. Thus the gum samples of Nigerian origin should be a better emulsifier for stabilizing oil-in-water emulsions than the Sudanese origin. Whether it is due to genotypic differences between the trees harvested or is merely due to climate difference or a combination of the two is unknown. We have also found that the gum arabic molecules associate readily in solution due to the presence of protein components and glucuronic acid residues. Such association increases with the increasing protein content in the molecules. But this association was inhibited by electrolyte, which indicated an ionic interaction between them. These results will be valuable for the soft drink industries, which are looking for the active flavor stabilizing gum arabic with high protein content.

Technical Abstract: The chemical and physicochemical characteristics of gum exudate samples harvested from mature trees of Acacia senegal at two specific locations in Nigeria have been investigated together with gum samples harvested from Acacia senegal and Acacia seyal originating from Sudan. The monosaccharide sugar compositions for all three Acacia senegal gum samples were found to be similar, but the protein contents for the Nigerian samples were significantly higher than recorded for the Sudanese sample. Gel Permeation Chromatography coupled to light scattering, refractive index and U.V. detectors, has shown the presence of arabinogalactan, arabinogalactan-protein and glycoprotein fractions within the Acacia senegal gums and has also shown the presence of an additional small proportion of very low molar mass proteinaceous material in all of the samples which has previously been ignored. The plot of radius of gyration, Rg, as a function of elution volume showed a discontinuity for one of the Nigerian gum samples and for the Acacia seyal gum sample at elution volumes corresponding to the AGP component suggesting a different molecular structure. Plots of Mw – v – Rg confirmed that the molecules had a highly compact structure. The hydrodynamic size of the molecules was followed using dynamic light scattering as a function of time and it was found that molecular association occurred in solution. The extent of association increased as the protein content in the sample increased and was inhibited in the presence of electrolyte, hence it was concluded that association was due to electrostatic interaction between the protein moieties and glucuronic acid groups on individual macromolecules.