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Six commercial gum arabic samples and authenticated samples of gum from Acacia Senegal and Acacia seyal have been characterized using a range of chemical and physicochemical techniques including determination of specific rotation, sugar composition, nitrogen and amino acid content, and molecular mass distribution. Although some of the gums have slightly different chemical and physicochemical characteristics, gel permeation chromatography shows that each consists of essentially three molecular mass fractions classified as an arabinogalactan, an arabinogalactan-protein complex, and a glycoprotein.The proportions of each varied for the individual samples. The gums were tested using a specifically developed enzyme-linked immunosorbent assay which can identify gum from A. Senegal and chemically related species. Their interaction with the antibody could be correlated with differences observed in their molecular compositions. Such evidence will assist in their chemotaxonomic classification.

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The physiochemical and immunological properties of three Sudanese gum arabic samples and four gum tahla samples (two Sudanese, one West African and one Tanzanian--Acacia seyal var. seyal) were compared. The optical rotation (ca -30 degrees) and rhamnose (12-14%), arabinose (24-29%), galactose (36-42%), glucuronic acid (16-17%), nitrogen (0.327-0.365%) and protein (2.16-2.41%) contents of the gum arabic samples were consistent with the FAO (1990) specification for Acacia gum. In contrast the gum tahla samples had positive [alpha]D values (+45 degrees to +54 degrees), lower rhamnose (3-4%) and higher arabinose (41-45%) contents and lower nitrogen (0.147-0.175%), and hence protein (0.97-1.15%), contents. All of the gum arabic samples precipitated with beta-glucosyl Yariv reagent and hence were shown to contain arabinogalactan-protein(s) (AGPs), whereas in all but one of the gum tahla samples AGPs were not detected. The strong interaction of gum tahla with a monoclonal antibody known to recognize arabinose residues present in AGPs and arabinogalactans (AGs) was consistent with the observed higher levels of arabinose present in the gum tahla samples relative to the gum arabic samples. The data presented confirm that there are a number of physicochemical and structural differences between gum arabic (A. senegal gum) and gum tahla (A. seyal gum), and that a quick and simple immunological technique (immunodot blots) using an antiAGP/AG monoclonal antibody (MAC 207) could be used to screen for the presence of gum tahla in gum arabic consignments.

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