Studies on acacia exudate gums. Part I: the molecular weight of Acacia senegal gum exudate

Sixty-seven samples of A. senegal var. senegal exudate gum, supplied by primary supplier, producer and user companies were analysed using gel permeation chromatography (GPC) coupled to a multi-angle laser light scattering detector, a refractive index detector and a UV detector operated at 214 nm. A set of 5 samples were fully authenticated A. senegal var. senegal and used to provide norms against which the other gum samples could be assessed. A standard unprocessed A. senegal var. senegal (hashab) from a 15-years old mature tree was used for comparison. A feature of the results is the extensive variation between individual samples, all of which were presented to the market as ‘gum arabic’. Of the samples, 15 were outside the selected norms. While certain of these can be attributed to natural variations, other unusually high molecular weight products found could have been the results of processing methods or blending with the higher molecular weight variant A. Senegal var. karensis. The instrumental method described provides a useful ‘fingerprint’ for rapidly evaluating the characteristics of the gum for specific applications. The method allows comparative measurements of protein content and its distribution throughout the various gum components, indicating for example, its emulsification and adhesion potential. A. senegal var karensis, a variant usually found in West Africa is shown to have almost double the weight average molecular weight of A. senegal var senegal. This is due almost exclusively to the greater proportion of the high molecular weight component, which satisfactorily accounts for its characteristic rheological and gelling behaviour.