Studies on acacia exudate gums.: Part V.: Structural features of Acacia seyal

An investigation of the molecular structure of Acacia seyal relative to Acacia senegal provides an indication of the components which influence emulsification effectiveness. Samples of A. seyal var. seyal and A. seyal var. fistula were fractionated using gel permeation chromatography. The fractions and the whole gum were analysed to determine the molecular weight, sugar content, amino acid, protein content, nitrogen, and intrinsic viscosity. The results confirm earlier findings that samples of A. seyal, as a broad grouping, have weight average molecular weights several times greater than A. senegal, due to the greater proportion of the high molecular weight component. Although the molecular weight of A. seyal is considerably greater than A. senegal, the intrinsic viscosity is less. The structure is, therefore, more compact, than the structure of A. senegal. The sugar composition and amino acids in each of the gums are identical but are present in different proportions, which is the main reason why A. seyal is dextrorotatory and A. senegal is laevorotatory. The distribution of the protein is different between the various components which constitute the gums. In A. senegal the protein is mainly located in association with the high molecular weight component (AGP-peak 1). Enzyme hydrolysis points to two components being associated with the high molecular weight material in A. seyal, only one of which is degraded by the enzyme pronase. In hydrophobic fractionation studies, a protein rich component of extremely high molecular weight (Fraction 3) was found in A. seyal but not in A. senegal. In size fractionation this would co-elute with the main component, which is further evidence for the presence of the two different high molecular weight components in A. seyal, unlike A. senegal. The adsorption of these high molecular weight protein fractions of A. seyal onto an oil droplet was tested and found not to be a highly efficient emulsifying component. Structurally, while A. seyal may have the same core structural linkages as A. senegal, the degree of branching is greater, with the protein distributed differently. (c) 2004 Elsevier Ltd. All rights reserved.