Composition and rheological properties of Albizia lebbeck gum exudate

The composition, structure and rheological properties of Albizia lebbeck gum from Brazilian plants were investigated. The content of galactose, arabinose and uronic acid is similar to those from Venezuelan and African gums. Mannose, present in gum from other origins, was not identified in Brazilian polysaccharide. 13C NMR spectra of intact and Smith-degraded polysaccharide allowed the chemical shift assignment to be made. The main chain is composed of galactan core (1->3) linked with approximately 33% of substitution at C-6. GPC indicated a major polysaccharide fraction of Mpk 2.4×106 g/mol and two other minor fractions of Mpk 3.6×105 and 4.5×104 g/mol. The whole gum is a low viscosity polysaccharide with an apparent flow activation energy of 16.6 and 17.2 kJ mol-1, for solutions of 2 and 3% (w/v) concentration, respectively. These values are characteristic of branched systems with little intra- and intermolecular interactions. NaCl, CaCl2 and AlCl3 affected reduced and intrinsic viscosities and Huggins constant of gum solution in different manners. The effect, due to the affinity between A. lebbeck polysaccharide and metal ions, follows the order: Al3+>Ca2+>Na+.