Composition and rheological properties of Albizia lebbeck gum exudate

The composition, structure and rheological properties of Albizia lebbeck gu m from Brazilian plants were investigated. The content of galactose, arabin ose and uronic acid is similar to those from Venezuelan and African gums. M annose, present in gum from other origins, was not identified in Brazilian polysaccharide. C-13 NMR spectra of intact and Smith-degraded polysaccharid e allowed the chemical shift assignment to be made. The main chain is compo sed of galactan core (1 --> 3) linked with approximately 33% of substitutio n at C-6. GPC indicated a major polysaccharide fraction of M-pk 2.4 x 10(6) g/mol and two other minor fractions of M-pk 3.6 x 10(5) and 4.5 x 10(4) 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 branche d systems with little intra- and intermolecular interactions. NaCl, CaCl2 a nd 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+. (C) 2001 Elsevier Science Ltd. All rights reserved.