Structural characteristics and rheological properties of locust bean galactomannans: a comparison of samples from different carob tree populations

Water-soluble galactomannans were isolated from the seeds of 12 carob tree populations grown in different regions of southern Greece. Their structures , molecular weight distributions and rheological properties were examined. The intrinsic viscosity values and estimated average molecular weights vari ed in the ranges 8.15-13.02 dl g(-1) and (2.3-3.9) x 10(6) respectively. Mo lecular sieve chromatography showed large variations in the molecular size distributions. The Manp/Galp ratio varied between 3.08 and 3.82, as determi ned by GC analysis of the alditol acetate derivatives of the monomeric cons tituents. There were also differences in the relative distribution of the f ree (Manp/Manp)), monosubstituted (Manp-Manp(Galp) and disubstituted (Manp( Galp)-Manp(Galp)) dyads of beta-(1-->4)-mannose, as revealed by C-13 NMR sp ectroscopy of the native polysaccharides. The pseudoplastic behaviour of ga lactomannan solutions (10-20 g l(-1)) was confirmed by steady shear and dyn amic rheological tests. Aqueous galactomannan (5 g l(-1))-xanthan gum (5 g l(-1)) mixed systems gave gels on cooling (G'much greater thanG "). Galacto mannans with high limiting viscosities, high Manp/Galp ratios and a low pro portion of the Manp(Galp)-Manp(Galp) dyad yielded the strongest mixed gel n etwork structures with xanthan, implying that interchain associations betwe en the two polymers occur via the unsubstituted segments ('smooth regions') of the carob galactomannan chains. (C) 2000 Society of Chemical Industry.