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 varied in the ranges 8.15–13.02 dl g−1 and (2.3–3.9) × 106 respectively. Molecular sieve chromatography showed large variations in the molecular size distributions. The Manp/Galp ratio varied between 3.08 and 3.82, as determined by GC analysis of the alditol acetate derivatives of the monomeric constituents. There were also differences in the relative distribution of the free (Manp-Manp), monosubstituted (Manp-Manp(Galp)) and disubstituted (Manp(Galp)-Manp(Galp)) dyads of β-(1→4)-mannose, as revealed by 13C NMR spectroscopy of the native polysaccharides. The pseudoplastic behaviour of galactomannan solutions (10–20 g l−1) was confirmed by steady shear and dynamic rheological tests. Aqueous galactomannan (5 g l−1)–xanthan gum (5 g l−1) mixed systems gave gels on cooling (G′ ≫ G″). Galactomannans with high limiting viscosities, high Manp/Galp ratios and a low proportion of the Manp(Galp)-Manp(Galp) dyad yielded the strongest mixed gel network structures with xanthan, implying that interchain associations between the two polymers occur via the unsubstituted segments (‘smooth regions’) of the carob galactomannan chains.