Investigation of molecular weight distribution of LBG galactomannan for flours prepared from individual seeds, mixtures, and commercial samples

The molecular weight distribution has been determined for the galactomannan solubilized from three types of locust bean gum (LBG) flours: single carob seeds, mixtures, and a range of commercial products. To prepare crude endosperm flours from carob seeds with minimal galactomannan degradation, a new extraction and milling method was developed. The method consists of applying a brief thermal shock to the seeds, followed by an extended 3-day swelling period, and manual separation of endosperms; particle size reduction to a flour is accomplished on hydrated endosperms using a centrifugal mill. This method was optimized so that redissolved LBG flours produced solutions with the highest possible viscosity and the least amount of galactomannan degradation as determined by SEC. For the three samples types, the molecular weight distribution, w(M), was found to be unimodal, appearing as a sharply defined main peak (Mp≈1.1×106 g/mol) with a small high molecular weight tail (up to 2.0×106 g/mol) and broad low molecular weight tail (down to 0.01×106 g/mol); polydispersities (Mw/Mn) were estimated to be 1.5–1.8. Variations in Mw and [η] for galactomannans extracted from individual seeds originating from the same carob tree were minimal and nearly indistinguishable from a bulk mixture (6 seeds, Mw=0.96–1.1×106 g/mol, [η]=14.2–15.1 dl/g). There was a higher variability in these molecular parameters for galactomannans solubilized from commercial LBG flours, which generally exhibited lower Mw and [η], broader distributions, and reduced solubilities (Mw=0.86–1.0×106 g/mol, [η]=12.4–13.6 dl/g). These side-effects were attributed to damage caused by industrial scale seed processing. The near constancy of Mp for the three sample types suggests that the average molecular size of LBG galactomannan varies only slightly due to natural or biological causes.