Studies on a purification method for locust bean gum by precipitation with isopropanol

Abstract

A commercial sample of locust bean gum was dissolved in hot water, centrifuged, and the supernatant and the sediment were recovered. Part of the supernatant was freeze-dried and the rest was poured into isopropanol to yield a precipitate of purified gum. This precipitate was recovered and a part of it was subjected to a second purification with isopropanol; an almost pure galactomannan sample was obtained. Another sample of the crude gum was fractionated on the basis of its solubility in water at different temperatures. Five fractions were obtained with mannose to galactose ratios (M/G) increasing with the temperature of fractionation. The purified samples exhibited higher M/G ratios and number average molecular weights (M n) than the crude gum, whereas the opposite was observed with the freeze-dried sample. The polydispersity index decreased quite significantly in purified samples, meaning that they were more homogeneous than the original sample. Intrinsic viscosities were lower for the purified samplesv The flow behaviour of aqueous solutions of the gums was studied in the concentration range 0.6–1.2 g/dl: the solutions of the crude gum had higher viscosities than those of the purified and the freeze-dried gums at similar concentrations, throughout the shear rate range.