Isolation and fractionation of hemicelluloses by graded ethanol precipitation from Caragana korshinskii

Caragana korshinskii hemicelluloses were isolated with 10% KOH at 25 °C for 10 h from the delignified materials. The alkali-extractable hemicelluloses were then successively sub-fractionated by graded precipitation at final ethanol concentrations of 10%, 20%, 30%, 45%, 60%, and 80%, respectively. Neutral sugars and molecular weight analyses of the six hemicellulosic subfractions revealed that the molecular weights and the distribution of branches along the xylan backbone are different among the hemicellulosic fractions obtained in various ethanol concentrations. The less branched hemicelluloses with large molecules were precipitated in lower ethanol percentages, while with the increasing ethanol concentrations, more branched hemicelluloses with low molecular weights were obtained. ¹H and ¹³C NMR studies revealed that the hemicellulosic subfraction precipitated at an ethanol concentration of 45% had a backbone of d-xylose residues and were branched mainly through 4-O-methyl-α-d-glucopyranosyl units.