Structural and physicochemical characterization of hemicelluloses isolated by alkaline peroxide from barley straw

Seven hemicellulosic preparations were obtained from barley straw by extraction with 1.5% H2O2 at pH 12.0 for 14h at 45degreesC with extractant to straw ratios of 10:1, 13:1, 15:1, 18:1, 20:1, 25:1 and 30:1. The respective yields of dissolved hemicelluloses were 28.4 wt%, 28. 8 wt%, 29.7 wt%, 30.1 wt%, 30.6 wt%, 30.8 wt% and 30.8 wt% or dry barley straw, corresponding to release of 78.7%, 79.8%, 82.3%, 83.4%, 84.8%, 85.3% and 85.3% of the original hemicelluloses. The polymers were mainly composed of xylose, arabinose, glucose, galactose and 4-0-methyl-D-glucuronic acid (59.3-62.6%, 19.7-22.0%, 8.8-12.5%, 5.6-6.9% and 5.8-6.7%, respectively) and had weight-average molecular weights of between 38 470 and 41 050 g mol(-1). (CNMR)-C-13 spectroscopy confirmed the results obtained by chemical methods, and showed that the hemicellulosic fraction was mostly by L-arabino-(4-0-methyl-D-glucurono)-D-xylan. More importantly, it was found that lignin provides some protection for the hemicelluloses in barley straw against attack by hydrogen peroxide. (C) 2002 Society of Chemical Industry.