Fractional and structural characterization of hemicelluloses isolated by alkali and alkaline peroxide from barley straw

Eight hemicellulosic fractions were obtained by sequential treatment of dewaxed barley straw with 0.1 M NaOH at 45 degreesC for 3 h, 0.25, 0.5, 1.0, 1.5, 2.0, and 3.0% H2O2 at 45 degreesC for 3 h at pH 11.5, and 10% KOH-1% Na2B4O7.10H(2)O at 28 degreesC for 15 h under continuous agitation. The yields of the fractions were 8.0, 3.1, 3.3, 3.3, 2.2, 2.0, 2.0, and 9.9%, respectively, of the initial amount of barley straw, corresponding to the dissolution of 21.6, 8.4, 8.9, 8.9, 5.9, 5.4, 5.4, and 26.7% of the original hemicelluloses. Meanwhile, the successive treatment also solubilized 29.1, 15.8, 14.6, 10.8, 4.5, 3.2, 2.7, and 3.7% of the original lignin, respectively. This sequential extraction together resulted in dissolution of 91.1% of the original hemicelluloses and 84.8% of the original lignin. The 0.1 M NaOH-soluble hemicellulosic fraction contained mainly xylose, glucose, and arabinose, 44.2, 15.7, and 15.2%, respectively, while the 10% KOH-1% Na2B4O7.10H(2)O-soluble fraction predominated in xylose, 75.0%. The six alkaline peroxide-soluble fractions were composed of 50.3-54.4% xylose, 14.7-16.9% arabinose, 6.8-10.7% glucose, 6.8-8.5% glucuronic acid or 4-0-methyl-D-glucuronic acid, 0.4-1.5% mannose, and 0.3-1.2% rhamnose. All the hemicellulosic fractions contained substantial amounts of glucuronoarabinoxylans and noticeable quantities of beta-glucans. In comparison, the six hemicellulosic fractions, isolated with alkaline peroxide, had much higher molecular weights (56,89063,810 g mol(-1)) than those of the two hemicellulosic preparations (28,000-29,080 g mol(-1)), isolated with alkali in the absence of hydrogen peroxide. The thermal stability of the hemicelluloses increased with an increment of their molar mass. (C) 2002 Elsevier Science Ltd. All rights reserved.