Comparative study of hemicelluloses from wheat straw by alkali and hydrogen peroxide extractions

As compared to traditional alkaline extractions, alkaline peroxide was used to isolate hemicelluloses from wheat straw. Yields of the solubilized hemicelluloses ranged from 18.9% (2% H2O2 extraction at 90 degrees C for 2 h at pH 11.5) to 26.6% (2% H2O2 extraction at 50 degrees C for 16 h at pH 12.5). The optimum hemicellulose yield (92% of the original hemicelluloses in water treated wheat straw) was obtained when the treatment was performed at 50 degrees C for 16 h at pH 12.5 by use of 2% H7O2. All the hemicellulosic preparations were much lighter in color than those obtained using traditional alkaline extractions in the absence of bleaching. The results, obtained by the destructive method such as acid hydrolysis, showed that extraction of wheat straw with aqueous 10% KOH following alkaline bleaching released the hemicellulosic fractions, which were enriched in xylose, whereas extraction of the straw with aqueous 2% H2O2 under alkaline conditions (pH 11.5-12.5) solubilized the hemicellulosic fractions, which were relatively higher in arabinose and glucose. The nine isolated hemicellulosic samples were further characterized by non-destructive methods such as Fourier transform infrared (Fr-TR), and carbon-13 magnetic resonance spectroscopy (C-13-NMR) as well as gel permeation chromatography (GPC). It has been demonstrated that the alkaline peroxide treatments under the conditions used do not affect the overall structure of hemicelluloses. The only one major change of the hemicelluloses was found to be degradation during the beaching of alkali-soluble hemicelluloses with 2% H2O2 at 60 degrees C for a period of 16 h at pH 11.5 or bleaching using a relatively higher concentration (5%) of hydrogen peroxide at 60 degrees C for 8 h at pH 11.5. (C) 1999 Elsevier Science Ltd. All rights reserved.