Successive alkali extraction and structural characterization of hemicelluloses from sweet sorghum stem.

Sweet sorghum stem was successively extracted with water at 90 °C, 0.3, 0.6, 1.0, 1.5, and 2.0% KOH aqueous solution, and 60% ethanol containing 2.5% KOH at 75 °C for 3 h, yielding 76.3% of the original hemicelluloses. Chemical composition and structural characterization of the seven hemicellulosic fractions obtained were comparatively investigated by a combination of HPAEC, GPC, FT-IR, (1)H-, (13)C-, HSQC NMR and TGA techniques. According to the spectral analysis, hemicelluloses from sweet sorghum stem are assumed to L-arabino-4-O-methyl-D-glucurono-D-xylan. In addition, the higher molecular weights of hemicelluloses resulted in a higher thermal stability of the samples. The present study suggests that successive alkali extraction is a promising approach for fractionation of hemicelluloses from sweet sorghum stem and to prepare hemicellulosic polymers with different branching and molecular weights.