Fractional Separation and Structural Features of Hemicelluloses from Sweet Sorghum Leaves

Six hemicellulosic samples were isolated from cell wall material of dewaxed sweet sorghum (Sorghum bicolor (L.) Moench) leaves by sequential extractions with distilled water, alkali, and organic alkali solvent. The samples were treated with water, 1% NaOH, and 60% ethanol. The organic alkali samples were treated with 1%, 3%, 5%, and 8% NaOH, which yielded 8.3%, 5.4%, 1.0%, 5.6%, 2.5%, and 4.9% hemicelluloses based on the dry initial sweet sorghum leaves, respectively, and resulted in a total release of 81% of all hemicelluloses originally present in the cell wall. The results indicated that water-soluble hemicelluloses contained noticeable amounts of glucose, arabinose, galactose, and xylose, and had a relatively lower molecular weight (17300 g/mol). The four alkali-soluble hemicellulosic fractions, rich in xylose, were more linear, and had higher molecular weights (48500128000 g/mol) than those of the alkali organic-soluble hemicellulosic fraction. With an increase of NaOH concentration from 1% to 8%, the ratio of arabinose to xylose decreased from 0.29 to 0.01, which implied that the hemicelluloses obtained by the higher concentration of alkali appeared to be more linear. Based on the sugar analysis, Fourier transform infrared (FT-IR), and nuclear magnetic resonance (NMR) results, 4-O-methylglucuronoarabinoxylans were the major constituents of the hemicellulosic polymers.