Graded Ethanol Fractionation and Structural Characterization of Alkali-Extractable Hemicelluloses from Olea europaea L.

Dewaxed Olea europaea L. was subjected to delignification followed by alkali extraction with 10% KOH containing 1% H 3 BO 3 . The released hemicelluloses were fractionated by precipitation through acidification and ethanol solutions with increasing concentrations from 20 to 30, 40, 50, 60, 70, and 90%. The structure of the subfractions obtained was comparatively characterized by sugar analysis, molecular weight, FT-IR, and NMR spectroscopy. Results indicated that 23.5% of hemicelluloses (% of the dewaxed material) were isolated by the alkaline extraction. An increase of ethanol concentration resulted in the precipitation of the more branched hemicelluloses as indicated by the increase of the ratios of arabinose to xylose and uronic acids to xylose. The hemicellulose subfractions Ha (precipitated by acidification to pH 5.5) and H30 (precipitated by 30% ethanol solution) had a similar structure, which was assumed to be glucuroxylan together with a small amount of -glucan, whereas the hemicellulose subfraction H70 (precipitated by 70% ethanol solution) had a more complicated structure, which was mainly composed of a (14)-linked -D-xylopyranosyl backbone with various side chains. The comprehensive structural characterization of the hemicelluloses of this species provides fundamental information for their potential applications in the fields of materials, chemicals, and energy production.