STRUCTURAL FEATURES OF ARABINOXYLANS FROM BARLEY AND MALT CELL-WALL MATERIAL

Arabinoxylans were extracted from water-insoluble cell wall material (WIS) of barley and malt with saturated barium hydroxide solution containing sodium borohydride (5 g/1). The residue obtained after this extraction was further extracted with 1 m and 4 m sodium hydroxide. The barium hydroxide extracts (BE fractions) were subfractionated by graded ethanol precipitation. Both the BE fractions obtained comprised more than 90% arabinoxylan with arabinose:xylose ratios of 0·72 and 0·68 for barley and malt, respectively. The remaining 10% was predominantly glucan. The BE fractions accounted for 63% (in barley) and 61% (in malt) of the arabinoxylan originally present in the WIS. Ethanol precipitation yielded a range of arabinoxylan fractions, in which the ratio of arabinose to xylose increased with ascending ethanol concentration. Methylation analysis showed that the main sugars found in all analysed fractions were 1,4-linked xylopyranose, in part substituted with single unit arabinoftiranose groups at positions 2, 3 or both. A novel feature was the presence of large amounts of xylopyranose, substituted at C2 (and not C3). Substitution of the xylopyranose backbone in the BE fractions of barley and malt and in the ethanol precipitates varied between 28% and 62% for barley fractions and between 22% and 56% for malt fractions. Most of the variation was due to differing levels of double substitution and single substitution at C2. No significant differences were observed between arabinoxylans extracted from barley WIS or from malt WIS. © 1992, Academic Press Limited. All rights reserved.