WATER-UNEXTRACTABLE CELL-WALL MATERIAL FROM WHEAT-FLOUR .2. FRACTIONATION OF ALKALI-EXTRACTED POLYMERS AND COMPARISON WITH WATER-EXTRACTABLE ARABINOXYLANS

Alkaline extracts of water-unextractable cell wall material (WUS-designated previously as water-insoluble cell wall material, WIS) from wheat flour were fractionated by DEAF anion exchange chromatography. Neutral sugar analysis showed that, for each extract, the arabinoxylans that did not bind to the iron exchanger had lower overall arabinose/xylose (Ara/Xyl) ratios than those that did bind. When the extract contained a mixture of polysaccharides, the fractions that were bound to the DEAE column contained more arabinoxylans than the unbound fractions. With increasing buffer strength, arabinoxylans with increasing Ara/Xyl ratios were eluted. The unbounded DEAE fraction (BE1-U) of the major alkaline extract was fractionated further by graded ethanol precipitation. Water-extractable arabinoxylans from the same wheat flour were fractionated similarly. This resulted in arabinoxylan fractions with Ara/Xyl ratios increasing from c. 0·4 to 1·1. Glycosidic linkage analysis showed that, as the Ara/Xyl ratio increased, so too did the ratio of branched to unbranched xylose and the ratio of double-branched to single-branched xylose. The single-branched xylose was mainly O-3-branched xylose, although, in fractions with high Ara/Xyl ratios, substantial proportions of O-2-branched xylose were present. No difference in linkage composition between arabinoxylans from the BE l-U fraction, representing 65% of the arabinoxylans present in the WUS, and water-extractable arabinoxylans was observed. HP-SEC/light-scattering analysis showed that, with increasing Ara/Xyl ratio, the weight-average molecular weights of BEl-U fractions increased from 260k to 640k. © 1992, Academic Press Limited. All rights reserved.