Structural characteristics of wheat flour arabinoxylans = Structuurkenmerken van arabinoxylanen uit tarwebloem

Abstract

A procedure has been developed for the isolation of highly purified water-unextractable cell wall material (WUS) from wheat flour. From this material arabinoxylans were extracted via sequential extraction with Ba(OH) 2 , KOH and NaOH solutions. The first extract comprised arabinoxylans only, which represented 80% of all arabinoxylans present in the WUS. The arabinoxylans in this extract were fractionated using anion exchange chromatography and graded- ethanol precipitation. Neutral sugar and glycosidic linkage analysis revealed great similarities between these alkali-extractable arabinoxylans and similarly fractionated water-extractable arabinoxylans, isolated from the same wheat flour. Enzymic analysis using different endo-(1-4)- β-D-xylanases from Aspergillus awamori showed, however, that the water-extractable arabinoxylans were degraded both faster and to a larger extent than the alkali-extractable arabinoxylans.Based on the structure and abundance of enzyme-resistant polymeric material and different oligosaccharides, the latter being identified with 1H-NMR analysis, present in the enzyme digests, a structural model is proposed for the arabinoxylans. This model shows that the arabinoxylan contains highly branched regions, interlinked with less branched regions which include subregions of unsubstituted xylose up to 7 contiguous residues. Variation in arabinose/xylose ratio between different arabinoxylan is due to variation in the relative proportion as well as the composition of the less branched regions. With increasing arabinose/xylose ratio of the arabinoxylans a decreasing extent of enzymic degradation observed. The enzymes used had different effects on the degradation of WUS and extracted arabinoxylans as well as on the baking performance of wheat flour