Isolation and chemical characterization of water-soluble arabinoxylans in rye grain

Abstract

Rye grain (cv. Kungs II) was ground and refluxed in 90% aqueous ethanol in order to inactivate endogenous enzymes and remove low molecular weight sugars. The residue was extracted with water at 40°C and a crude arabinoxylan isolated from the extract by precipitations with ammonium sulphate and 67% aqueous ethanol. The crude arabinoxylan constituted 0·74% of the whole-grain and contained 72% arabinoxylans. The crude arabinoxylan was fractionated on a DEAE-cellulose column. The main fraction was eluted with water and contained arabinose and xylose residues in a ratio of 1 : 2·1, together with traces of other components. The quantitatively second most important fraction was eluted with weak borate and contained arabinose and xylose residues in a ratio of 1 : 1·8, together with significant amounts of glucose residues and non-carbohydrate components. Methylation and 1H- and 13C-NMR analysis revealed that the water-eluted arabinoxylan contained a main chain of 4-linked β- d-xylopyranosyl residues of which about 50% were substituted at position three with terminal α- l-arabinofuranosyl residues. About 2% of the xylose residues were double-substituted at positions two and three by terminal α- l-arabinofuranosyl residues. The same structural units were found in the fraction eluted with weak borate but the branched and double-branched units were more abundant.