Variation in the degree of D-Xylose substitution in arabinoxylans extracted from a European wheat flour

Abstract

Total water-extractable arabinoxylan from flour of the European bread making wheat cultivar Camp Remy was fractionated by ethanol precipitation. Both 1H-NMR spectroscopy and methylation analysis of the isolated arabinoxylan fractions showed wide variation in the degree of xylose substitution. More highly substituted arabinoxylan fractions were precipitated at higher ethanol concentrations. At ethanol concentrations of 10–30% (v/v) (1→3),(1→4)-β- d-glucan co-precipitated with arabinoxylan. Re-precipitation of the polymer mixture resulted in a partial separation of the arabinoxylan from the mixture. Arabinoxylans with wide structural variation were also isolated by extraction of the flour with aqueous ethanol solutions of decreasing concentations. By this isolation technique, fractions with the highest degree of substitution were extracted with the most concentrated (50%) ethanolic solutions. The range of structural variation in the fractions was quite similar for both isolation methods. A decrease in the proportion of unsubstituted xylose residues occurred concomitantly with a decrease in the proportion of monosubstituted xylose units and with an increase in the proportion of disubstituted xylose units. An increase in the proportion of paired disubstituted xylose units as the arabinose to xylose ratio of the arabinoxylan fractions increased, and the presence of monosubstituted xylose residues next to disubstituted xylose residues in the highly substituted fractions, were illustrated by 1H-NMR spectroscopy. Methylation analysis indicated an increase in traces of side chains of arabinose residues and in the levels of O-2 substituted xylose residues as the arabinose to xylose ratio increased.