Structural variation and rheological properties of water-extractable arabinoxylans from six Greek wheat cultivars

Abstract

Aqueous extractions (at 25°C) of flours from six Greek wheat cultivars that differed in their bread-making quality, followed by porcine pancreas α-amylase digestion to remove starch contaminants and precipitation with ammonium sulphate (saturation at 95%), yielded arabinoxylan isolates with notable differences in molecular weight, ranging from 146,500 to 397,000, and in arabinose-to-xylose ratios (Ara/Xyl, varying between 0.57 and 0.71). The arabinoxylan isolates contained only small amounts of proteins and were relatively free of other polysaccharide contaminants. Generally a very high ratio of trans to cis-ferulic acid content was detected for all samples. The genotype also had an impact on the molecular characteristics of these polysaccharides. 1H NMR spectroscopy revealed variations in the un-substituted (61.1–67.9%) and di- (19.4–28.1%) and mono-substituted xylose residues (10.0–16.6%). Using 2D NMR spectroscopy, it was feasible to identify the spin sequence of individual monosaccharide residues and elucidate their structure, despite the extensive overlapping of 1H and 13C resonances. Differences in the physicochemical properties of the arabinoxylans, such as the critical concentration values (c∗ and c∗∗), viscosity, shear-thinning behaviour, as well as the gelling ability, could not be fully explained by differences in molecular size and structure of the isolated polysaccharides.