Structure and physicochemical properties of barley non-starch polysaccharides -: I.: Water-extractable β-glucans and arabinoxylans

Water-soluble non-starch polysaccharides were extracted from a Canadian malting barley (cv. Harrington) by sequential treatment with water at 40 degrees C (WE40) and 65 degrees C (WE65). The yields were 1.4 and 1.3% (w/w), respectively, of the dry barley grist. The WE40 extract was composed of 82.5% glucose, 8.9% xylose, and 7.0% arabiose residues, whereas WE65 contained 93.3% Glc, 3.3% Xyl, and 2.5% Ara. Only minute amounts of mannose and galactose residues were found in either fraction. Both extracts were further fractionated by stepwise (NH4)(2)SO4 precipitation into several polysaccharide populations. Subfractions from both extracts, obtained up to 45% saturation With (NH4)(2)SO4, contained mostly beta-glucans, whereas subfractions precipitated at increasing saturation levels of (NH4)(2)SO4 (45-100%) contained progressively more arabinoxylans and less beta-glucans. Compared to WE40, the WE65 extract was enriched in beta-glucan populations with higher molecular size, higher limiting viscosity values, and higher content of beta-(1 --> 4) linkages. The ratio of tri-/ tetrasaccharide oligomers was also higher in beta-glucans extracted at 65 degrees C than those extracted at 40 degrees C. Arabinoxylans in both extracts, WE40 and WE65, were highly substituted and contained large proportions of doubly substituted xylose residues. (C) 1998 Elsevier Science Ltd. All rights reserved.