ISOLATION AND CHEMICAL CHARACTERIZATION OF WATER-SOLUBLE MIXED-LINKED BETA-GLUCANS AND ARABINOXYLANS IN OAT MILLING FRACTIONS

Dehulled oats were separated into fractions of bran, outer starchy endosperm and inner starchy endosperm, after and without prior steam-flaking. The bran had significantly higher contents of protein, fat, ash and dietary fibre and lower content of starch than the endosperm fractions. An improved procedure was developed for the isolation of pure (92-99%) water-soluble mixed-linked (1-3), (1-4)-beta-D-glucans, involving fat extraction, enzymatic removal of starch and protein. and subsequent precipitation of water-soluble polysaccharides with 60% aqueous ethanol and 20% (w/v) aqueous (NH4)2SO4. High-field C-13-NMR analysis of the isolated mixed-linked beta-glucans showed the polysaccharide structure to be similar in all oat fractions. The proportion of isolated (1-3)-beta-linkages in the polymer was on average 28.3% (SD 0.8) according to H-1-NMR analysis. H-1-NMR also showed that water-soluble oat arabinoxylans were composed of a main chain of (1-4)-linked beta-D-xylopyranosyl residues, substituted by terminal alpha-L-arabinofuranosyl residues predominantly at O-3 but also at both O-2 and O-3. The proportion of xylose residues substituted solely at O-3 was 35% higher in bran than in endosperm arabinoxylans.