Structural characteristics of water-soluble feruloyl arabinoxylans from rice (Oryza sativa) and ragi (finger millet, Eleusine coracana): Variations upon malting

Water-soluble feruloyl arabinoxylans (feraxans), isolated from native and malted (96 h) rice (Oryza sativa) and ragi (Eleusine coracana) grains, were fractionated on DEAE-cellulose, followed by purification on Sephacryl S-300 and the homogeneity was ascertained by high performance size exclusion chromatography, cellulose acetate and capillary electrophoresis. Structural characterization of the purified polysaccharides by methylation, followed by GLC–MS, and also by ¹H NMR and ¹³C NMR spectroscopy, indicated very high branching and presence of high amounts of O-2 substituted xylans. The amount of O-2, 3 disubstituted xylopyranosyl residues and the arabinose:xylose ratio was higher in malt feraxans. All feraxan samples consumed almost equal amounts of periodate (4.02–4.30 μmol/mg). High amount of xylose (∼40%), as identified by Smith degradation, further substantiated the high branching of feraxans. A model is presented depicting the structure of water-soluble feraxans from rice and ragi and their changes upon malting.