Autohydrolysis of agricultural by-products for the production of xylo-oligosaccharides

Six agricultural residues of different botanic origin, namely corncobs (CC), almond shells (AS), olive stones (OS), rice husks (RH), wheat straw (WS), and barley straw (BS), were tested as feedstocks for the production of xylo-oligosaccharides (XOs) by autohydrolysis at 179 °C for 23 min. The yield of XOs depended on the content of xylan and its accessibility, and was proportional to the acetyl content of the raw materials. It was higher for CC (60%) and AS (55%), while RH provided a low yield (30%) in accord with its lowest acetyl content. The compositional and GPC analyses of the hydrolysis products indicated that they contained partially acetylated oligomeric and polymeric xylan fragments, and some monosaccharides and degradation products. Combined 1D (13C and 1H) and 2D (HSQC) NMR spectra of the dialyzed XOs samples revealed that the acetyl groups were located in the xylose residues mainly in position 3 (between 60 and 67 mol%), whereas the occurrence of acetyl groups in position 2 and in both positions 2 and 3 was similar (19–30 mol% and 8–25 mol%, respectively). The NMR analyses showed the presence of 4-O-methylglucuronic acid (MeGA) residues in all XOs samples, as indicated by the MeGA/Xyl mole ratios ranging from 2.5:100 for CC up to 9.1:100 for OS. Regardless of the structural differences of the xylan-types present in the parent plant materials, all XOs showed structural features of a partially O-acetylated 4-O-methylglucuronoxylan.