Purification of xylo-oligosaccharides from almond shells by ultrafiltration

摘要

Ultrafiltration using commercial thin-film polymeric membranes was studied to purify the xylo-oligosaccharides (XOs) obtained by autohy-drolysis of almond shells.Almond shells are agricultural residues with a high content of xylan that are produced abundantly in some regions with Mediterranean climate.Autohydrolysis was performed at 179 °C for 23 min in a 10 L stirred batch reactor at an almond shells-to-water mass ratio of 1-6,and the crude XOs were recovered by spray drying.Xylo-oligosaccharides only accounted for 58.3% of the recovered non-volatile products,and the rest was composed of low molar mass lignin (16.0%),monosaccharides (4.95%),inorganic compounds (or ashes,4.80%) and a 15.0% of other unidentified impurities originated from the extractives of the almond shells.The product had a wide distribution of molar mass,ranging from around 0.1 kDa (monomers) to 70 kDa (dextran-equivalent).Ultrafiltration of the XOs solution was studied with commercial membranes (Osmonix,G-series) of molecular weight cut-off (MWCO) of 1,2.5,3.5 and 8 kDa,at pressures from 2.6 to 9 bar.The selectivity towards preferential permeation of the lignin-related impurities was larger for the 1 kDa membrane,and it decreased linearly with the flux of permeate (or trans-membrane pressure) for all membranes.The flux of lignin-related impurities varied between 10g/hm2 for the 1 kDa membrane at 2.7 bar and 264g/hm2 for the 8 kDa membrane at 8.6 bar,while the flux of xylo-oligosaccharides changed from 19 to 579g/hm2 at the same conditions.The apparent MWCO of the membranes,determined from GPC analysis of the XOs in the permeate,were above their nominal values.The 1 kDa membrane had an apparent MWCO of 2.75 ± 0.70 kDa,and for the 8 kDa membrane it was 10.9 ± 0.5.Membranes had a good resistance to fouling;water permeability for the used membranes was close to 93% of that of a new,unused membrane.