Purification of xylo-oligosaccharides from almond shells by ultrafiltration

Ultrafiltration using commercial thin-film polymeric membranes was studied to purify the xylo-oligosaccharides (XOs) obtained by autohydrolysis 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 10 g/h m2 for the 1 kDa membrane at 2.7 bar and 264 g/h m2 for the 8 kDa membrane at 8.6 bar, while the flux of xylo-oligosaccharides changed from 19 to 579 g/h m2 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.