Autohydrolysis of Almond Shells for the Production of Xylo-oligosaccharides:  Product Characteristics and Reaction Kinetics

Almond shells are agricultural residues with a high content of xylan that are produced abundantly in some regions with a Mediterranean climate. We have studied the production of xylo-oligosaccharides from almond shells by autohydrolysis at 150−190 °C. The yield, composition, and molar mass distribution of the xylo-oligosaccharides were dependent on temperature and time: the maximum yield of xylo-oligosaccharides increased from 42% at 150 °C and 300 min to 63% at 190 °C and 19 min, while their anhydroarabinose-to-anhydroxylose and acetyl-to-anhydroxylose mass ratios were 0.039 and 0.076 at 150 °C and 0.129 and 0.125 at 190 °C, respectively. Experimental data was used to fit the parameters of a kinetic model for xylan autohydrolysis, which describes the yields of the different reaction products and accounts for the changes in the chemical composition of xylan and xylo-oligosaccharides. The recovery of the xylo-oligosaccharides by spray drying was also evaluated. Spray drying of the autohydrolysis liquor obtained at 179 °C and 23 min gave a yield of nonvolatile products of 29.4 g/(100 g of dry almond shells). The composition of this product was 58.3% xylo-oligosaccharides, 2.4% xylose, 1.5% arabinose, 0.78% glucose, 0.27% HMF, 16% Klason-type lignin, 4.8% ash, and 14.9% nonidentified products, which probably included organic extractives and protein present in almond shells, and products formed by the degradation of carbohydrates.