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

Abstract

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.

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“…Kinetics of hemicellulose decomposition into oligomers and monomers [25] Corn stover Hydrothermal treatments for fractionation (optimal temperature, 190 8C)…”

“…The main soluble products from autohydrolysis reactions are usually referred as ''oligosaccharides'', even if they may present a broad distribution of molecular weights [25]. The term ''oligosaccharide'' is usually reserved for DP in the range 3-10 [26], whereas fractions of higher molecular weight can been considered as ''dietary fiber''.…”

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“…There are models describing hot water extraction as a homogeneous system with pseudo-first-order reactions and the extracted xylan having two different reactivities [5,[17][18][19]. Nabarlatz et al proposed a modified pseudo-first-order model that xylan is composed of xylose, arabinose, and acetic acid [20,21]. Other models tend to ignore the possible biphasic nature of xylan and propose more general linear sequence first-order kinetic model [22,23].…”

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“…However, different from its starch counterpart, the highly ordered crystalline structure of cellulose itself, together with the protective sheath (lignin and hemicellulose layers) around it, requires some form of pretreatment to open up the structure to effectively convert it to glucose. Pretreatment processing generally includes treating lignocellulosic materials with dilute sulfuric acid, followed by delignification with various organo-solvents, combined dilute alkaline treatment and homogenization, autohydrolysis, and steam explosion [21][22][23][24][25]. Recently, National Renewable Energy Laboratory researchers developed an advanced pretreatment technology that uses a mixture of an organic solvent and water to cleanly fractionate chemical-grade cellulose, hemicellulose sugars, and lignin [26].…”