Toward Renewable-Based, Food-Applicable Prebiotics from Biomass: A One-Step, Additive-Free, Microwave-Assisted Hydrothermal Process for the Production of High Purity Xylo-oligosaccharides from Beech Wood Hemicellulose

Xylooligosaccharides (XOS) produced from biomass offer a plethora of excellent physicochemical and physiological properties to be used as natural prebiotic nutraceuticals. Herein, this work first addresses and optimizes a novel one-pot, additive-free, microwave-assisted process to produce high purity XOS from beech wood hemicellulose, studying the influence of the temperature, reaction time, and solid loading. These variables exerted a significant influence, allowing the transformation of hemicellulose into a gas (0-19%), an XOS-rich liquid product (9-80%) and a spent solid material (17-90%). The liquid phase consisted of a mixture of XOS with a degree of polymerization (DP) DP > 6 (75-100 C-wt %) and DP 3-6 (0-10 C-wt %), together with mono/disaccharides (0-1 C-wt %), carboxylic acids (0-5 C-wt %), ketones (0-12 C-wt %) and furans (0-12 C-wt %). A good compromise between the liquid yield (81%) and XOS purity (96 C-wt %) was achieved at 172 °C using a solid loading of 5 wt % for 47 min. This time could be reduced (33 min) and the solid loading increased (25 wt %) without substantially altering the XOS (98 C-wt %) purity, although the liquid yield was reduced. The liquid yield could be increased up to 97% at the expenses of XOS purity (90 C-wt %) at 177 °C using a 5 wt % solid loading for 60 min. For these optima, the microwave production costs shifted between 1.42 and 6.50 €/kg XOS, which is substantially lower than the XOS market price, thus highlighting the high potential of this emerging technology.