Production of prebiotic xylooligosaccharides from alkaline extracted wheat straw using the K80R-variant of a thermostable alkali-tolerant xylanase

Abstract

Agricultural by-products are raw materials of importance for increased utilization of renewable biomass. Wheat straw is a raw material of significant production volume and is in this work used for production of xylooligosaccharides (XOS). Extraction of xylan by dilute alkali was followed by hydrolysis using a variant of the alkali-tolerant Bacillus halodurans S7 endoxylanase A mutated at K80R. The xylan yield was on average 56.5g xylose equivalents per kg dried, ground wheat straw, with 1 arabinose per 12 xylose residues. The K80R variant, which displayed higher specific activity than the wild type enzyme, was added at a load of 96U/g extracted xylan. The XOS-yield (xylobiose–xylopentaose) was evaluated at time intervals in the temperature range of 50–65°C, at pHs from 7 to 10. The enzyme was optimally active at 60°C up to pH 9. Hydrolysis was completed within 7h, resulting in 36% conversion of the xylan to predominantly xylobiose. Xylose content was low (2.4%) despite extended incubation, which is desirable for XOS-production. The XOS-containing hydrolysate was confirmed as a suitable carbon source for the putative probiotic strain Lactobacillus brevis DSM 1269, showing the applicability of the method to obtain prebiotic XOS.