Hydrolysis of grass xylan by xylanases from Aureobasidium pullulans for xylooligosaccharide and xylitol productions

Abstract

Ten grass samples were screened for potential as a xylan source and five including sugarcane leaf, purple guinea grass, Napier grass, sabi grass and vetiver grass were selected for optimization of xylan extraction. After optimization, sugarcane leaf gave the highest xylan recovery at 99.42% when it was extracted by 14.32% (w/v) NaOH at 13.25:1 liquid: solid ratio with 32.36-min steaming time. From the xylan structural and sugar composition analyses, the major xylan from sugarcane leaf was arabinoxylan whereas those from the other grass samples were (glucurono)arabinoxylan. Xylans from sugarcane leaf, purple guinea grass and Napier grass were selected for further enzymatic hydrolysis and xylooligosaccharide and xylitol production based on their high xylan recovery. After screening for the best xylanolytic enzymes producers, crude endoxylanase and β-xylosidase were produced from Aureobasidium pullulans NRRL 58523 and A. pullulans CBS 135684, respectively, and used for the partial and complete xylan saccharification. After optimization, the maximum yield of xylooligosaccharides at 0.27 g/g initial xylan was received from sugarcane leaf through partial hydrolysis with 65.32 U/g xylan endoxylanase dosage and 52.89-h incubation time. Sugarcane leaf also gave the maximum xylose yield at 0.20 g/g initial xylan through the optimized completed hydrolysis with a cocktail of 69.93 U/g xylan endoxylanase and 67.85 U/g xylan β-xylosidase and 17.71-h incubation time. Xylooligosaccharides from all digested xylans showed significant species–specific prebiotic activity toward Lactobacillus spp. Insignificant antioxidant activity was also observed from all grass xylans via DPPH assay. Xylose obtained from the complete hydrolysis was used for xylitol production by Candida tropicalis FS 10. The highest xylitol conversion efficiency at 28.78% was obtained when xylose from sugarcane leaf was added at 56.69 g/L to the optimized production medium containing 5.44 g/L glucose and 4.71 g/L yeast extract.