Optimization of xylanase production by Cryptococcus flavescens LEB-AY10 from steam exploded sugarcane bagasse

Abstract

The production of a thermostable xylanase was evaluated using steam exploded sugarcane bagasse as substrate. Initially, the selected yeast isolate was identified as Cryptococcus flavescens LEB-AY10 (CCT 7725) by sequencing the ITS1-5.8S-ITS2 region and D1/D2 domains of the large subunit (26S) ribosomal DNA. Then, sugarcane bagasse submitted to three different conditions of steam explosion (188.5oC, 198oC or 210oC) and their corresponding soluble C5 fractions were assayed as enzyme inductors, while raw bagasse and xylan from beech wood were used as controls. Molasses or compounds from synthetic media (peptone, yeast extract, magnesium and ammonium sulphate) were used to supplement the cultivation broth. Additionally, two-phase fermentations or activated charcoal treatments were also assayed with the purpose of removing inhibitors. After selection of C5 fractions from the mild pretreatment and their supplementation with synthetic compounds, an experimental design was used to study and optimize the fermentation conditions, as well as to enhance the production of xylanase. At the optimized conditions, according to the results of the experimental design, the enzyme activity after 96 h of fermentation was 4.67±0.23 U/mL (at 50oC) or 8.33±0.36 U/mL (at 80oC), which is 5.6 times higher compared to the activity in the preliminary tests using bagasse and sugarcane molasses, and 2.4 times higher than the activity in the initial assays using C5 fractions. Therefore, mild pretreatment of sugarcane bagasse showed to be a suitable C5 source for the production of xylanase by C. flavescens LEB-AY10, instead of expensive purified xylan.