Bioconversion of Non-Detoxified Hemicellulose Hydrolysates to Xylitol by Halotolerant Yeast Debaryomyces nepalensis NCYC 3413

Abstract

Lignocellulosic materials are one of the most abundant renewable resources whose exploitation for the production of biochemicals and biofuels is the major challenge in the area of industrial biotechnology due to inhibition of growth and product formation by the toxic compounds released upon their hydrolysis. Indeed the bioprocess that can produce industrial products from hemicellulose hydrolysates in the presence of toxic compounds is economical than the process which involves detoxification. In this study, the ability of halotolerant strain Debaryomyces nepalensis NCYC 3413 to convert non-detoxified xylose enriched hemicellulose hydrolysates from corn cobs, rice straw, sugarcane bagasse and wheat straw to xylitol was evaluated. It was found that this strain has the capability to grow in all hemicellulose hydrolysates and convert xylose to xylitol without detoxification of hydrolysates. The maximum xylitol concentration of 14.6 g L-1 was obtained from corn cobs and wheat straw with productivities of 0.16 and 0.20 g L-1 h-1 respectively at a yield of 0.30 g g-1. Whereas sugarcane bagasse and rice straw gave xylitol yields of 0.31 and 0.32 g g-1 respectively with 14.2 g L-1 maximum xylitol and productivities were calculated to be 0.20 and 0.15 g L-1 h-1 respectively. The presence of high glucose hindered xylitol production by producing ethanol. Based on our findings, we suggest that (i) D. nepalensis is a promising strain for ecofriendly xylitol production as it exhibits broad specificity to lignocellulose substrates, fermentation of mixed sugars and (ii) tolerance towards lignocellulosic inhibitors making the process more economical.