Evaluation of the detoxification of brewery's spent grain hydrolysate for xylitol production by Debaryomyces hansenii CCMI 941

The hemicellulose fraction of brewery's spent grain was selectively hydrolyzed by two sequential steps, autohydrolysis and post-hydrolysis (with diluted sulfuric acid), in order to obtain a fermentable hydrolysate. The concentrations of monosaccharides and inhibitory by-products generated by these sequential processes were evaluated and several detoxification methods for inhibitors removal were tested. These methods included pH adjustment to 5.5, overliming, adsorption into activated charcoal and treatment with anion- and cation-exchange resins at different pH. The treatment of the raw hydrolysate with anion-exchange resins without previous pH correction was found to be the method that removed more inhibitory compounds. Detoxification effects in both non-concentrated and concentrated hydrolysates were evaluated for Debaryomyces hansenii biomass and xylitol production, respectively. When supplemented with inorganic salts and vitamins, the detoxified hydrolysates (through anion-exchange resins) showed lower biomass yield and only slightly higher volumetric productivity (11%) compared to the non-detoxified (pH-adjusted) hydrolysates. The best xylitol yields (0.51 and 0.50 g g−1) and productivities (0.29 and 0.33 g l−1 h−1) were found both in non-detoxified and detoxified with activated charcoal hydrolysates, respectively.