Comparison of two posthydrolysis processes of Brewery's spent grain autohydrolysis liquor to produce a pentose-containing culture medium

A readily fermentable pentose-containing hydrolysate was obtained from Brewery's spent grain by a two-step process consisting of an auto-hydrolysis (converting the hemicelluloses into oligosaccharides) followed by an enzymatic or sulfuric acid-catalyzed posthydrolysis (converting the oligosaccharides into monosaccharides). Enzymatic hydrolyses were performed with several commercial enzymes with xylanolytic and cellulolytic activities. Acid-catalyzed hydrolyses were carried out at 121°C under various sulfuric acid concentrations and reaction times, and the effects of treatments were interpreted by means of a corrected combined severity factor (CS 1), which varied in the range of 0.80–2.01. Under the tested conditions, chemicalhydrolysis allowed higher pentose yields than enzymatic hydrolysis. Optimized conditions (defined by CS 1=1.10) allowed both complete monosaccharide recovery and low content of inhibitors. Liquors subjected to posthydrolysis under optimal conditions were easily fermented by Debaryomyces hansenii CCMI 941 in semiaerobic shake-flask experiments, leading to xylitol and arabitol as major fermentation products. The bioconversion process was improved by hydrolysate concentration and supplementation of fermentation media with casamino acids.