Valorization of Hemicelluloses: Production of Bioxylitol from Poplar Wood Prehydrolyzates by Candida guilliermondii FTI 20037

Abstract

In this study on the valorization of hemicelluloses (a co-product generated during cellulosic bioethanol production), prehydrolyzates obtained from poplar woodchips pretreated in an industrial experimental steam-explosion pilot-plant facility were evaluated for the production of bioxylitol using the yeast, Candida guilliermondii FTI 20037, employing both batch and fed-batch fermentation modes in shake flasks on defined nutrient medium. The prehydrolyzates consisted of monosaccharides (pentose and hexose sugars) as well as xylo-oligosaccharides and undegraded hemicellulose. Xylose (31.6 ± 0.57 g/L) was the major sugar in the prehydrolyzates that also contained acetic acid and degradation products of lignin and sugars (phenolic and furanic compounds). Xylose in the prehydrolyzates could be further increased (106.4 ± 0.02 g/L) through an acid hydrolysis step (0.6 % (w/v) H2SO4). Compounds of a toxic nature in both the acid hydrolyzates and prehydrolyzates were removed by treatment with Amberlite IRA-400 resin (chloride form). Batch fermentation of pure xylose and poplar prehydrolyzate resulted in bioxylitol production of 9.9 ± 0.01 and 4.9 ± 0.17 g/L, respectively, indicating that the poplar prehydrolyzates exhibited an inhibitory effect on fermentation. After detoxification of the poplar prehydrolyzates, bioxylitol production increased to 8.9 ± 0.01 g/L. Fed-batch fermentation of the prehydrolyzate increased the bioxylitol production to 12.39 ± 0.33 g/L, while acid hydrolysis followed by detoxification resulted in a maximum bioxylitol production of 22.0 ± 0.01 g/L, a 348 % increase. The results demonstrated that acid hydrolysis and detoxification followed by fed-batch fermentation was an efficient way to produce bioxylitol from poplar prehydrolyzates.