Wood hydrolysis and hydrolysate detoxification for subsequent xylitol production

This study deals with two different aspects of the transformation of lignoc ellulosics into xylitol: the optimization of conditions for wood hydrolysis and the setting-up of an adequate hydrolyzate detoxification procedure nec essary to obtain high xylitol yields in the successive fermentation process . A comparison between the processes of wood autohydrolysis (steam explosio n) and pre-hydrolysis with dilute sulfuric acid, carried out batch-wise in laboratory scale, shows comparable yields, either in terms of final concent rations of xylose and pentose sugars in the hydrolyzate or of solubilised f raction of wood. On the other hand, notwithstanding the longer time require d, the pre-hydrolysis with dilute sulfuric acid produced acid hydrolyzates with lower contents of inhibiting substances (furfural, acetic acid, etc.). In order to obtain satisfactory xylitol yields from the hydrolysate produc ed by steam explosion, samples of this hydrolyzate were submitted to differ ent detoxification techniques and then fermented batch-wise by a Pachysolen tannophilus strain previously adapted to this substrate. The best detoxifi cation was performed by adding to the traditional overliming with Ca(OH)(2) and sulfite reduction, three steps of a) filtration to remove insoluble su bstances, b) stripping of acetic acid and furfural, and c) lignin-derived c ompounds removal by adsorption on charcoal. The fermentation of this hydrol yzate was very effective, achieving a final xylitol concentration of 39.5 g /l from 89.0 g/l xylose after 96 h, corresponding to a volumetric productiv ity of 0.41 g/lh and a product yield of 0.63 g/g.