SELECTION OF HEMICELLULOSIC HYDROLYSATE PRETREATMENTS AND FERMENTATION CONDITIONS TO STIMULATE XYLITOL PRODUCTION BY ETHANOL-PRODUCING YEASTS

Xylitol production from hardwood hemicellulosic hydrolysates by well-k nown ethanol-producing yeasts was stimulated through an experimental s chedule including pretreatments of the hydrolysate, the choice of the best xylitol producer and the selection of the optimum fermentation co nditions. The xylitol or ethanol yields obtained on consumed xylose de monstrated that their production was stimulated under completely diffe rent conditions, as to be expected by the fact that these catabolites are the final products of different metabolic pathways. In particular, the catabolism of Pachysolen tannophilus, that is the best ethanol pr oducer from this natural substrate, could be targeted towards xylitol rather than towards ethanol production by ensuring a strongly reducing environment through a suitable pretreatment of the hydrolysate. The f inal removal of fermentation inhibitors by adsorption onto highly adso rbing substances allowed a further 20% xylitol yield increase.