Xylitol is a five carbon sugar alcohol with established commercial use as an alternative sweetener.  Xylitol can be produced from hemicellulose hydrolysate, which is a promising raw material source that may compete with petroleum feedstocks.  However, there are difficulties with microbiological growth and xylitol biosynthesis on hydrolysate due to the inhibitors formed from hydrolysis of hemicellulose.  This research focused on the effect hemicellulose derived inhibitors on xylitol production from xylose by Candida guilliermondii. 

Experiments were performed in shake flasks and bioreactors at various pH. Xylitol biosynthesis was measured in the presence of an inhibitor (furfural, syringaldehyde, or vanillin), under fully aerobic conditions, and at different cell densities.  The experiments consisted of microbial growth on a synthetic media containing xylose.  Bioreactor studies included a batch cell growth phase without inhibitors, followed by a resuspension phase where lower oxygen was supplied to encourage xylitol synthesis over biomass synthesis. We found that pH 5 was optimum for growth and xylitol production in both shake flask and bioreactor experiments. Both the batch and reactor experiments indicated that the presence of furfural caused a delay in xylitol biosynthesis.  This delay was approximately 6 hours in bioreactor experiments and independent of furfural concentration.  The presence of syringaldehyde also resulted in a delay in xylitol biosynthesis of approximately six hours, but the presence of vanillin did not cause a delay. As stated earlier, xylitol is produced under semi-aerobic conditions.  The experiments conducted here support that finding in that xylitol was produced more efficiently under semi-aerobic conditions then with fully aerobic conditions.  The effect of furfural, vanillin, and syringaldehyde concentrations on both biomass growth and xylitol biosynthesis were examined in shake flasks as well as bioreactors.  It was shown that although each inhibitor had an effect on biomass growth in shake flasks, the degree of increasing toxicity is vanillin < syringaldehyde < furfural.  This order was also followed in the case of xylitol biosynthesis in shake flasks.  However the order is different when the effect on xylitol biosynthesis decoupled from biomass growth was the focus.  In the order of increasing toxicity, the effect on xylitol biosynthesis in bioreactor experiments is furfural < vanillin < syringaldehyde. Cell density was a variable studied in the synthesis of xylitol.  It was seen in every case that cell density impacted the amount of inhibitor the cells could tolerate. This suggests that a higher cell density is preferred for xylitol biosynthesis. 

  <>These findings have implication for design of processes to use biomass raw material. A high cell density should be used to lessen toxicity of the inhibitors.  Furthermore, a higher cell density is more favorable for xylitol production because of lower oxygen flux.  In cases of low cell density, more oxygen is available for cell growth as opposed to xylitol biosynthesis. For maximum xylitol production, biomass growth should be decoupled from xylitol biosynthesis.  Biomass growth is achieved more efficiently when the cells are fully aerated.  However, xylitol biosynthesis is favored under semi- aerobic conditions, therefore resulting in a lesser xylitol yield.  Furthermore, when xylitol biosynthesis is coupled with biomass growth, not all xylose will be available for xylitol biosynthesis as some would be utilized for biomass growth.  Another growth source could be used to establish biomass.  In the presence of furfural and syringaldehyde, a lag phase of approximately 6 hours was observed.  This lag phase must considered in the process design.