Monday, April 30, 2007

The effect of inhibitors on xylitol biosynthesis

Christine J. Kelly and Curtis A. Lajoie. Chemical Engineering, Oregon State University, 103 Gleeson Hall, Corvallis, OR 97333

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 due to the inhibitors formed from hydrolysis of hemicellulose. 

 Experiments were performed in bioreactors with the yeast Candida guilliermondii.  The experiments consisted of yeast growth on a synthetic media containing xylose.  In the bioreactors, after batch phase, the cells were centrifuged and returned to the bioreactor with new xylose medium to decouple growth from xylitol biosynthesis. This phase is termed the resuspension phase, and was performed at low dissolved oxygen transfer rates to minimize metabolism of the xylitol. The effect of the synthetic inhibitors furfural, syringaldehyde, vanillin on growth and xylitol production was investigated independently.

 Experiments demonstrated that each inhibitor caused an inhibition of cell growth with furfural exhibiting the greatest toxicity, followed by syringaldehyde, and finally vanillin. The inhibition of both growth and xylitol accumulation was cell density dependant in that for all inhibitors, higher cell density experiments tolerated higher inhibitor concentrations. Some inhibitors caused a lag in xylitol synthesis.  Furfural and syringaldehyde consistently caused a six hour lag in xylitol accumulation, and the duration of the lag phase was independent of inhibitor concentration. However, the presence of vanillin did not result in xylitol biosynthesis lag phase. Increasing toxicity to xylitol synthesis was observed with increasing concentration of furfural, vanillin, and syringaldehyde.