(438g) Effect of Furfural, Syringaldehyde and Vanillin on Yeast Growth and Xylitol Biosynthesis

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.