YRH400, which may require further genetic modification to
decrease xylitol production and increase ethanol yield.
We also sought to determine whether increased time of
bioabatement would improve xylose fermentation by YRH400.
Some improvement may be expected if the increased abatement
incubation time resulted in increased consumption of
acetate and other inhibitors affecting xylose utilization. In a
series of fermentations, a time course of inhibitor abatement
ranging from 0 h (unabated) to 96 h of bioabatement was followed
by YRH400 fermentation. Xylose consumption
increased up to 56%, with a final ethanol concentration of
0.83% w/v (Table 5). However, the increased metabolism of
xylose in these experiments cannot be tied definitively to
removal solely of acetate, since C. ligniaria also metabolizes
numerous additional inhibitory compounds present in
biomass dilute acid hydrolyzates [36]. As in earlier fermentations,
a significant portion of xylose was converted to xylitol
rather than ethanol.
In experiments containing rice hull solids, bioabatement
resulted in SSF ethanol yields ranging from 41 to 65% of the
theoretical maximum based on saccharified sugars whereas
SSF of unabated RHH using E. coli FBR5 yielded no ethanol.
However, SSF ethanol yields for both yeast strains in unabated
RHH were close to those obtained in bioabated RHH.
For the D5a and YRH400 SSF reactions, the impact of bioabatement
on SSF can be seen in reduced fermentation lag
times (Fig. 3) although the effect for YRH400 is modest due to
relatively short lag times observed in unabated RHH. Xylose
reductase from Pichia stipitis has been shown to reduce furaldehyde
compounds in lignocellulosic hydrolyzates [38].
Xylose reductase from Zymomonas mobilis was also recently
shown to exhibit activity toward furfural and HMF [39]. The
improved ability of YRH400 to ferment the unabated RHH is
most likely due to increased expression of xylose reductase
engineered into this strain, which required less time to
complete fermentations than D5a (Fig. 3). These data also
indicate that it may be possible to use harsher pretreatment
conditions and/or higher solids loading (i.e., increased inhibitor
concentrations) when using bioabatement and
YRH400.
YRH400, which may require further genetic modification to
decrease xylitol production and increase ethanol yield.
We also sought to determine whether increased time of
bioabatement would improve xylose fermentation by YRH400.
Some improvement may be expected if the increased abatement
incubation time resulted in increased consumption of
acetate and other inhibitors affecting xylose utilization. In a
series of fermentations, a time course of inhibitor abatement
ranging from 0 h (unabated) to 96 h of bioabatement was followed
by YRH400 fermentation. Xylose consumption
increased up to 56%, with a final ethanol concentration of
0.83% w/v (Table 5). However, the increased metabolism of
xylose in these experiments cannot be tied definitively to
removal solely of acetate, since C. ligniaria also metabolizes
numerous additional inhibitory compounds present in
biomass dilute acid hydrolyzates [36]. As in earlier fermentations,
a significant portion of xylose was converted to xylitol
rather than ethanol.
In experiments containing rice hull solids, bioabatement
resulted in SSF ethanol yields ranging from 41 to 65% of the
theoretical maximum based on saccharified sugars whereas
SSF of unabated RHH using E. coli FBR5 yielded no ethanol.
However, SSF ethanol yields for both yeast strains in unabated
RHH were close to those obtained in bioabated RHH.
For the D5a and YRH400 SSF reactions, the impact of bioabatement
on SSF can be seen in reduced fermentation lag
times (Fig. 3) although the effect for YRH400 is modest due to
relatively short lag times observed in unabated RHH. Xylose
reductase from Pichia stipitis has been shown to reduce furaldehyde
compounds in lignocellulosic hydrolyzates [38].
Xylose reductase from Zymomonas mobilis was also recently
shown to exhibit activity toward furfural and HMF [39]. The
improved ability of YRH400 to ferment the unabated RHH is
most likely due to increased expression of xylose reductase
engineered into this strain, which required less time to
complete fermentations than D5a (Fig. 3). These data also
indicate that it may be possible to use harsher pretreatment
conditions and/or higher solids loading (i.e., increased inhibitor
concentrations) when using bioabatement and
YRH400.
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