Optimization of seed production for a simultaneous saccharification cofermentation biomass-to-ethanol process using recombinantZymomonas |
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Authors: | Lawford Hugh G Rousseau Joyce D McMillan James D |
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Institution: | (1) Bio-engineering Laboratory, Department of Biochemistry, University of Toronto, M5S 1A8 Toronto, Ontario, Canada;(2) Biotechnology Center for Fuels and Chemicals, National Renewable Energy Laboratory, 1617 Cole Boulevard, 80401-3393 Golden, CO |
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Abstract: | The five-carbon sugard-xylose is a major component of hemicellulose and accounts for roughly one-third of the carbohydrate content of many lignocellulosic
materials. The efficient fermentation of xylose-rich hemicellulose hydrolyzates (prehydrolyzates) represents an opportunity
to improve significantly the economics of large-scale fuel ethanol production from lignocellulosic feedstocks. The National
Renewable Energy Laboratory (NREL) is currently investigating a simultaneous saccharification and cofermentation (SSCF) process
for ethanol production from biomass that uses a dilute-acid pretreatment and a metabolically engineered strain ofZymomonas mobilis that can coferment glucose and xylose. The objective of this study was to establish optimal conditions for cost-effective
seed production that are compatible with the SSCF process design.
Two-level and three-level full factorial experimental designs were employed to characterize efficiently the growth performance
of recombinantZ. mobilis CP4:pZB5 as a function of nutrient level, pH, and acetic acid concentration using a synthetic hardwood hemicellulose hydrolyzate
containing 4% (w/v) xylose and 0.8% (w/v) glucose. Fermentations were run batchwise and were pH-controlled at low levels of
clarified corn steep liquor (cCSL, 1-2% v/v), which were used as the sole source of nutrients. For the purpose of assessing
comparative fermentation performance, seed production was also carried out using a “benchmark” yeast extract-based laboratory
medium. Analysis of variance (ANOVA) of experimental results was performed to determine the main effects and possible interactive
effects of nutrient (cCSL) level, pH, and acetic acid concentration on the rate of xylose utilization and the extent of cell
mass production. Results indicate that the concentration of acetic acid is the most significant limiting factor for the xylose
utilization rate and the extent of cell mass production; nutrient level and pH exerted weaker, but statistically significant
effects. At pH 6.0, in the absence of acetic acid, the final cell mass concentration was 1.4 g dry cell mass/L (g DCM/L),
but decreased to 0.92 and 0.64 g DCM/L in the presence of 0.5 and 1.0% (w/v) acetic acid, respectively. At concentrations
of acetic acid of 0.75 (w/v) or lower, fermentation was complete within 1.5 d. In contrast, in the presence of 1.0% (w/v)
acetic acid, 25% of the xylose remained after 2 d. At a volumetric supplementation level of 1.5–2.0% (v/v), cCSL proved to
be a cost-effective single-source nutritional adjunct that can support growth and fermentation performance at levels comparable
to those achieved using the expensive yeast extract-based laboratory reference medium. |
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Keywords: | Index Entries" target="_blank">Index Entries RecombinantZymomonas seed production via co-fermentation of glucose and xylose corn steep liquor pH acetic acid synthetic hemicellulose hydrolyzate |
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