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Continuous fermentation studies with xylos-utilizing recombinant Zymomonas mobilis.
H G Lawford1, J D Rousseau, A Mohagheghi
1Department of Biochemistry, University of Toronto, Ontario, Canada. hugh.lawford@utoronto.ca
Applied Biochemistry and Biotechnology
|June 13, 2000
Summary
This study optimized continuous ethanol fermentation using a recombinant Zymomonas strain, finding corn steep liquor cost-effective. Glucose was fully utilized, while xylose remained, achieving 94% theoretical ethanol yield.
Area of Science:
- Biotechnology and Bioengineering
- Industrial Microbiology
- Renewable Energy
Background:
- Recombinant Zymomonas 39676:pZB4L was previously studied in pH-stat batch fermentations for ethanol production.
- Optimizing continuous fermentation performance is crucial for cost-effective bioethanol production from lignocellulosic feedstocks.
Purpose of the Study:
- To evaluate the continuous cofermentation performance of a prehydrolysate-adapted recombinant Zymomonas strain.
- To assess the impact of substituting yeast extract with corn steep liquor (CSL) and magnesium on fermentation efficiency and cost.
- To determine the effects of varying sugar ratios and acetic acid concentrations on maximum dilution rate (Dmax) and ethanol productivity.
Main Methods:
- Continuous cofermentation experiments were conducted using a dilute-acid prehydrolysate-adapted recombinant Zymomonas strain.
- Yeast extract was replaced with corn steep liquor (CSL) and magnesium, and different sugar compositions were tested.
- Maximum dilution rate (Dmax) was determined based on 80% sugar utilization, and Pirt plot analysis was used to assess sugar utilization kinetics under inhibitory conditions.
Main Results:
- Substitution of yeast extract with 1% CSL and 2 mM Mg did not negatively impact cofermentation performance.
- Glucose utilization was complete due to higher transporter affinity, while xylose remained in the effluent, achieving 94% of theoretical ethanol yield.
- Increasing glucose concentration and decreasing xylose concentration shifted Dmax from 0.07/h to 0.08/h; acetic acid significantly reduced Dmax and volumetric ethanol productivity.
Conclusions:
- Corn steep liquor is a cost-effective alternative to yeast extract for continuous ethanol fermentation.
- The recombinant Zymomonas strain exhibits efficient glucose and xylose cofermentation, achieving high theoretical ethanol yields.
- Acetic acid poses a significant challenge to continuous fermentation performance, increasing the maintenance coefficient and reducing productivity.