Volume 92 Issue 40 | p. 9 | News of The Week
Issue Date: October 6, 2014 | Web Date: October 3, 2014

Bumping Up Ethanol Yields

Biotechnology: Simple alterations in fermentations could boost yeast productivity
Department: Science & Technology | Collection: Green Chemistry, Sustainability, Life Sciences
News Channels: Biological SCENE, Environmental SCENE
Keywords: ethanol, biofuel, yeast, biotechnolgy, cell membrane
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Normal yeast cells die once ethanol reaches a high level, (dead cells shown stained blue at left). But increasing potassium levels and increasing pH can strengthen cell membranes to keep the yeast alive, shown at right.
Credit: Felix Lam
Normal yeast cells producing ethanol die (left, stained blue) once ethanol reaches high levels. With a few modifications, yeast can be strengthened (right) to produce more ethanol without damage.
 
Normal yeast cells die once ethanol reaches a high level, (dead cells shown stained blue at left). But increasing potassium levels and increasing pH can strengthen cell membranes to keep the yeast alive, shown at right.
Credit: Felix Lam
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Yeast cells grown under controlled conditions in bioreactors, like those shown here, are constantly monitored in studies to improve yeast vitality and ethanol production.
Credit: Martina Butorac
Yeast cells grow under controlled conditions in bioreactors.
 
Yeast cells grown under controlled conditions in bioreactors, like those shown here, are constantly monitored in studies to improve yeast vitality and ethanol production.
Credit: Martina Butorac

At 25 billion gal produced globally each year, ethanol has made a crucial contribution to transportation fuel supplies. But without much effort, a few tweaks to yeasts and the reaction conditions they operate under could boost ethanol production further and reduce costs, according to two research reports.

Both approaches address ethanol production’s biggest problems: Yeasts used to ferment sugars into alcohols become easily overheated and inebriated. Yeasts are sensitive to the high temperatures they generate while processing sugar in large fermentors. The microorganisms also have low tolerance to the alcohol they are making. The heat and alcohol both affect the integrity of the yeast cell membrane, gradually impairing ethanol production and eventually killing the biocatalytic yeast cells.

“A major challenge in this field is the wide variety of inhibitory conditions encountered by the biocatalyst throughout the production process,” comment Clint Cheng and Katy C. Kao of Texas A&M University in a perspective accompanying the reports. “These two papers exemplify the importance of robust biocatalyst design for industrial fermentation, while also illustrating the need to use a spectrum of approaches to address the challenge.”

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In temperature-tolerant yeast strains, the bent steroid alcohol fecosterol is more abundant than ergosterol and helps strengthen the yeast cell membrane.
Credit: Martina Butorac
Fecosterol
 
In temperature-tolerant yeast strains, the bent steroid alcohol fecosterol is more abundant than ergosterol and helps strengthen the yeast cell membrane.
Credit: Martina Butorac

In one report, Jens Nielsen of Chalmers University of Technology, in Sweden, and coworkers identified yeast strains that are tolerant to higher processing temperatures (Science 2014, DOI: 10.1126/science.1258137). Operating at higher temperature has the advantage of increasing fermentation efficiency and reducing the energy costs of cooling industrial fermentation reactors.

Most yeasts thrive at 30 °C. But Nielsen’s team found yeast strains that tolerate a temperature close to 40 °C. Through genomic analysis the researchers showed that the heat tolerance is mostly a result of altering the composition of yeast sterols—the equivalent to cholesterol in people—that help control cell membrane fluidity. They specifically found that heat-tolerant yeasts have a mutation in a sterol desaturase gene that leads to high levels of feco­sterol, a molecule with a bent shape. Most yeasts instead contain high levels of ergosterol, a linear molecule. Fecosterol bolsters the integrity of the yeast cell membrane, enabling continued fermentation at higher temperature.

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Credit: Martina Butorac
Ergosterol
 
Credit: Martina Butorac

In a second report, Gerald R. Fink of the Whitehead Institute for Biomedical Research, Gregory Stephanopoulos at Massachusetts Institute of Technology, and coworkers show how increasing potassium ion concentration and hydroxide ion concentration (preventing pH from dropping) in the fermentation medium can modify electrochemical gradients across yeast cell membranes (Science 2014, DOI: 10.1126/science.1257859). By controlling the gradients they can strengthen cell membranes to improve yeast tolerance to higher levels of ethanol and other alcohols.

When the team tried to further improve yeasts by modifying them with genetic deletions that increase the activity of potassium and hydrogen ion protein pumps in their membranes, ethanol production exceeded that of industrial strains without altering potassium and pH levels.

 
Chemical & Engineering News
ISSN 0009-2347
Copyright © American Chemical Society
Comments
Henrique Amorim (Fri Oct 03 15:14:38 EDT 2014)
Dear Steve, you did not mention my comments abou both papers.
As I mentioned to you these papers are half true. You mentioned just the good part.
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