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.
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.
Source: www.cen.acs.org