Researchers at the University of Georgia
have developed a “super strain” of yeast that can efficiently ferment
ethanol from pretreated pine—one of the most common species of trees in Georgia and the United States. Their research could
help biofuels replace gasoline as a transportation fuel.
“Companies are interested in
producing ethanol from woody biomass such as pine, but it is a notoriously
difficult material for fermentations,” says Joy Doran-Peterson, associate
professor of microbiology in the Franklin College of Arts and Sciences.
“The big plus for softwoods,
including pine, is that they have a lot of sugar that yeast can use,” she
says. “Yeast are currently used in ethanol production from corn or
sugarcane, which are much easier materials for fermentation; our process
increases the amount of ethanol that can be obtained from pine.”
Before the pinewood is fermented
with yeast, however, it is pre-treated with heat and chemicals, which help open
the wood for enzymes to break the cellulose down into sugars. Once sugars are
released, the yeast will convert them to ethanol, but compounds produced during
pretreatment tend to kill even the hardiest industrial strains of yeast, making
ethanol production difficult.
Doran-Peterson, along with
doctoral candidate G. Matt Hawkins, used directed evolution and adaptation of Saccharomyces cerevisiae, a species of
yeast used commonly in industry for production of corn ethanol, to generate the
Their research, published online
in Biotechnology for Biofuels, shows
that the pine fermented with the new yeast can successfully withstand the toxic
compounds and produce ethanol from higher concentrations of pretreated pine
than previously published.
“Others before us had
suggested that Saccharomyces could adapt to harsh conditions. But no one had
published softwood fermentation studies in which the yeast were pushed as hard
as we pushed them,” says Doran-Peterson.
During a two-year period,
Doran-Peterson and Hawkins grew the yeast in increasingly inhospitable
environments. The end result was a strain of yeast capable of producing ethanol
in fermentations of pretreated wood containing as much as 17.5% solid biomass.
Previously, researchers were only able to produce ethanol in the presence of 5
to 8% solids. Studies at 12% solids showed a substantial decrease in ethanol
This is important, says
Doran-Peterson, because the greater the percentage of solids in wood, the more
ethanol that can be produced. However, a high percentage of solids also places
stress on the yeast.
“Couple that stress with the
increase in toxic compounds, and the fermentation usually does not proceed very
well,” she says.
Pine is an ideal substrate for
biofuels not only because of its high sugar content, but also because of its
sustainability. While pine plantations account for only 15% of Georgia’s trees,
they provide 50% of harvested timber, according to Dale Greene, professor of
forest operations in UGA’s Warnell School of Forestry and Natural Resources. The
loblolly pine that Doran-Peterson and Hawkins used for their research is among
the fastest growing trees in the American South.
“We’re talking about using
forestry residues, waste and unsalable timber,” says Peterson,
“Alternatively, pine forests are managed for timber and paper
manufacturing, so there is an existing infrastructure to handle tree-farming,
harvest, and transportation for processing.
“The basic idea is that
we’re trying to get the yeast to make as much ethanol as it can, as fast as it
can, while minimizing costs associated with cleaning or washing the pretreated
pine. With our process, no additional clean-up steps are required before the
pine is fermented,” she says.
Saccharomyces cerevisiae strain AJP50 is patent pending (PCT/US2009/043358).