New yeast can ferment more sugar, make more cellulosic ethanol

June 7, 2010

WEST LAFAYETTE, Ind. - Purdue University scientists have improved a strain of yeast that can produce more biofuel from cellulosic plant material by fermenting all five types of the plant's sugars.

Nathan Mosier, an associate professor of agricultural and biological engineering; Miroslav Sedlak, a research assistant professor of agricultural and biological engineering; and Nancy Ho, a research professor of chemical engineering, used genes from a fungus to re-engineer a yeast strain Ho developed at Purdue. The new yeast can ferment the sugar arabinose in addition to the other sugars found in plant material such as corn stalks, straw, switchgrass and other crop residues.

"Natural yeast can ferment three sugars: galactose, manose and glucose," Ho said. "The original Ho yeast added xylose to that, and now the fifth, arabinose, has been added."

The addition of new genes to the Ho yeast strain should increase the amount of ethanol that can be produced from cellulosic material. Arabinose makes up about 10 percent of the sugars contained in those plants.

In addition to creating this new arabinose-fermenting yeast, Mosier, Sedlak and Ho also were able to develop strains that are more resistant to acetic acid. Acetic acid, the main ingredient in vinegar, is natural to plants and released with sugars before the fermentation process during ethanol production. Acetic acid gets into yeast cells and slows the fermentation process, adding to the cost of ethanol production.

"It inhibits the microorganism. It doesn't produce as much biofuel, and it produces it more slowly," Mosier said. "If it slows down too much, it's not a good industrial process."

Mosier, Sedlak and Ho compared the genes in the more resistant strains to others to determine which genes made the yeast more resistant to acetic acid. By improving the expression of those genes, they increased the yeast's resistance.

Mosier said arabinose is broken down in the same way as the other four sugars except for the first two steps. Adding the fungus genes allowed the yeast to create necessary enzymes to get through those steps.

"This gave the yeast a new tool set," Sedlak said. "This gives the yeast the tools it needs to get arabinose into the chain."

The team's findings on acetic acid were published in the June issue of the journal FEMS Yeast Research. The findings on arabinose were published in the early online version of the journal Applied Microbiology and Biotechnology.

Mosier, Sedlak and Ho will continue to improve the yeast to make it more efficient during industrial ethanol production and more resistant to inhibitors. The. U.S. Department of Energy funded their research.

Writer: Brian Wallheimer, 765-496-2050, 

Sources: Nathan Mosier, 765-496-2044,

                  Nancy Ho, 765-494-7046,

                  Miroslav Sedlak, 765-494-3699,

Ag Communications: (765) 494-2722;
Keith Robinson,
Agriculture News Page



Effect of Acetic Acid and pH on the Cofermentation of Glucose and Xylose to Ethanol by a Genetically Engineered Strain of Saccharomyces Cerevisiae

Elizabeth Casey, Miroslav Sedlak, Nancy W.Y. Ho and Nathan S. Mosier

A current challenge of the cellulosic ethanol industry is the effect of inhibitors present in biomass hydrolysates. Acetic acid is an example of one such inhibitor that is released during the pretreatment of hemicellulose. This study examined the effect of acetic acid on the cofermentation of glucose and xylose under controlled pH conditions by Saccharomyces cerevisiae 424A(LNH-ST), a genetically engineered industrial yeast strain. Acetic acid concentrations of 7.5 and 15 g L−1, representing the range of concentrations expected in actual biomass hydrolysates, were tested under controlled pH conditions of 5, 5.5, and 6. The presence of acetic acid in the fermentation media led to a significant decrease in the observed maximum cell biomass concentration. Glucose- and xylose-specific consumption rates decreased as the acetic acid concentration increased, with the inhibitory effect being more severe for xylose consumption. The ethanol production rates also decreased when acetic acid was present, but ethanol metabolic yields increased under the same conditions. The results also revealed that the inhibitory effect of acetic acid could be reduced by increasing media pH, thus confirming that the undissociated form of acetic acid is the inhibitory form of the molecule.



Establishment of L-Arabinose Fermentation in Glucose/Xylose Co-fermenting Recombinant Saccharomyces Cerevisiae 424A(LNH-ST) by Genetic Engineering

Aloke Kumar Bera, Miroslav Sedlak, Aftab Khan and Nancy W.Y. Ho

Cost-effective and efficient ethanol production from lignocellulosic materials requires the fermentation of all sugars recovered from such materials including glucose, xylose, mannose, galactose, and L-arabinose. Wild-type strains of Saccharomyces cerevisiae used in industrial ethanol production cannot ferment D-xylose and L-arabinose. Our genetically engineered recombinant S. cerevisiae yeast 424A(LNH-ST) has been made able to efficiently ferment xylose to ethanol, which was achieved by integrating multiple copies of three xylose-metabolizing genes. This study reports the efficient anaerobic fermentation of L-arabinose by the derivative of 424A(LNH-ST). The new strain was constructed by over-expression of two additional genes from fungi L-arabinose utilization pathways. The resulting new 424A(LNH-ST) strain exhibited production of ethanol from L-arabinose, and the yield was more than 40%. An efficient ethanol production, about 72.5% yield from five-sugar mixtures containing glucose, galactose, mannose, xylose, and arabinose was also achieved. This co-fermentation of five-sugar mixture is important and crucial for application in industrial economical ethanol production using lignocellulosic biomass as the feedstock.