Pretreatment Releases Sugar Content
Badal Saha, lead scientist at the fermentation biotechnology research unit of the USDA’s Agricultural Research Service (ARS), began his talk with a status report on the fuel ethanol industry. He described the production of ethanol from corn as a mature technology, with U.S. ethanol production reaching nine billion gallons, or about 4.2% of gasoline usage in 2008.
Almost all of this derives from corn, with one bushel of corn yielding about 2.7 gallons of ethanol and about 28% of the U.S. corn crop going toward ethanol production. Even if all the corn grain available were converted to ethanol, however, it would meet only 15–18% of current transportation fuel needs.
Alternative feedstocks are needed to boost ethanol production, Saha said, and the 1.3 billion tons of biomass available in the U.S. would be sufficient to produce the total amount of oil the country imports each year. Advanced biofuels can be derived from lignocellulosic feedstocks, such as agricultural waste (e.g., corn stover, wheat straw, rice hulls), agricultural processing byproducts (e.g., corn fiber or sugar cane bagasse), forestry and wood processing waste, the paper portion of municipal solid waste, or dedicated energy crops such as switchgrass. The main challenge in converting lignocellulosic biomass to biofuel is breaking down the cellulose, hemicellulose, and lignin components to their basic sugars.
A variety of pretreatment strategies have been devised, some of which can disrupt the lignin structure and solubilize the cell wall. Saha described a method of extracting sugar from hemicellulose, which comprises 35% of corn fiber and is made up of 70% carbohydrates.
In this method, pretreatment of corn fiber in a dilute acid solution at moderate temperature is followed by enzymatic saccharification of cellulose, yielding up to 100% of the sugar content of the hemicellulose without generating fermentation inhibitors. With this strategy, saccharification and fermentation are combined in a single process. Saha described similar success applying this strategy to other feedstocks including wheat straw, barley straw, and rice hulls. Technological challenges remain, such as the need for more efficient enzymes for saccharification, new microbes that can ferment multiple sugars, better integration of process steps, and improved methods for recovery of dilute ethanol.
Stephen Hughes, Ph.D., research molecular biologist at the USDA-ARS, described an automated process for high-throughput transformation (with bacterial xylose isomerase and xylose kinase genes), mutagenesis, and screening of yeast to select for fast-growing strains optimized for anaerobic growth on xylose.
A key advantage of using yeast for cellulosic ethanol production is their ability to work over a broad temperature (<44oC) and pH (3.0–8.0) range to produce large amounts of sugar. GMAX-L is a genetically engineered yeast strain able to convert both xylose and glucose to ethanol. It produces 10–15% more ethanol than a strain that utilizes glucose alone.
Dr. Hughes also reported ongoing work to achieve stable expression of a biocatalyst in yeast, which enables the fungi to produce biodiesel after ethanol production has completed. “Ethanol alone is not profitable; valuable coproducts will make biorefineries sustainable,” he concluded, explaining that advanced biorefineries would be able to use engineered yeast such as GMAX for simultaneous production of ethanol, alternative biofuels, plastics, chemicals, and animal feed.
Algae is a viable feedstock for first-generation biodiesel and ethanol production plants, and is a promising resource for producing aviation fuels and biocrude for biogasoline, said Will Thurmond, president of Emerging Markets Online.
“The U.S. and Europe cannot produce enough plant feedstocks to meet targets” for biofuel production—even with cellulosic corn—as defined by government mandates, which are largely being driven by a growing demand for energy independence and national security concerns, said Thurmond.