Better barley for biofuels
April 01, 2008
by World Grain Staff
Researchers are studying how to make barley, a cereal crop that is grown throughout the world, a more efficient feedstock for biofuels production.
Compared to maize (corn), typical barley has much lower starch content and the mash created while converting it into ethanol has a high viscosity, making the process difficult and expensive, said Kevin Hicks, with the Eastern Regional Research Center, Agricultural Research Service (ARS), U.S. Department of Agriculture (USDA). But barley, along with other grains such as wheat, pearl millet, sorghum and field peas, can play a role in bridging the gap between current methods and next-generation ethanol production. In the U.S. alone, barley could produce 3.8 billion liters (1 billion gallons) of ethanol.
ARS and Genencor International researchers working together in a cooperative research and development agreement are investigating new varieties of barley, including "hull-less" types, and processing methods that mitigate barley’s shortcomings as a feedstock.
BARLEY ATTRIBUTES According to the U.S. Grains Council, of all the cereal crops, barley has been adapted to the widest variety of climates. It is grown in Russia, Australia, Germany, Turkey and North America. The leading exporters are the European Union (E.U.), Australia and Canada.
The crop is grown for feed, food or malting. It has higher protein and digestible amino acids, especially lysine, as compared to maize. Those are desirable attributes in the distillers grains, a co-product of the ethanol process, that is fed to livestock.
Annual world barley output is about 130 million to 155 million tonnes, with the E.U., Russia and Canada producing about half of the total. Due to low market demand, the U.S. now produces only 3% to 5% of the total barley output.
Along with its higher protein and amino acid content, barley is a good biofuel feedstock because it is cheaper than maize and can be grown in areas where maize cannot, Hicks said. In some areas with mild winters, it is grown as a winter crop, allowing double cropping with soy.
"You can get more grain from the same land in a sustainable way," Hicks said. "This is one way to get more biofuel feedstock without competing with food and feed markets."
Barley also presents some challenges, including an abrasive hull, he said. The phytoliths all over the hull act just like sandpaper, which can be destructive to grain handling and grinding equipment. The standard feed barley available today has a low starch content of about 50% to 55%, much less than the 70% of maize.
The mash produced with barley has a high viscosity due to beta-glucans, Hicks said. This makes processing difficult and expensive and limits the feed use of the distillers grains.
The composition and type of barley is critical for economical production of ethanol, he said. A typical hulled feed barley might have a starch content of 55%, a beta-glucan content of 5% and a low test weight of 48.1 pounds per bushel (maize has a test weight of about 56 pounds). In comparison, an elitehulled barley developed by collaborators Carl Griffey and Wynse Brooks at Virginia Tech has a higher starch content of 60%, a lower beta-glucan content of 4% and a better test weight of 53 pounds.
Even better is a hull-less barley, developed by Virginia Tech, with a starch content of nearly 64%, a beta-glucan content of 4% and a test weight of 61 pounds, which rivals wheat.
Any type of barley can be made hullless by breeding in the genetic trait, Hicks said. It grows like regular barley but the hull is not held as tightly to the grain. During harvest or cleaning, 90% to 95% of the hull can be removed, he said.
PRODUCTION METHODS The process for producing ethanol from barley is similar to that used for maize. During the pre-liquefaction stage, milled barley along with a beta glucanase and a thermal stable beta glucanase is heated to 60 degrees C (140 degrees F) for an hour. The mash then goes through a three-hour liquefaction process before heading to fermentation. During fermentation, additional beta glucanase is added. The beta glucanase lower the viscosity of the barley mash by partial hydrolysis of the betaglucans.
Hicks said new studies done by Dr. John Nghiem in his lab have shown that complete conversion of the beta-glucans to glucose is possible with additional amounts of a key enzyme, beta glucosidase. This further reduces the viscosity and produces additional glucose that can be fermented to ethanol, thereby increasing yields.
Researchers are calling this the enhanced dry grind enzymatic or EDGE process. The same enzymes used in the standard conversion of barley to ethanol are added during pre-liquefaction. During fermentation, beta-glucosidase is added.
In studies, the EDGE process produced an ethanol yield of 402 liters per dry tonne of feedstock (2.25 gallons per bushel) as compared to the 395 liters (2.21 gallons) produced in the conventional process.
A third process called STARGEN also has been studied by Genencor collaborators Mian Li and Jay Shetty and has shown higher yields than the standard conversion method. The STARGEN process doesn’t require the high temperature cooking step, Hicks said. A mixture of acid alpha amylase, beta glucanase and protease are added to the mash, which starts converting the solid starch into glucose. During fermentation, a combination of alpha amylase and protease are added.
The process is simpler and requires less energy and equipment, Hicks said. Viscosity is not a problem because the process only goes to a high temperature of 57 degrees C.
"Under these conditions, we are able to hydrolyze the beta-glucans, and since we’re not gelatinizing the starch, there is little viscosity from that either," he said.
STARGEN produced high levels of ethanol from barley similar to the EDGE process with a starch fermentation efficiency of nearly 96%.
In conclusion, Hicks said the EDGE process leads to higher yields of ethanol than the conventional process and the distillers grains from both contain low levels of beta-glucans. The STARGEN process has a higher ethanol yield, the distillers grains contain less beta-glucan, and it is simpler.
STARGEN and the EDGE process can produce high yields of ethanol per bushel when high quality hulled barley is used. Even higher yields could be possible with hull-less barley, currently being studied.