areas of biology, fermentation processing, chemistry and end uses for biobutanol.
The challenge to improve the process technology and the microbes that carry out the fermentation drives academic and governmental researchers as well. Qureshi, for instance, has been studying biobutanol production for more than 20 years. He came to the United States from New Zealand to develop a membrane process for more effectively recovering butanol from fermentation broth. He’s also worked to develop efficient butanol bioreactors. In the past few years, however, his research has taken a different direction, one that focuses on optimizing the process for more economical substrates such as wheat straw, barley straw, switchgrass and corn stover. “We need to move toward more economical substrates,” Qureshi says. “But it’s not as simple as it looks.”
First of all, there’s an inherent paradox in the microbial fermentation of butanol: although butanol-producing bacteria produce the enzymes that convert simple sugars into the alcohol, butanol itself is toxic to those same bugs. This butanol inhibition results in a lower alcohol concentration in the fermentation broth, which leads to lower yields of butanol and higher recovery costs. These are the challenges that surface when highly pure feedstocks are used. When a cheaper, biomass substrate is used, additional microbial inhibitors are generated during the pretreatment process.
Strategies for reducing butanol toxicity and improving yield, including integrating several steps in the process and manipulating the microbial cultures, are advancing. “We’ve made good progress with raw materials, removing inhibitors and product separation,” Qureshi says. The overall process that Qureshi’s team has developed for the production of butanol from agricultural residues involves four steps: pretreatment, which opens the cell wall structure and removes lignin; hydrolysis of hemicellulose and cellulose into simple hexose and pentose sugars using enzymes; fermentation of simple sugars into butanol using a pure culture of Clostridium beijerinckii P206, an anaerobic bacterium; and recovery of the butanol. The unique aspect of the process is that the last three steps are combined and performed in a single reactor. “We’ve integrated the process and it appears to be very effective economically,” Qureshi says. His team is currently in the process of filing a patent on the process.
In addition, Qureshi has teamed with Lars Angenent, an environmental engineer at Washington University, as well as other USDA-ARS researchers to improve the economics of the hydrolysis step. The idea is to replace the need for enzymes, which are often expensive, with a mixed culture of bacteria. “The real tenets of my lab involve studying nondefined mixed cultures and seeing what they can do,” Angenent explains. In the collaboration with Qureshi, Angenent will use microbes collected from the sludge of an anaerobic digester as well as microbes from sheep rumen to ferment pretreated corn fiber to butyric acid, a chemical found in rancid butter, parmesan cheese and vomit. The solution containing the acid will be sent to Qureshi’s lab where it will be fermented into butanol by his pure cultures of Clostridium.
The collaboration is in its infancy, financed by a $425,000 grant from the USDA. Currently, Angenent’s team is working to optimize the butyric acid production by tweaking conditions like pH and temperature. “We try to steer the community to produce one product over another,” he explains. Once conditions are right for the production of significant levels of butyric acid, Qureshi will take over.
Engineering Butanol-Fermenting Bugs
Whereas the approach spearheaded by Qureshi and Angenent involves optimizing butanol production by microbes that naturally produce it, a team of chemical and biomolecular engineers from the University of California, Los Angeles, recently