Grow It on Glycerin

By Jerry W. Kram
What are biodiesel producers to do with all that glycerin? That's been the question ever since the biodiesel industry ramped up production and saturated the market with the three-carbon polyol byproduct.

Glycerin has a multitude of uses in the food, pharmaceutical and chemical industries, among others. However, with the growth of the biodiesel industry, there is more glycerin than the current market can easily absorb. This presents an opportunity to create new processes that use glycerin to produce more valuable chemicals, says Katherine Taconi, assistant professor in the Department of Chemical and Materials Engineering at The University of Alabama in Huntsville. "This is a huge issue, not just for biodiesel but for biorefining in general," she says. "If we want biorefining to replace traditional refining to any significant extent, you can't just make ethanol and biodiesel. Only about 60 percent of a barrel of crude goes to making fuel. The rest goes to making all sorts of other stuff."

Taconi, whose work is funded by the USDA, is investigating strains of bacteria that can ferment glycerin into other chemicals, a process she calls "bioconversion." Her current project is attempting to make butanol from glycerin. Her lab is using both pure and mixed culture methods to transform the glycerin into butanol and a variety of byproducts. "Organisms never produce just one product, unfortunately," she says. "They produce several, but if you are lucky, it's not more than three or four."

Taconi says from an engineering perspective, glycerin is a three-carbon compound, and carbon should never be wasted. It can be converted to other chemicals through a catalytic process, but as a biologist, she first looked to fermentation. "There aren't a lot of organisms you can use to ferment glycerol into value-added chemicals," she says. "Almost all organisms can ferment glycerol as an intermediate compound, but they use it for growth."

Research led Taconi to investigate a handful of anaerobic organisms that could ferment glycerin into other chemicals. These include the genuses Clostridium, Klebsiella and Enterobacter. One species of glycerin-fermenting bacteria-Clostridium pasteurianum-just happened to be good at producing butanol, 1,3 propanediol and ethanol. "This strain of Clostridium is not very well-studied," Taconi says. "People have generally looked at sugar as the main feedstock. The species I am looking at can use glycerol."

Taconi's current work involves screening mixed cultures of "wild-type" anaerobic organisms for the production of useful chemicals and optimizing the environmental conditions of pure cultures to maximize butanol production. "These are very fundamental studies, just looking at what parameters affect product distribution and substrate utilization," she says. "The biggest is pH. [Other anaerobic fermentations] are highly pH-dependent, but nobody has really investigated C. pasteurianum enough to know if that pathway is pH-dependent or not." She is also investigating whether trace metals can influence butanol production.

Nothing in scientific literature indicates whether this organism could produce butanol from crude glycerin, which would contain water, methanol and salts from the neutralized caustic catalyst, and this is one of the things Taconi wants to learn. Other future research topics she is interested in include elucidating the enzymatic pathway that C. pasteurianum uses to convert glycerin to butanol. "For better or worse, the pathway that the industry is taking is metabolic engineering, but you won't get there until you understand what enzymes are in the pathway and what enzymes are active in the pathway," she says.