Production of medium-chain volatile fatty acids by mixed ruminal microorganisms is enhanced by ethanol in co-culture with Clostridium kluyveri

Authors: Weimer, Paul; NERDAHL, MICHAEL - University Of Wisconsin; BRANDL, DANE - University Of Wisconsin

Submitted to: Bioresource Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/12/2014
Publication Date: 11/1/2014
Publication URL: https://handle.nal.usda.gov/10113/60083
Citation: Weimer, P.J., Nerdahl, M., Brandl, D.J. 2015. Production of medium-chain volatile fatty acids by mixed ruminal microorganisms is enhanced by ethanol in co-culture with Clostridium kluyveri. Bioresource Technology. 175:97-101.

Interpretive Summary: In an effort to find efficient and economic ways to turn biomass into biofuels via fermentation, researchers have studied the very efficient rumen fermentation process and microbial communities in cattle. The rumen fermentation converts fibrous and non-fibrous plant material to a mixture of volatile fatty acids that serve as the primary energy source for the animal. Most of the acids contain 2 to 4 carbon atoms (short-chain). The microbial community of the rumen has been shown to be useful for converting biomass materials to these same short-chain acids in bioreactors, but these acids have a much lower potential to produce quality fuels than do acids that contain 5 and 6 carbon atoms. We demonstrated that these 5- and 6-carbon acids can be produced in good yield in fermentations of biomass by mixed rumen microbes upon addition of a dilute solution of ethanol and an inoculum of a strain of another bacterium, Clostridium kluyveri, that we had previously isolated from the rumen. The amounts of the 5- and 6- carbon acids produced over a two-day incubation period were equal to or greater than those produced in other fermentation systems that are operated at longer incubation times (5 to 115 days). The results provided a potentially improved route to industrial production of 5- and 6-carbon acids.

Technical Abstract: Fermentative production of medium-chain (C5-C8) volatile fatty acids by the carboxylate platform has several potential advantages as a route to biofuel precursors. However, its practicality is limited by the relatively slow synthesis of these acids from shorter precursors (C2-C4) that accumulate during modified anaerobic digestion processes. Production of medium-chain acids is stimulated by addition of ethanol (potentially obtainable at low cost from cellulosic fermentations that produce ethanol at concentrations too low for economical distillation). But such combined fermentations still proceed relatively slowly and require substantial run times to produce medium-chain acids in quantity. We found that co-culture of mixed ruminal microbes with a rumen-derived strain of the bacterium Clostridium kluyveri converted a combined feedstock (cellulosic biomass plus ethanol) to volatile fatty acid mixtures that include valeric (C5) and caproic (C6) acids as the major fermentation products over a 48-h run time. Concentrations of caproate reached 6.1 g/L, similar to or greater than those reported in conventional carboxylate fermentations that employ substantially longer run times.