Effect of pressurized solvents on ethanol production by the thermophilic bacterium Clostridium thermocellum

Inhibition of microbial metabolism due to the presence of metabolic products leads to reduced yields in some fermentation processes. In-situ extraction of inhibitory fermentation products is one method to increase the product yield. Solvent compatibility with the microorganism is an important criterion in the selection of extractant solvents. Although supercritical CO₂ has been investigated for the post-fermentation extraction of metabolic products, it has generally been rejected for in-situ extraction due to its inhibition of the metabolism of numerous microorganisms. The objective of this study was to assess the impact of dense gases and supercritical fluids (nitrogen, CO₂, and ethane) on the carbohydrate consumption and ethanol formation by a model organism, Clostridium thermocellum, a fibrolytic thermophilic bacterium. Non-growing cells capable of metabolism were incubated at 60°C with cellobiose as a substrate in the presence of the three pressurized fluids. The fermentation broth was sampled with time for residual cellobiose and ethanol concentration. The rate and extent of ethanol production were similar in cell suspensions maintained at atmospheric pressure under nitrogen (conventional method) and at 6.9 MPa under nitrogen. Ethane at 6.9 MPa reduced the extent of ethanol production by less than 20% relative to the atmospheric control, whereas CO₂ at the same pressure reduced ethanol formation by more than 80%. These results suggest that pressurized hydrocarbons may have benefits over supercritical CO₂ for the in-situ recovery of volatile fermentation products. (C) 1999 Elsevier Science B.V.