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

Inhibition of microbial metabolism due to the presence of metabolic product s leads to reduced yields in some fermentation processes. In-situ extractio n of inhibitory fermentation products is one method to increase the product yield. Solvent compatibility with the microorganism is an important criter ion in the selection of extractant solvents. Although supercritical CO2 has been investigated for the post-fermentation extraction of metabolic produc ts, it has generally been rejected for in-situ extraction due to its inhibi tion of the metabolism of numerous microorganisms. The objective of this st udy was to assess the impact of dense gases and supercritical fluids (nitro gen, CO2, and ethane) on the carbohydrate consumption and ethanol formation by a model organism, Clostridium thermocellum, a fibrolytic thermophilic b acterium. Non-growing cells capable of metabolism were incubated at 60 degr ees C with cellobiose as a substrate in the presence of the three pressuriz ed fluids. The fermentation broth was sampled with time for residual cellob iose and ethanol concentration. The rate and extent of ethanol production w ere similar in cell suspensions maintained at atmospheric pressure under ni trogen (conventional method) and at 6.9 MPa under nitrogen. Ethane at 6.9 M Pa reduced the extent of ethanol production by less than 20% relative to th e atmospheric control, whereas CO2 at the same pressure reduced ethanol for mation by more than 80%, These results suggest that pressurized hydrocarbon s may have benefits over supercritical CO2 for the in-situ recovery of vola tile fermentation products. (C) 1999 Elsevier Science B.V. All rights reser ved.