Continuous two-stage ABE-fermentation using Clostridium beijerinckii NRRL B592 operating with a growth rate in the first stage vessel close to its maximal value.

A two-stage continuous cultivation experiment with Clostridium beijerinckii NRRL B592 is described. The experiment was designed to mimic the two phases of batch culture growth of the organism in a two-stage continuous process. Thus in the first stage turbidostat the organism was grown acidogenically as rapidly as possible, and transferred to the second stage at the 'acid break point'. The second stage was designed to mimic the solventogenesis of the batch culture when it enters late exponential/early stationary phase. The volume of the second stage vessel was calculated to provide the necessary residence time for complete sugar utilization. It was hoped that the experimental set-up chosen would show whether data obtained from batch fermentation could be transferred directly to continuous culture. The culture maintained its ability to produce acetone, 1-butanol and ethanol at a dilution rate of 0.12 h(-1) for the first stage and 2.2 x 10(-2) h(-1) for the second stage and achieved an average overall solvent concentration of 15 g/l and an overall solvent productivity of 0.27 g/l/h for a period of steady-state operation of more than 1600 hours. The productivity of solventogenesis in the first stage was dependent on the value of the growth rate of the culture which was in turn determined in part by the organism employed but also by the medium composition.