Importance of agitation in acetone-butanol fermentation

The specific rates of anaerobic solvent production by Clostridium acetobutylicum increased with increasing fermentor impeller speed from 190 to 340 rpm (N(Re) = 3.93 x 10(4)). The maximum values were 5.54, 3.85, and 0.8 mmol/h . g cell for butanol, acetone, and ethanol, respectively. Corresponding rates for respective gases produced were 11.60 and 15.88 mmol/h . g cell for H(2) and CO(2). Further increases in agitation speed resulted in generally decreasing specific production rates to the point of inactive fermentation at 560 rpm. A competition observed between the cellular subsystems for butanol + butyric acid and biomass biosynthesis was evaluated through expressing the energetic yield coefficients. An imbalance between the production and outflux of the former metabolites is apparently further enhanced by a mechanical damage of the cells at high shear rates. A correlation was developed between the production of gases and solvents pointing at both H(2)-to-solvent as well as CO(2)-to-solvent ratios following the same pattern, peaking at 410 rpm.