Acetone-butanol fermentation and its variants.

Recent intensive research on the acetone-butanol-ethanol and the isopropanol-butanol-ethanol fermentation has increased the basic understanding of these processes substantially. Metabolic investigations on Clostridium acetobutylicum, and Clostridium beijerinkii show that enzyme activities necessary for solvent production are induced only in solvent-producing cells. Although produced, or added, acetic and butyric acid have significant effects on the metabolic activities, the transition from acid to solvent production cannot as yet be fully explained. Based on studies in continuous cultures, the kinetics of product formation can be described. Knowledge of the mechanism of butanol toxicity is accumulating but no dramatic increase in butanol tolerance has so far been obtained. Successful results, approaching the limitations determined by biological and technological possibilities, have been obtained in batch and continuous cultures, and in continuous processes based on immobilized cells. Continuous processes are superior to batch cultures in respect of their productivity.