Com medium is used for fermentation by C. acetobutylicum, while molasses medium is fermented by C. saccharo acetobutylicum. The fermentation passes through three phases:
(i) rapid bacterial growth and production of acetic and butyric acids, and evolution of CO2 and H2 (initially, H2 is the major component of gas) for 13-17 hr after incubation,
(ii) rapid conversion of the acids into acetone and butanol leading to a fast decline in the acidity of the broth (this is called "acid break"), and
(iii) marked decrease in gas evolution, and acetone and butanol production.
The final broth contains about 2% acetone, ethanol and butanol, which represents about 30% conversion of the carbohydrates. Typically, com medium yields butanol, acetone and ethanol in the ratio 6, 3 and I, while for molasses medium these values are 6.5, 3 and 5.
Recovery of the products is by a distillation process usually by a "beer still" which operates in a continuous mode; this yields approximately 40% (w/w) total mixed solvents. Individual solvents are separated by fractional distillation.
