In the NaOAc buffering, the highest specific butanol rate (0.58 h −1) was associated with the highest undissociated butyric acid (1.92 g/L). CaCO 3 supplementation extended cell growth to 36 h and resulted in higher butyrate yield under 8 g/L of CaCO 3. By neutralizing the fermentation pH to ~5.0 at different time, we observed that neutralization should take place at the beginning of exponential phase (12 h), and otherwise resulting in lower concentrations of undissociated butyric acid, cell biomass and final butanol. In this study, with the aim of further understanding the role of undissociated butyric acid in butanol production, we investigated the correlation between undissociated butyric acid concentration and specific butanol production rate in batch fermentation of Clostridium acetobutylicum by comparing three pH control approaches: NaOH neutralization (at 12, 24 or 36 h), CaCO 3 supplementation (2, 5, or 8 g/l) and NaOAc buffering (pH 4.6, 5.0 or 5.6). Furthermore, contradictory results have been reported about required acidic conditions for initiation of solventogenesis. However, it is not very clear at what pH level the acid crash will occur, at what pH level butanol production will be dominant, and at what pH level butyric acid production will be prevailing.
PH control has been essential for butanol production with Clostridium acetobutylicum.
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