Abstract
pH control has been essential for butanol production with Clostridium acetobutylicum. 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. Furthermore, contradictory results have been reported about required acidic conditions for initiation of solventogenesis. 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), CaCO3 supplementation (2, 5, or 8 g/l) and NaOAc buffering (pH 4.6, 5.0 or 5.6). 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. CaCO3 supplementation extended cell growth to 36 h and resulted in higher butyrate yield under 8 g/L of CaCO3. 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). The linear correlation of the undissociated butyric acid with the specific butanol production rates suggested the undissociated butyric acid could be the major driving force for butanol production.
Highlights
Butanol is one of the promising advanced biofuels and an important intermediate in chemical synthesis
With the aim of improving butanol production yield, we investigated the effects of different pH control methods on butanol and butyric acid production in batch fermentation by C. acetobutylicum
Effect of NaOH neutralization on cell growth, butanol and butyric acid production by C. acetobutylicum To determine at what time and pH level the acid crash will occur, we added 5 mL of 0.5 M NaOH at 12 h, 24 h, and 36 h individually during the fermentation to control the pH between 4.7 and 5.3
Summary
Butanol is one of the promising advanced biofuels and an important intermediate in chemical synthesis It is being pursued by industry and the U.S government (the DOE, USDA and NSF) for the generation of alternative fuels (Hess 2006; Li et al 2011a; Li et al 2011b; Milne et al 2011; Qureshi and Ezeji 2008). Initiation of solventogenesis has been correlated well with the concentration of undissociated butyric acid (Huang et al 1986; Husemann and Papoutsakis 1988; Monot et al 1984). It has been suggested that solvent production initiated at the minimum of 1.5 g/L of undissociated butyric acid (Monot et al 1984). A linear correlation has been found between butanol and undissociated butyric acid concentration at the onset of solvent formation (Husemann and Papoutsakis 1988). The solvent formation could be initiated at neutral pH (6.8–7.0) with the supplementation of high concentration of acetate plus butyrate (George and Chen 1983; Holt et al 1984)
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