Bioconversion of syngas to organic acids (e. g., acetic and butyric acids) and alcohols (e. g., ethanol and butanol) follows the acetyl-CoA pathway (Henstra et al., 2007; Klasson et al., 1990; Phillips et al., 1994). The most common acidogenic microorganisms include Clostridium thermoaceticum, C. Ijungdahlii, Peptostreptococcus productus, A. woodii, Eubacterium limosum, and B. methylotrophicum. Many of the reported fermentation studies have shown a high acetic acid production compared to the other organic acids. Younesi et al. (2005) reported an acetate con­centration of 1.3 g/L at 1.4 atm pressure using C. ljungdahlii.

Butyrate is synthesized by the chemical intermediate acetyl-CoA reacting with butyryl — CoA (Brown, 2006). Acetic and butyric acid yields are highly dependent on the types of microbe and the substrate. Worden et al. (1989) reported that the production of butyrate was increased by 10-folds at the expense of acetate yield when the pH shift was from 6.8 to 6.0. Recovery of organic acids produced during syngas fermentation may provide oppor­tunity for additional revenue generation from coproduct.