Acetic acid (CH3COOH) and ethanol (C2H5OH) are the two major products from syngas fermentation. Equations (1)-(4) show the four basic reactions producing acetic acid and ethanol. In this case, the gaseous substrates CO and H2 follow the acetyl-CoA pathway to produce acetic acid and ethanol under strict anaerobic conditions.

From Equation (1), it is clear that about one third of the carbon from CO is utilized in the product yield. The overall ethanol production, combining Equations (1) and (2), reveals that two thirds of the carbon from CO is converted to ethanol. During the acetyl-CoA pathway, hydrogen provides the required reducing equivalents and electrons when hydrogenase enzyme is present in the fermentation media (Equation (5)).

H2 ! 2H+ + 2e — (5)

If the hydrogenase enzyme is inhibited or hydrogen is not present in the fermentation broth, the required electrons are obtained from CO in the presence of CODH enzyme. In other words, CO is used in supplying electrons, rather than in the biofuel production. This obvi­ously results in a drastic reduction in biofuel yields. It is therefore, important to maintain healthy concentrations of both hydrogen and CO during syngas fermentation.

Syngas-fermenting microorganisms are critical in biofuel production. Under optimum growth conditions, most of the known syngas-fermenting microbes tend to produce more acetate than alcohol products (e. g., ethanol, butanol, etc.). Vega et al. (1989) reported acetate to ethanol product ratio of 20:1. In order to improve the product formation from acidogenesis to solventogenesis, researchers investigated nutrient limitations, pH shifts, reducing agent addition (Klasson et al., 1992), and hydrogen addition.