Corn steep liquor (CSL) is rich in carbohydrates, vitamins, minerals, and trace metals. It is also a lower cost nutrient compared to yeast extract (YE), the primary media constituent currently being used in syngas fermentation. Syngas is primarily a mixture of CO, CO2, and H2. Yeast extract was employed at a concentration of 1 g/L, while two CSL concentrations (10 g/L and 20 g/L) were investigated. Initially, growth and product profiles of Clostridium strain P11 in YE and CSL media were followed during fermentation in 250-mL serum bottles fed with syngas every 24 h at 239 kPa (absolute). In subsequent runs, the syngas fermentation was scaled-up to 7.5-L fermentor, in which growth, product profile and redox potential were closely monitored during the fermentation process. The temperature and pressure in the bioreactor were controlled at 37°C and 143 kPa (absolute). Modeled syngas composed of 20% CO, 15% CO2, 5% H2, and 60% N2 (by volume) was used.
After 600 h of fermentation, ethanol concentrations in 250-mL serum bottles with 1 g/L YE, 10 g/L and 20 g/L CSL were 1.3 g/L, 1.5 g/L, and 2.7 g/L, respectively. However, the maximum ethanol concentrations after 360 h of fermentation in 7.5-L fermentor with 10 g/L and 20 g/L CSL media were 8.6 g/L and 9.6 g/L, respectively, which represent 57% and 60% of the theoretical ethanol yields based on CO consumed. Only about 6.1 g/L of ethanol was obtained in the medium with 1 g/L YE after 360 h, which represents 53% of the theoretical ethanol yield based on CO consumed. These results demonstrate that corn steep liquor is a source of nutrients for strain P11 and can replace yeast extract as the primary medium component.