3.3.2. Product profiles
Acetic acid and ethanol were the major products formed during syngas fermentation in culture media (Fig. 3b). Acetic acid is a primary metabolite, and its production is associated with cell growth. The production of acetic acid was higher during the growth phase and gradually increased till the cells entered the stationary phase.
A maximum acetic acid concentration of 1.67 g L_1 was reached at 192 h (Fig. 3b). Acetic acid concentration then decreased to 1.3 g L_1 at the end of the fermentation. Acetic acid concentrations were comparatively lower between 120 and 168 h. Consumption of acetic acid by mixed culture TERI SA1 was observed at later stages
of fermentation. Acetic acid consumption was associated with production of ethanol after 192 h of fermentation. This was also observed in other syngas fermentation studies using Clostridium strain P11 (Panneerselvam et al., 2010) and C. carboxidivorans P7 (Hurst and Lewis, 2010). Ethanol production started in the exponential phase (Fig. 3b). The rate of ethanol production increased in the early exponential phase and then slightly decreased till
the cell entered in stationary phase. During stationary phase the ethanol concentration again start increasing and decreased with the decline in cell concentration. This trend in ethanol production was due to the effect of syngas pressure in reactor headspace (Hurst and Lewis, 2010). The maximum ethanol concentration was 2.3 g L_1 at 72 h, which was 49% higher than achieved during batch fermentation conditions (Fig. 3b). Higher ethanol production in semi-continuous fermentation mode could possibly be due to the sparging of fresh syngas after every 24 h as compared to batch fermentation (sparged and pressurized with syngas just in the beginning of fermentation).