The Impact of Initial pH on the Performance of Enterobacter aerogenes S012 to Convert Glycerol to Ethanol at Optimum Temperature and Agitation Speed

Abstract

Aims: (i) To determine the effect of higher initial pH on the effectiveness and efficiency of E. aerogenes S012 in converting glycerol to ethanol; (ii) To identify the best pH range in which the bacterium produces the highest results. Place and Duration of Study: Department of Biological Engineering, North Carolina A & T State University, Greensboro, USA, between March and December 2012. Methodology: Five initial pH values, 7.5, 8.0, 9.0, 9.5 and 10.0, were tested. Fermentations were also performed at controlled pH, where the pH was restored to 6.5 every 6 h. Fermentations were performed in a shaker incubator for 72-96 h, while the products were analyzed by HPLC. Results: The results showed that a higher initial pH of 9.5 was best and that pH 6.2-6.5 was optimum for the organism. We also found that a pH drop below 6.0 stalled fermentation when the organism reached the stationary growth phase, but fermentation continued even after log growth phase when the pH remained at or above 6.0. Optimized fermentation gave ethanol amount, yield and average productivity of 43.5 g/l, 1.11 mol/mol-glycerol and 0.45 g/l/h, respectively, in 96 h. This represents more than a 200% improvement in the effectiveness and efficiency of the organism in converting glycerol to ethanol. Productivity of ethanol was highest during the logarithmic growth of the bacterium. Conclusion: Maintained at slightly acidic pH, E. aerogenes S012 could be an efficient biocatalyst for conversion of biodiesel production “waste” to ethanol, a biofuel.