Ethanol Production in Batch and Continuous Culture from Some Carbohydrates with Clostridium thermosaccharolyticum LMG 6564

Abstract

Summary pH-Controlled batch and continuous fermentations were carried out with Clostridium thermosaccharolyticum LMG 6564. Growth in batch cultures was studied over a broad range of glucose concentrations (50 to 250 mM). The pH effect was the same for all glucose concentrations tested: at pH 7.0, ethanol and carbon dioxide were the main end-products; lowering the pH decreased the ethanol production and increased the acetate, hydrogen gas, butyrate, butanol and lactate formation. With 250 mM glucose only 74% of the glucose was fermented at pH 7.0, perhaps because ethanol reached a toxic level of 0.8%. Steady state growth of continuous cultures in glucose limitation (D = 0.11 h-1) in the pH range between 5.0 and 7.0 revealed a similar pH effect on the metabolite pattern as for batch fermentations. Higher dilution rates resulted in higher lactate and lower ethanol concentration for all glucose concentrations tested (50 to 250 mM). Only for the lowest glucose concentration (50 mM) in the inflow medium, the ethanol yield was the same as in the corresponding batch experiments. For higher glucose concentrations (150 and 250 mM), batch fermentations were favorable for ethanol recovery. Batch fermentations at pH 7.0 with 150 mM glucose turned out to be the best conditions for the highest ethanol yield of 1.2 mol per mol glucose. In continuous culture, the product pattern was the same when fructose was used as fermentable sugar. With sucrose a higher lactate yield and with mannitol a higher ethanol yield was obtained. However, this latter fermentation was very weak.