The pH-stat Butyric Acid Feeding Strategy Coupled with Gas-Stripping for n-Butanol Production by Clostridium beijerinckii

Organic acids are produced as by-product in food waste by acid fermentation process for biogas production. Particularly, butyric acid is one of the main organic acids and can be converted to n-butanol via the butyrate kinase pathway. However, bacterial toxicity of butyric acid limits its use as a major carbon source. Here, we produced n-butanol from butyric acid using Clsotrdium beijerinckii KCTC 2203, which was the most suitable strain among the six Clostridia strains tested. We produced up to 9.84 g/L of n-butanol using a mixed carbon source composed of 25 g/L of glucose and 10 g/L of butyric acid in a flask culture. The pH-stat fed-batch fermentation method was also performed to increase the production rate. To maximize the effect of pH-stat feeding, substrate feeding solutions with various butyrate/glucose (B/G) ratios were fed. As a result, 9.69 g/L of n-butanol could be produced at a B/G ratio of 0.5, and the pH-stat fed-batch feeding strategy exhibited an enhanced n-butanol production rate of 0.40 g/L/hr without substrate depletion. Finally, we integrated this method with gas stripping using the volatility of n-butanol to prevent product inhibition, resulting in the production of 41.27 g/L of n-butanol in 124 h.