Fermentation of sweet sorghum derived sugars to butyric acid at high titer and productivity by a moderate thermophile Clostridium thermobutyricum at 50 °C

• Sugars from sweet sorghum juice and bagasse were fermented to butyric acid at 50 °C.
• A butyrate titer of 44 g L−1 was achieved in ∼40 h at 50 °C with sorghum juice.
• Butyrate yield from sorghum juice and bagasse was 0.41 and 0.4 g g−1, respectively.
• Average butyrate productivity with sorghum juice over 48 h was 0.9 g L−1 h−1.

In this study, a moderate thermophile Clostridium thermobutyricum is shown to ferment the sugars in sweet sorghum juice treated with invertase and supplemented with tryptone (10 g L−1) and yeast extract (10 g L−1) at 50 °C to 44 g L−1 butyrate at a calculated highest volumetric productivity of 1.45 g L−1 h−1 (molar butyrate yield of 0.85 based on sugars fermented). This volumetric productivity is among the highest reported for batch fermentations. Sugars from acid and enzyme-treated sweet sorghum bagasse were also fermented to butyrate by this organism with a molar yield of 0.81 (based on the amount of cellulose and hemicellulose). By combining the results from juice and bagasse, the calculated yield of butyric acid is approximately 90 kg per tonne of fresh sweet sorghum stalk. This study demonstrates that C. thermobutyricum can be an effective microbial biocatalyst for production of bio-based butyrate from renewable feedstocks at 50 °C.