Simultaneous synthesis of lactic acid and hydrogen from sugars via capnophilic lactic fermentation by Thermotoga neapolitana cf capnolactica

This study investigated the effect of the salinity level, buffering agent and carbon source on the hydrogen (H₂) and lactic acid synthesis under capnophilic (CO₂-assisted) lactic fermentation (CLF) by Thermotoga neapolitana cf capnolactica (DSM 33003). Several series of batch fermentation experiments were performed either in 0.12 L serum bottles for selection of the best performing conditions or in a 3 L fermenter for the best possible combination of conditions. The serum bottle study revealed that change in the salinity level of the culture medium from 0 to 35 g L⁻¹ NaCl increased lactic acid synthesis by 7.5 times without affecting the H₂ yield. Use of different buffers (MOPS, TRIS or HEPES) did not affect the average H₂ yield of 3.0 ± 0.24 mol H₂ mol⁻¹ of glucose and lactic acid synthesis of 13.7 ± 1.03 mM when the cultures were sparged by CO₂. Among the carbon sources investigated, glucose was found to be the best performing carbon source for the CLF fermentation with 35 g L⁻¹ of NaCl and 0.01 M of phosphate buffer. Hence, an up-scale experiment using a 3 L fermenter and the combination of the best performing conditions showed a 2.2 times more lactic acid synthesis compared to the 0.12 L serum bottle experiments. The study reveals the robustness and flexibility of the CLF-based technology using T. neapolitana cf capnolactica fermentation under various operating environmental conditions.