Fermentation of syngas to ethanol without media replacement

Many fermentation processes have traditionally used continuous media replacement to replenish nutrients or to minimize toxic effects. Currently, the fermentation of syngas is being studied for the production of ethanol. Two novel microbial catalysts were used for the studies and the key performance parameters of the organisms, such as growth rates and production rates, were compared. Studies were carried out in both 100 ml bottles with daily replacement of headspace gas as well as in 3-liter bioreactors with continuous gas flow. In all studies, both microbial catalysts were observed to first produce acetic acid as a fermentative product while in the growth phase. As expected, a switch was observed from acetic acid to ethanol as the cell concentration reached steady state. Surprisingly, cells remained healthy and productive with no liquid replacement for more than two months while the ethanol concentration continued to increase with time at a nearly constant rate. Furthermore, it was observed that the acetic acid decreased, indicating that the cells could be converting the acid to ethanol. These findings suggest that syngas fermentation to ethanol can occur over long periods without media replacement.