Continuous fermentation studies for significant increase in succinic acid productivity in Actinobacillus succinogens cultures

Succinic acid (SA), an anaerobic fermentation product biosynthesized by Actinobacillus succinogenes, has been reported as one of the top 10 valuable building block chemicals. In order to enhance volumetric productivity of succinic acid, continuous fermentations without, and also with cell-recycling were carried out at various dilution rates and production medium compositions. A drip tube installed in the inlet stream of the continuous fermentation system provided a barrier in the medium flow path, preventing the microorganisms from growing into the feed line. Therefore, it was possible to successfully operate the continuous fermentation process for more than 300 hours without any contaminations. Cell-recycled continuous operations were possible, since the efficient decantor system (i.e., cell separator) developed in our laboratory could separate high density of the suspended cells effectively from the outlet stream, thus overcoming the wash-out phenomenon encountered at relatively low dilution rate in the continuous process without cell-recycling. It was found that the decantor system could recover more that 65% of the suspended cells present in the outlet culture broth of the continuous fermentation system. Notably, a strategy of feeding increased amount of MgCO₃ through the inlet stream of the cell-recycled continuous fermentation system(CRCFS) turned out to be very efficient for enhanced production of succinic acid, resulting in about 10 fold, and more than 6 fold higher volumetric productivity (DP) (i.e., g succinic acid/L/hr) than the corresponding batch and continuous fermentation systems without cell-recycling, respectively.