Enhanced Propionate Formation by Propionibacterium freudenreichii subsp. freudenreichii in a Three-Electrode Amperometric Culture System

In order to influence the fermentation pattern of Propionibacterium freudenreichii towards enhanced propionate formation, growth and product formation with glucose and lactate as energy sources were studied in a three-electrode poised-potential amperometric culture system. With anthraquinone 2,6-disulfonic acid (E0′ = −184 mV; poised electron potential = −224 mV) or cobalt sepulchrate (E0′ = −350 mV; −390 mV) as mediator and an activated platinum working electrode, reduction of bacterially oxidized mediator occurred fast enough to keep more than 50% of the respective mediator (in minimum 0.4 mM) in the reduced state, up to a current of 2 mA. With glucose as substrate, 90.0 or 97.3% propionate was formed during exponential growth in the presence of 0.5 mM anthraquinone 2,6-disulfonic acid or 0.4 mM cobalt sepulchrate, respectively. Growth yields of 56.3 or 53.8 g of cell material per mol of substrate degraded were calculated, respectively, and the electrons were transferred quantitatively from the working electrode to the bacterial cells. With l-lactate, only 68.6 or 72.9% propionate was formed with the same mediators. The results are discussed with respect to energetics, electron transfer potentials, and potential application of the new technique in technical propionate production.