Model of the employed fermentation system. A 2.5 L benchtop bioreactor was used for anaerobic cultivation of Bacillus subtilis DSM 10T. The bioreactor was equipped with two Rushton turbines, a temperature sensor, pH and pO2 electrodes, peristaltic pumps for pH control, an exhaust cooler and attached exhaust gas analysis, which were connected to a computer for online analysis. To adjust anaerobic conditions in the liquid medium and the head space of the bioreactor valve 1 was opened to allow a N2 flow through the sparger. During fermentation valve 1 was closed and N2 was allowed to flow through valve 2, enabling a constant gas flow through the head space.