2.3. Fermentation process
Each media was inoculated with 2%(v/v) Bacillus subtilis natto and incubatedat 40 °C in aerobic conditions for 48 h(at120rpm).A 3-L fermentor(BioFlo/CelliGen115,New Brunswick Scientific Co., USA)was equipped with a top-driven stirrer with two six blade Rushton impellers(i.e.diameter 45 mm,width14mm,length 14mm)and four baffles(i.e.width14mm).Filtered ai rwas injected through a perforated pipe sparger located under the bottom impeller. The levels for aeration and stirrer controllers
were set at 1vvm and 200 rpm,respectively.The DO probe was calibrated using a standard two-point calibration method.The 0% calibration was achieved by sparging nitrogen into the medium at approximately 1 vvm until the value stabilized near zero;the 100% calibration was achieved by increasing agitation to approximately 50 rpm and increasing airflow to 5 vvm. After calibration,dissolved oxygen values were reported based on these calibration set points.