2.2 Measurement of overall oxygen mass transfer coefficient (kLa)
The measurement of probe response time for a polarographic dissolved oxygen (DO) probe and kLa in a benchtop bioreactor and a pilot-scale bioreactor were performed as previously described (Leth and McDonald, 2017) with slight modifications. The step change from pure N2 (medical grade) to pure O2 (medical grade) was used to measure the response time of a DO probe (Mettler Toledo, Billerica, MA) in deionized (DI) water in two Erlenmeyer flasks saturated with pure N2 in one flask and pure O2 in the other flask. To evaluate kLa inside a bioreactor, the DO probe is inserted into the bioreactor containing DI water where temperature was maintained at 27˚C. The DI water (4 L in the benchtop bioreactor and 30 L in the pilot-scale bioreactor) was sparged with pure N2 until the %DO reached 0. Then the gas inlet was switched to industrial air, and the changes of %DO were recorded until a steady state %DO was reached (~ 100% DO air saturation). Three replicate trials were performed for finding probe response time and kLa values. By using the dynamic method with significant oxygen probe response, the collected data points were fitted in the equations suggested by Blanch and Clark (1997) using the method of least squares in MATLAB® (MathWorks Inc, Natick, MA).