3.2 Determination of the volumetric oxygen mass transfer coefficient (kLa)
Even though we succeeded in producing consistent rrBChE in the cyclical semicontinuous single-stage culture operation in the 40-L STB described in previous section, the rrBChE production levels and productivities were still lower than what was obtained in a single-stage benchtop bioreactor culture under simplified bioreactor processing (Macharoen et al., 2020). Our hypothesis was that maintaining kLa, as a scale up parameter, and bioreactor configuration such as eliminating baffles between the lab-scale and pilot-scale bioreactors would provide comparable growth kinetics and rrBChE production kinetic parameters between the two scales likely due to similar hydrodynamic stress and oxygen transfer capabilities.
The dynamic gassing out method as described in Blanch and Clark (1997) was used to determine the kLa values in a lab-scale bioreactor and a pilot-scale bioreactor using different polarographic DO probe and DI water. The DO probe response times used in the lab-scale and pilot-scale bioreactors were 25.3 ± 1.5 s and 30.9 ± 1.9 s, respectively. Figure 2 shows the kLa values obtained in a 5-L STB with constant agitation rate at 75 rpm (Figure 2a) and a 40-L STB with a constant aeration rate 0.4 vvm (Figure 2b) where baffles were not equipped in the two scales. Under normal aeration rates used in the 5-L STB, 0.2-0.4 vvm during the growth phase and 0.2 vvm during the induction phase, the kLa values were found in the range of 8.7 ± 0.3 h-1 to 13.2 ± 0.1 h-1(Figure 2a), while the kLa values in the range of 8.1 ± 0.2 h-1 to 9.7 ± 0.1 h-1 in the 40-L STB were found using different agitation rates between 75-250 rpm at a constant aeration rate 0.4 vvm (Figure 2b). Generally kLa values around 10 h-1 are adequate during the growth phase of various plant cell cultures (Takayama and Akita, 2006). Increasing agitation rate in the 40-L STB does not appear to have a strong effect on kLa in this case probably due to a slightly undersized impeller compared to bioreactor diameter (Di/Dt of 0.38). Within the testing agitation range, the single undersized impeller may not have a chance to break up rising-up bubbles outside of the impeller due to comparable sparger ring diameter to impeller diameter. The agitation rate of 150 rpm and the aeration rate of 0.4 vvm were used to investigate the growth kinetics and rrBChE production kinetics in a single stage batch operation in the 40-L STB without baffles to compare to the results in the 5-L STB.