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.