m−^3 (Figure 14.4), combined with a model proposed by Jüsten et al. (1998) for fungal
mycelial breakage. The aeration rates of 0.13 vvm (Rushton) and 0.17 vvm (upward-
pumping) are specified so as to prevent oxygen mass transfer limitation in the broth.
From the flow regime maps, it is apparent that the Rushton turbine may be unsuitable for
this application. The minimum speed for complete solids and gas dispersion, which
simultaneously satisfies oxygen mass transfer requirements, exceeds the limit for shear-
related damage. In contrast, the upward-pumping impeller would appear to offer a wider
range of potential operating conditions. Moreover, the minimum stirrer speeds required
for solids suspension and gas dispersion are largely insensitive to aeration rate. Although
there is experimental evidence of effective gas dispersion, stable operation and
diminished power reduction on gassing with upward-pumping, axial flow agitators
(Junker et al., 1998a; Nienow, 1998), there is also some data (Junker et al., 1998b) to
suggest that such impellers cannot deliver the mass transfer rates required for high
oxygen demand fermentations. To date, however, there are no reports of upward-
pumping impellers employed with plant cell suspensions and they are certainly worthy of
investigation.
Aeration in Plant BioreactorsTo meet the oxygen demands of respiring plant cells in agitated bioreactors, aeration rates
in the range 0.05–0.1 vvm are generally required, although values as high as 0.5 vvm are
commonly reported. When higher rates are employed, gas flow dominates circulation
patterns in the vessel, the air is poorly dispersed and the vessel essentially operates as an
agitated bubble column. With a view to minimising agitation rates, spargers generating
large numbers of small bubbles are more effective than single orifice nozzles (e.g. Treat
et al, 1989). High aeration rates, in addition to exacerbating foaming (Section on
Foaming and Wall Growth), are associated with gas-stripping effects known to be
inhibitory to plant cell systems. By recirculating the exhaust gas from a STR, Schlatmann
et al. (1993) observed ajmalicine production profiles in C. roseus similar to those
obtained in shake flasks, suggesting that valuable metabolites were removed during
normal aeration. Carbon dioxide has been identified as an important gaseous component
(e.g. Kobayashi et al., 1991), although its effects are unrelated to photosynthetic activity.
The volatile plant hormone, ethylene, is also susceptible to stripping and has been the
subject of a number of related studies (Kim et al., 1991; Mirjalili and Linden, 1995).
Ethylene effects appear to be highly system specific, while both CO 2 stripping and CO 2
accumulation have been observed to have a negative influence on growth of Cyclamen
persicum suspensions in bioreactors (Hone et al., 1999). Schlatmann et al. (1994a)
suggested that stripping of some other, unidentified volatile metabolite(s) was responsible
for the reduction in secondary metabolism observed on scale-up from a shake flask to a
sparged bioreactor. Currently available information on the effects of carbon dioxide,
ethylene and oxygen accumulation
Bioreactor design for plant cell suspension cultures 437