Figure 14.5 Flow regime maps for (a)
Rushton turbine (D=0.5T) and (b)
upward-pumping, 6-blade, pitched-
blade turbine (D=0.57). Redrawn, with
permission, from Doran (1999). ©
ACS
is sparse and line specific and has, in general, been collected under conditions which
make it difficult to draw comparisons between the results of different studies.
Extended oxygenation of plant bioreactors is most commonly achieved via direct
sparging with air. Bubble-free aeration via porous membranes (e.g. Su and Humphrey,
1991) or surface aeration (Tanaka et al., 1983; Jolicoeur et al., 1992) and in a loop
fluidised bed reactor (Dubuis et al., 1995) has proven successful in laboratory/pilot-scale
systems. In sparged, laboratory-scale bioreactors, the contribution of surface aeration to
overall mass transfer may be significant (Singh and Curtis, 1994a) and head-space
gassing with air or oxygen-enriched air may be used to reduce bulk sparging rates.
Although more work is required in this area to develop a rational approach to
optimisation of aeration conditions, current knowledge suggests that gassing rates in
sparged systems should be maintained at the lowest level compatible with satisfying
cellular oxygen demands and, where appropriate, mixing requirements. Oxygen
supplementation may be beneficial.
Foaming and Wall GrowthIn aerated, laboratory/pilot-scale plant bioreactors, foaming is a common occurrence and
can constitute a significant nuisance. Cells become entrained in a foam “meringue” which
forms above the active broth level and which is fed by splashing. Although necrosis is
inevitable in regions beyond the splash zone, significant cell/callus growth may occur on
internal vessel surfaces. System productivity is reduced and, in severe cases, foam
overflow may foul the exhaust filter and increase the risk of contamination. However,
unlike animal cell systems, there is no evidence to suggest that plant cells are damaged by
interactions with bursting bubbles, either during the gas disengagement process (Singh
and Curtis, 1994a) or in the foam layer itself (Wongsamuth and Doran, 1994).
Foaming is strongly dependent on the aeration and agitation conditions and on the
properties of the cell-free broth, particularly surface tension. Foaming studies with A.
belladonna (Wongsamuth and Doran, 1994) and M. citrifolia (Cusack, 1998) broths in
bubble columns have shown that, at a given aeration rate, foam formation is strongly,
positively correlated with extracellular protein concentration, which is typically growth
associated. Absence of foaming in aggregated, self-immobilised cultures (Xu et al., 1998)
has been attributed to reduced levels of metabolite secretion associated with cellular
differentiation. Foaming typically commences during the exponential growth phase;
release of additional proteinaceous material by cell lysis towards the end of the growth
cycle may exacerbate the condition. Foaming patterns will be influenced by broth pH,
which is generally uncontrolled in plant cell suspensions and which typically varies
Bioreactor design for plant cell suspension cultures 439