Liquid Culture Systems for in vitro Plant Propagation

Application of bioreactor systems 97
2. Bioreactor design
Bioreactors devoted to mass propagation includes systems for cultivating
cells, tissues, somatic embryos or organogenic propagules (e.g. bulblets,
corms, nodules, microtubers, and shoot clusters) in liquid suspensions. Until
the mid 1970s, traditional microbial technology provided the main source of
knowledge and equipment for the cultivation processes, almost exclusively
in the form of stirred tank reactors (STR) with flat blade turbines for
agitation. Today, a relatively large number and variety of reactor systems are
available, allowing a rational selection of an appropriate reactor for a given
process. Still, most of the standard equipment designed for microbial
cultivation does not meet the special requirements for cultivation of fragile
plant cells or cell aggregates.
Takayama and Akita (1994), Heyerdahl (1995), Walker (1995), Lee
(1997), Sajc et al. (2000), Honda et al. (2001), Paek et al. (2001), Paek and
Chakrabarty (2003) reviewed different reactor configurations for plant cell
suspensions, plant tissue and organ cultures. The relative advantages and
selection criteria for various reactor configurations were discussed for
specific process applications.
Those bioreactors are fundamentally classified by agitation methods and
vessel construction into: a) mechanically agitated (stirred tank bioreactor,
rotating drum bioreactor, spin filter bioreactor), b) pneumatically agitated
and non-agitated bioreactors (simple aeration bioreactor, bubble column
bioreactor, air-lift bioreactor, balloon type bubble bioreactor-BTBB).
Numerous modifications of the conventional STR with bubble aeration
have been developed that have a variety of impeller designs (Honda et al.,
2001). Notwithstanding, the STR presents several limitations such as high
power consumption, high shear forces and problems with sealing and
stability of rotating shafts in tall bioreactors. Air-lift bioreactors combine
high loading of ‘solid’ particles and good mass transfer, which are inherent
for three-phase fluidized beds. Air bubbles, using internal or external
recirculation loops, generate efficient mixing in the liquid phase. The main
advantages of air-lift bioreactors are low shear forces, low energy
requirements, and simple design. Rotary drum reactors have significantly
higher surface area to volume ratios than other reactor types. As a
consequence, mass transfer is achieved with comparably less power
consumption, according to Danckwert’s surface renewal theory (Dankwert,
1951). These features are favorable for bioprocesses utilizing shear-sensitive
tissues, as well as for photo bioreactors (Sajc et al., 2000).
In a bubble column bioreactor the bubbles create less shear forces, so that
it is useful for plant organ cultures especially for propagation of various
species through tissue culture of shoots, bulbs, corms and tubers. The