6.2. Air driven bioreactors
A bubble column bioreactor (Figure 36.2.Ba) is a reactor, in the shape of a column, in which the reaction medium is kept mixed and aerated by the introduction of air at the bottom (IUPAC, 1997). The major advantages of bubble column bioreactors are the low capital costs, uncomplicated mechanical configurations and less operational costs due to low energy requirements. Alternatively they are less suitable for the processes where highly viscous liquids exist. In an airlift bioreactor (Figure 36.2.Bb) the reaction medium is agitated and aerated by the introduction of air or another gas mixture and the circulation is improved by internal draught tubes or external loops. Thus, the reactor volume is separated into gassed and degassed regions generating a vertically circulating flow (IUPAC, 1997). Airlift bioreactors fulfill the low O2 demands of plant cell cultures with low shear effects. Airlift bioreactors have a number of advantages, such as combining high loading of solid particles, providing good mass transfer, relatively low shear rate, low energy requirements, and simple design. The main disadvantage is their unsuitability for high density plant cultures. Hence stirred tank bioreactors are preferred for culturing plant cell suspensions at high densities. A further problem in air lift bioreactors is extensive foaming which can clog the air exhaust filters and increase the risk of contamination. To overcome these problems, sparger rings for plant cells at high aeration rates, bubble free aeration, antifoam agents, etc. can be applied.
6.3. Non-agitated bioreactors
For the immobilization of a large amount of cells per unit volume packed bed (Figure 36.2.Ca) and membrane reactors (Figure 36.2.Cc) are advantageous. However, diffusional limitations of mass transfer and difficulties in handling gaseous components can limit the use of both configurations (Sajc et al., 2000). Fluidized bed reactor (Figure 36.2.Cb) is based on the utilization of the energy of the flowing fluid to suspend the particles. This type of reactors provides the major benefit of mass transfer of the small particles.
7. Bioreactors for hairy roots
While designing a suitable bioreactor for hairy root cultures the physiology and morphology of the hairy roots should be taken into consideration. The major problem in bioreactor cultivation of hairy roots is their tendency to form clumps resulting from the bridging of primary and secondary roots. This results in densely packed root beds and reduces mass transfer (both oxygen and nutrients). Root thickness, root length, the number of root hairs and root branching frequency are some of the factors which should be taken into consideration for hairy root cultures in bioreactors. Immobilization of hairy roots by horizontal or vertical meshes as well as by cages or polyurethane foam promotes their growth in submerged stirred bioreactors, bubble columns, air lift reactors and drum reactors where the roots are immersed in the culture medium. Isolation of the roots from the impeller also rules out the possibility of root damage even at low tip speeds in stirred bioreactors. Also the oxygen transfer limitation in hairy root cultures in bioreactors can be reduced by growing them in gas phase bioreactors, spray or droplet reactors and mist reactors. Here the roots are exposed to humidified air or a gas mixture and nutrients are delivered as droplets by spray nozzles. Spray and mist reactors also provide the added advantage of low hydrodynamic stress.
8. Conclusion
High productivity, high product yield and high product concentration are the major objectives of plant tissue process development. A variety of bioreactor types providing growth and expression of bioactive substances are available today for plant cell and tissue cultures. Low biomass and product level can be achieved in any type of bioreactors. However, an improved understanding of the manifold interactions between cultivated cells, product formation and the specific designs for different bioreactor types will enhance and sustain high productivity and also reduce the process costs.