Apart from the choice of a suitable bioreactor system for cultivation of plant cell and tissue cultures, the operational strategy is of significance. Optimal results will be expected only from the combination of the most suitable reactor type and operational mode. Unfortunately, the possibilities of modern bioprocess engineering are not applied rigorously to mass propagation procedures based on plant cell and tissue cultures. This section gives a general survey of operational strategies of plant cell and tissue bioreactors. It is common knowledge that the mode of reactor operations can vary. Production methods have been developed based on batch, fed-batch, and continuous systems.
Cells (biocatalysts) and medium are added to a closed system (bioreactor) in a batch or discontinuous culture. In terms of the cultivation process, only the cultivation parameters—for example, temperature, aeration rate, and light—will be changed. Medium or biomass is not added or removed during total cultivation time. The culture is harvested after an appropriate period of cultivation, i.e., when growth has stopped because of lack of nutrient.
The medium is continuously fed into the bioreactor and biomass is continuously removed to keep the liquid level constant in continuous cultures. The steady-state level of the cell population, which is determined by the nutrient feed rate, is reached as long as the principle of the chemostat [growing biomass and dilution rate D (ratio of volumetric flow rate to reaction volume) < specific growth rate] is followed. Theoretically, this method allows continuous exponential growth of the biocatalyst (20,24).
Compared with continuous cultures, batch systems have the advantages of simplicity, reliability, and flexibility as well as the drawback of a lower space-time yield. However, they can also cause significant disadvantages for systems with substrate as well as product inhibition in that the total substrate concentration has to be received at the beginning and the formed products have to stay inside the reactor during the total cultivation time. In contrast, continuous systems can define the substrate concentration through the regulation of the dilution rate as well as the contact time of the product by choosing the continuous reactor route. Drawbacks of continuous strategies are a high level of instrumentation, inflexibility, and susceptibility to failure related to infections by contaminants.
The fed-batch operation of a bioreactor is a compromise between an ordinary batch and a continuous operation. Fresh nutrient medium is added during the stationary phase of the biocatalyst growth in fed-batch operations, and as a result, the cell growth will continue. The volume of liquid medium is increasing during the cultivation time. Fed-batch cultivations are preferred for processes involving substrate inhibitions and biotransformations. The precursor represents the feeding medium in the course of biotransformation processes. It is also common practice to feed elicitors in fed-batch processes. In contrast to fed-batch processes, the nutrient medium is removed periodically and fresh medium is added during repeated fed-batch cultivations. The bioreactor is then left to run as a batch until the cycle is repeated. Such an operational route ensures prolongation of the process time as long as active biomass is available. A more difficult inoculum scale-up will not be necessary (102).
The two-stage batch mode is a modified fed-batch operation. In 1977 Zenk and co-workers successfully introduced a modified discontinuous procedure in which growth and product formation do not take place simultaneously, the so-called two-step procedure (20). The two-step or two-stage mode, which is also used commercially for shikonin production, works with two optimized media used separately. In reality, it is also possible to vary parameters such as oxygen supply, pH value, and light intensity to influence growth and product formation.
However, additional process strategies and reactors with external or internal (in situ) reactor-separator systems do exist. Special growth and product formation processes have been developed through reactor arrangements such as cascade operation (reactors in series) or through the combination of stirred tank and plug flow reactors. Here, depending on the growth and product formation phase, each bioreactor can be operated independently while using separate media or process parameters, for example temperature, pH, and oxygen supply. External or internal reactor-separator systems allow enrichment (increasing the cell concentration) or removal of biomass and metabolites in cultivation processes. It has been found that these assemblies are of special interest for slowly growing cultures where an increase of process efficiency can be achieved by high biomass concentrations as well as uninhibited product formation. Therefore, plant cell and tissue cultures have to be enriched and metabolites, medium components, or the complete medium have to be exchanged periodically or continuously inside the bioreactor. Typical cell culture configurations that meet these demands are membrane reactors and bioreactors with perfusion systems usually applied in the form of internal (spin filter) or external (centrifuge) cell retention devices (103). They guarantee the establishment of high-density cultures (e.g., commercial production of berberine). To our knowledge, other known technical reactor-separator systems are multiphase reactors, vacuum bio-reactors, bioreactors with integrated adsorbers or ion exchangers, distillation and extraction bioreactors, and reactors with gas stripping (104).
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