Industrial Biotransformations

Fed-batch
Continous
stirred tank
Continuous
plug flow
Unclear
Batch
0 20 40 60 80
Number of processes
Fig. 7.6 Types of reactors in industrial biotransformations.
where immobilization is applied, the reactor is not continuous but the biocatalyst is
reused in a batch or fed-batch reactor. Considering the abundance of fine chemicals as
biotransformation products, it is not surprising that such batch reactors are more popular
than continuous reactors (see Figure 7.6). Batch processing provides the flexibility
that is usually required in fine chemistry.
Fed-batch reactors mainly involve processes with free whole-cells, where substrate toxicity
can be a major issue. Continuous reactors are more common for enzymes than for
whole cells, in particular when considering redox biotransformations. The origin of this
may be the aforementioned diffusion limitations due to immobilization of living cells,
but also the genetic instability of living cells.
In general, water is used as the solvent in industrial biotransformations. Figure 7.7
indicates that only a modest proportion of the industrial processes are carried out in
organic solvents (without a separate water phase). These involve (trans)esterifications
and amidations. More often, a water-miscible or water-immiscible organic liquid is added
to an aqueous phase, leading a monophasic or biphasic liquid, respectively. The latter
category is the more common one. Not only does it include processes where an organic
solvent is used as a reservoir for the substrate and product, but also a significant number
of processes where the organic phase is actually composed of just the liquid substrate or
product. No organic solvent is added in such cases.
aqueous
solvent
not reported
suspended
product
organic solvent
biphasic liquid
monophasic
liquid mixture
Fig. 7.7 Characteristics of solutions in industrial biotransformations.
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