Industrial Biotransformations

. immobilization methods
. substrate and product concentrations
. addition of antioxidants or stabilizers
. reactor material or coating
. physical treatment (stirring, pumping, gas–liquid phases, etc.).
5.7 Recent Developments and Trends
During optimization of the space–time yield it is necessary to consider the catalyst
costs. In particular, the combination of stability and activity has to be considered. Sometimes
it is desirable to work at very low temperatures with low reaction rates, which have
to be compensated for by a large amount of biocatalyst if the turnover number has thus
to be increased. In other cases the turnover number will have a lower priority because as
much product as possible has to be synthesized. No pragmatic rule exists for the best
strategy to adopt to optimize the reaction conditions. Empirical methods based on statistical
methods have good chances of being successful (e.g., genetic algorithms).The most
important improvements can be made by finding and constructing an optimal catalyst.
5.6
Scale-up of Bioreactors
One of the primary goals of bioprocess engineering is to achieve large-scale production
levels by scaling-up processes that were originally developed in the laboratory on a
bench-scale. It implies an increase in the volume handled in a production cycle, or in
other words, a decrease in the surface to volume ratio. This fact prohibits a direct scalingup
of a process, because mass and energy transport limitations arise.
Scaling-up involves scientific, engineering and economic considerations, in addition
to correct judgments and experience. It also results in the introduction of larger apparatus,
whose effects on the process must be analyzed and optimized. The possible major
effects on physical and biotransformation parameters as a result of scaling-up are summarized
in Table 5.2.
5.7
Recent Developments and Trends
New challenges in the reaction engineering of biotransformations can be foreseen in various
areas. In this section we will briefly discuss recent developments and techniques
which have already been applied successfully on a laboratory scale but have not found
widespread technical application.
In the field of redox biotransformations [36, 37] a great deal of effort has been spent
on establishing methods for efficient cofactor regeneration. Recent research has elucidated
the potential of using cheap redox equivalents such as molecular hydrogen with
hydrogenase enzymes [38, 39] or electrons in indirect electrochemical cofactor reduction
for the production and regeneration of reduced [40, 41] and oxidized cofactors [42, 43].
Even enzymes that are not cofactor dependent can be combined with electrochemical
steps, e.g., for the synthesis of a cosubstrate [44].
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