Fed-Batch Cultivation - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 688

of the specific growth rate. Also, in modern fed-batch processes for yeast production, the feed
is strictly under control based on the measurement of traces of ethanol in the exhaust gas of
the fermentor.

13.7.5. Inductive, Enhancer or Enrichment Components
In certain fermentations, it is of interest to continuously add either an inductive or fast
consumed components and not just a limiting substrate. An example is the continuous
addition of an antibiotic in recombinant microorganims bearing an antibiotic marked
plasmid. Genetically altered microorganisms often belong to this category. Another example
is given by the production of glutathione by high-glutathione-accumulating S. cerevisiae , the
microorganims commonly used for commercial production. Cysteine was found to be the
only amino acid that enhanced glutathione formation. However, the growth inhibition
occurred and it was related to the concentration of cysteine. This problem was then
resolved by an adequate addition of cysteine in exponential fed-batch culture without
growth inhibition.
Fed-batch operation is an appropriate mode of fermentation in microorganisms that are
producing heterologous proteins and whose elevated protein expression results in product
degradation by activation of proteases. A general insight on this subject was the study of
a recombinant E. coli for production of chloramphenicol acetyltransferase. A gradual induc-
tion with Isopropyl- -D-thio-galactoside (IPTG) and phenylalanine (rate-limiting precursor)

addition strategies were able to reduce the physiological burden imposed on the bacterium,
thereby avoiding cellular stress responses and enhancing bioreactor productivity. In this case,
IPTG and phenylalanine were the driving parameters that dominated the feed.
The addition of precursors or inducers should take into account if the product of interest is
growth associated or not. For example, the use of a tyrosine-deficient strain of E. coli in the
production of phenylalanine requires a balanced feed of tyrosine that, if not provided in
low quantities, is used as carbon source with subsequent production of excessive biomass
synthesis at the expense of phenylalanine synthesis. This limitation on biomass production
is possible because the phenylalanine production was not growth associated.

b

13.7.6. Respiratory Quotient
Gas analyzers, especially mass spectrophotometers, are relatively fast. Respiratory
quotient (RQ), the ratio between the moles of carbon evolved per moles of oxygen consumed,
has been a general method used to determine indirectly the lack of substrate in the growth
medium. It is a fairly rapid method of measurement that is useful because the gas analyses
can be related to crucial process variables. The method is not “universal” to all bioprocesses
since some biosystems can produce by-products that affect the productivity of the process
without affecting the RQ, such as the production of acetic acid by E. coli .
Usually, the signal is characterized by a sharp rise in dissolved oxygen. Based on the
concept of RQ, there are the so-called DO-stats, in which the feed is regulated in accordance
with the dissolved oxygen. The analysis of the dissolved oxygen or carbon dioxide evolution
rate can also be used to control or prevent the production of by-products. The RQ is often