Fed-Batch Cultivation - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 690

13.7.9. Fluorescence
A common parameter to monitor cell culture is optical density. A linear relationship exists
between the culture fluorescence and the dry cell weight concentration up to 30 g-dwc/L,
due to the optical interference of the intracellular NAD(P)H pool. Thus, fluorescence can
be used to estimate online the biomass concentration and be a controlling parameter in the
feed provision.
13.8. PARAMETERS TO START AND FINISH THE FEED AND STOP
THE FED-BATCH FERMENTATION
The times at which the feeding should start and finish, as well as the criteria to stop a fed-
batch fermentation is very much dependent on the specific cultivation kinetics and the oper-
ator's interest. For example, in substrate-limited processes, the feed should start immediately
after the substrate reached the low limit, otherwise the process may be difficult to control, for
example, because of a lag phase due to previous starvation. The most commonly used crite-
rion to start the feed is the depletion of the substrate, which can be measured by a multitude
of techniques, from specific enzymatic assays, HPLC (high-performance liquid chromatog-
raphy) and NMR (Nuclear Magnetic Resonance) spectroscopy to indirect methods such as
the exhaust gas analysis. Still related with the amount of the substrate in the medium, the
operator might not find necessary to reach the complete depletion but to be below a predeter-
mined set point (eventually related with historical data, growth models and known yields).
The fed-batch fermentation should be halted when the production slows down because of
cell death, because the metabolic potential of the culture becomes inadequately low or
because by-product excretion starts at significant levels. Some other criteria can be an
increase in viscosity that implies an increased oxygen demand until the oxygen limitation
is achieved, which is the case for penicillin production.
13.9. SUMMARY
Batch operations are not controllable as all the substrate is added into the reactor at the
start. Fed-batch reactor is based on feeding of a growth-limiting nutrient substrate to
a culture. Cell growth and fermentation can be controlled by the feeding strategy. The fed-
batch strategy is typically used in bio-industrial processes to reach a high cell density in
the bioreactor. Mostly the feed solution is highly concentrated to avoid dilution of the biore-
actor. In essence, fed-batch reactor is applied in such a fashion that a chemostat or CSTR is
simulated with a seemingly batch operation. The controlled addition of the nutrient directly
affects the growth rate of the culture and allows avoiding overflow metabolism (formation of
side metabolites, such as acetate for E. coli , lactic acid in cell cultures, ethanol in S. cerevisiae )
and oxygen limitation (anaerobiosis).
Substrate limitation offers the possibility to control the reaction rates to avoid technological
limitations connected to the cooling of the reactor and oxygen transfer. Substrate limitation
also allows the metabolic control to avoid osmotic effects, catabolite repression and overflow