Biochemical Reactions - A First Look
49
These observations - which as stated initially are not commonplace - do, however, show that
experimental studies of bioreactions in principle try to extract useful information from a system, which is
not at all observable. It is vastly more complex than the gas-phase reaction catalyzed by a solid catalyst,
which is a standard topic in textbooks on chemical reaction engineering. An almost infinite possibility of
reaction paths with a hierarchal control structure which fine-tunes the active paths in response to a
changing environment, to the age of the culture, or to signals that we have not even begun to explore is
the standard scenario of cell reaction studies.___________________________________________________
Besides setting the substrate concentrations to certain values in the liquid and the gas feed
streams the attainment of a steady state continuous culture requires that vg, and especially v, the
feed rate of the liquid feed, are set to constant values relative to the medium volume
V
in the
reactor. Using gas flow meters
vJV
is controlled at a given set point to obtain a certain gas flow
rate - often specified in terms of v.v.m. - volume gas per volume liquid per minute. The ratio
between v and V is one of the most important input variables in bioreactors. The ratio is called
the
dilution rate
and is measured in units of reciprocal time, usually h'1.
v
Dilution rate =
D
= —
V
(3.1)
D
is the space-time or the reciprocal of the holding time, the usual term in chemical reaction
engineering.
To attain a constant
D
several different control strategies may be used:
The volume of the reaction medium or the weight of reactor and medium is measured
with a frequency of e.g. 10 m in'1
and the liquid feed rate v is controlled to give a certain
set-point for
V.
When
D
is fixed in this way, i.e. by controlling one input variable v by
means of measuring another input variable
V
the reactor is said to operate as a
chemostat.
This is the mode o f operation for a vast majority of laboratory continuous stirred tank
reactors - and for many industrial reactors.
Control of
D
can also be achieved by measurement of one of the output variables:
In the
turbidostat
v is manipulated at a constant
V
to obtain a constant biomass
concentration x (g L 1) in the effluent. In this way a certain value of
D
is obtained which
corresponds to the set point value of
x.
The feed of a nutrient (e.g. glucose) can be manipulated to obtain a certain pH in the
effluent. Many bioreactions produce or consume protons and by separating the nutrient
feed from the alkali/acid feed used to neutralize the proton production the rate of the
bioreaction can likewise be controlled. This is the
pH-auxostat.
Measurement of the effluent concentration of one of the metabolic products, e.g. ethanol
in fermentations with
Saccharomyces cerevisiae,
can also be used to obtain the
D
value,
which corresponds to a given set point for the effluent concentration (or the rate of
production) of one of the products. This is called a
productostat
(Andersen
et al.,
1997).
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