Biochemical Reactions - A First Look
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
in the
reactor. Using gas flow meters
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
dilution rate
and is measured in units of reciprocal time, usually h'1.
Dilution rate =
= —
is the space-time or the reciprocal of the holding time, the usual term in chemical reaction
To attain a constant
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
is fixed in this way, i.e. by controlling one input variable v by
means of measuring another input variable
the reactor is said to operate as a
This is the mode o f operation for a vast majority of laboratory continuous stirred tank
reactors - and for many industrial reactors.
Control of
can also be achieved by measurement of one of the output variables:
In the
v is manipulated at a constant
to obtain a constant biomass
concentration x (g L 1) in the effluent. In this way a certain value of
is obtained which
corresponds to the set point value of
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
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
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
et al.,
previous page 72 Bioreaction Engineering Principles, Second Edition  read online next page 74 Bioreaction Engineering Principles, Second Edition  read online Home Toggle text on/off