Biochemical Reactions - A First Look
59
Figure 3.4 Anaerobic glucose-limited continuous culture of
S. cerevisiae.
Production rate as a function
of glucose consumption rate at steady state: total ethanol (liquid and gaseous) (■ ); CO; ( • ) ; biomass
('*'); glycerol (□). The lines are calculated by linear regression. The data are taken from Duboc
et ai
(1998).
Note 3.2 How to treat ions in the black box model.
Unless otherwise stated the stoichiometries discussed in this book are written with undissociated
substrates and reactants, but the stoichiometry could easily be extended to consider charged compounds.
Hereby there will be introduced an additional balance, namely the overall charge, but there will also be
introduced an additional compound, namely protons. In reaction (3.15) and in the specific example of
reaction (3.23) the nitrogen source (NH3) is likely to be almost completely protonated at the usual
medium pH of 5.5-6 in a yeast cultivation since pKa is 9.25 for the acid
N H
A. The ammonium ion is
transported across the cell membrane by an active, ATP consuming mechanism (Section 2.1.2.3.) and
delivered to the cytosol of the cell. The pH of the cytosol is close to neutral and again the dissociation of
n h ;
is quite small. Nitrogen is, however, incorporated into the cell mass as NH
3
and with this drain of
NH
3
away from the equilibrium all the protons of
N H A
are liberated and transported back to the
medium.
This can be used to determine the specific growth rate P. In a stoichiometry such as (3.23) with a
negligible production of organic acids
qx
= Px is nearly proportional to the volumetric rate of proton
production which can be determined by titration of the medium with (usually 1
-5 M) NaOH or K.OH to
keep the pH of the medium constant at e.g. 5.5. Using a separate feed of NH
3
rather than the commonly
applied mixed feed containing (NH^iSCh or some other ammonium salt one may if desired avoid the
high cation concentration of Na+ or Kf which is a side effect of titration with the strong bases.__________
At the end of this section we shall again emphasize that any given stoichiometry such as reaction
(3.23) is true only for steady state operation and usually only for a given set of environmental
previous page 82 Bioreaction Engineering Principles, Second Edition  read online next page 84 Bioreaction Engineering Principles, Second Edition  read online Home Toggle text on/off