Biom asi (g L'1)
Biochemical Reactions - A First Look
57
Tima (h)
Tima (h)
Figure 3.3 Anaerobic batch fermentation with
S. cerevisiae.
After 20 hours the nitrogen source is exhausted
and nitrogen limitation sets in.
A. The biomass concentration and the cellular content of the storage carbohydrates trehalose and glycogen
B. The cellular content of protein and the biomass concentration. Anaerobic fermentation of
S. cerevisiae
with NHj as the nitrogen source. The data are taken from Schulze (1995) and Schulze
et at
(1996),
biomass is synthesized after depletion of the N-source the original cells just become bloated with
glycogen (CHi 6700.8j) and some trehalose (CH17gO0g9). After 40 hours the biomass contains 25 wt %
glycogen and 6 wt % trehalose. At the end of the batch fermentation the formula weight of
X
is
24.57 gL 1
and its composition has changed from (3.19) to
^f-CHj.^Oo^No.usSo.oonPo.t
(3.21)
Accumulation of storage carbon compounds in procaryotes may amount to 70-80% of the total cell
weight. A typical example is the accumulation of polyhydroxybutyrate (PHB) in
Ralstonia
eutropha
(Lee and Choi, 2001) when nitrogen, phosphate or some other nutrient needed for growth
of an active cell is denied to the culture. In these extreme cases the biomass composition
approaches that of the storage polymer, e.g. CH] 50o.5 for PHB.
3.3 Black Box Stoichiometries
Equation (3.15) is an attempt to represent all the chemical reactions by which the substrates are
converted to biomass and metabolic products by a single chemical equation, a so-called “black
box” model. This is clearly an immense simplification as all the biochemical reactions responsible
for the overall conversion of substrates to metabolic products and biomass are lumped into a single
overall reaction. For certain biological systems the yield coefficients may, however, be constant,
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