Design of Fermentation Processes
401
Dimensionless time 0
Dimensionless time 0
Figure 9.14. Washout of the slowest-growing species. In A:
DK
0.3 h'1, and X2 is washed out. In
B:
D
>
Q.3 hand
is washed out. Infection:
&X; =
0,005 in both A and
B.
D
= 0.20 h 1
in A and 0.32 h'1
in
B.
Figures 9.14 A and B show what happens after infection of the reactor with the growth-favored microbial
species. At
D <
0.3, the specific growth rate of
xj
is higher than that of
x2.
A stable steady-state cultivation
ofx, can be maintained only when
xs
is absent, in which case Eq. (9.113a) is automatically satisfied. Even a
minute infection of the chemostat by
x,
leads to washout of
x2
,
and
S
decreases from 2/15 to the lower
steady-state value 1/10 associated with growth of
x,
alone at the specified
D -
0.2 h'1. For
D
> 0.3 ft1,
infection of a monoculture of
xt
will correspondingly lead to a takeover by
x2,
as illustrated in Fig. 9.13B for
D = 032
h’1.
Reversion of a producing strain to a nonproducing wild type strain of the same microorganism is a
well-known cause of disappointment in the industrial application of microorganisms. The reversion
may occur by mutation, or a valuable plasmid may be lost from a recombinant microorganism.
Kiipekar
et al.
(1985) give an example of a Cephamycin-C producing strain which reverts to a
nonproducing strain, and their analysis of experimental data from a chemostat culture to find a
kinetic model for the metamorphosis reaction is discussed in Example 9.15.Another case study is
discussed in Problem 9.8.
For growth kinetics of the monotonic Monod type and a dilution rate where species
x,
is
growth-favored, even a small net rate of conversion of
x,
to
x2
by metamorphosis reactions will lead
to washout of
Xj,
even when the culture was initially free of
x2.
Conversely, a metamorphosis
reaction in the other direction may help to stabilize the growth-handicapped species in the
continuous culture.
The most frequent outcome of competition or parasitism in a stirred tank continuous reactor is that
one of the microbial species is washed out except at a certain value of the dilution rate. The
transient of a perturbation usually follows a straight (exponentially damped) path toward the final
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