Continuous Cultivation - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 619

800

700

600

500

X

400

300

200

100

S

0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Dillution rate, h -1

FIGURE E12-1.1 Variations of biomass and substrate concentrations with dilution rate in a chemostat.

We can then calculate the maintenance coefficient, which is given by m s ¼
k d /YF X/S ¼ 0.04967/h.

Figure E12-1.1 shows the agreement between the data set (symbols) and the model predic-
tions (lines). One can clearly see the match between the experimental data and the model
predictions. The washout condition is shown on the right.

12.1.3. The Chemostat as a Tool
The chemostat can be used as a tool to study the mutation and selection of cultures and
also to study the effect of changes in the environment on cell physiology. The molecular
aspects of mutation and selection will be discussed in Chapter 14.
Natural or induced mutations can take place in a chemostat culture. Errors in DNA repli-
cation take place with an average frequency of about 10 6 to 10 8 genes per generation. With
a cell concentration of 10 12 cells/L in culture, the probability is high in a chemostat that
a wide variety of mutant cells will be formed. The vast majority of natural mutations in a che-
mostat are of little significance, unless the mutation alters the function of a protein involved
in growth in the chemostat environment. If the specific growth rate of the mutant is larger
than that of the wild type, then the mutant outgrows the wild type in a chemostat. This selec-
tion for a variant cell type can be accomplished by creating a more favorable environment for
growth of the mutant organism.
A chemostat culture can be used for the selection of special organisms. Selection or enrich-
ment nutrient media need to be used for this purpose. For example, if it is desired to select an
organism growing on ethanol, a nutrient medium containing ethanol and mineral salts is
used as a feed to a chemostat culture. An organism capable of oxidizing some toxic refractory
compounds can be selected from a mixed culture by slowly feeding this compound to