Biomedical Engineering Reference
In-Depth Information
optimum temperature. The profile is quite similar to that of an enzyme-catalyzed reaction in
Chapter 8.
11.10. EFFECT OF P H
Hydrogen-ion concentration (pH) affects the activity of enzymes and therefore the micro-
bial growth rate. The optimal pH for growth may be different from that for product forma-
tion. Generally, the acceptable pH range varies about the optimum by 1 e 2 pH units. Different
organisms have different pH optima: the pH optimum for many bacteria ranges from
pH
¼
3 e 8; for yeast, pH
¼
3 e 6; for molds, pH
¼
3 e 7; for plant cells, pH
¼
5 e 6; and for
animal cells, pH
6.5 e 7.5. Many organisms have mechanisms to maintain intracellular
pH at a relatively constant level in the presence of fluctuations in environmental pH.
When pH differs from the optimal value, the maintenance energy requirements increase.
One consequence of different pH optima is that the pH of the medium can be used to select
one organism over another.
In most fermentations, pH can vary substantially. Often the nature of the nitrogen source
can be important. If ammonium is the sole nitrogen source, hydrogen ions are released into
the medium as a result of the microbial utilization of ammonia, resulting in a decrease in pH.
If nitrate is the sole nitrogen source, hydrogen ions are removed from the medium to reduce
nitrate to ammonia, resulting in an increase in pH. Also, pH can change because of the
production of organic acids, the utilization of acids (particularly amino acids), or the produc-
tion of bases. The evolution or supply of CO 2 can alter pH greatly in some systems
(e.g. seawater or animal cell culture). Thus, pH control by means of a buffer or an active
pH control system is important. Variation of specific growth rate with pH is depicted in
Fig. 11.8 , indicating a pH optimum.
¼
1
0.9
0.8
0.7
0.6
µ
µ max
0.5
0.4
Wild type
0.3
With adaptation
0.2
0.1
0
3
4
5
6
7
8
9
pH
FIGURE 11.8 A fictitious variation of specific growth rate with pH. With some microbial cultures, it is possible
to adapt cultures to a wider range of pH values if pH changes are made in small increments from culture transfer to
transfer.
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