How Cells Grow - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 581

At high temperatures, the thermal death rate exceeds the growth rate, which causes a net
decrease in the concentration of viable cells.

m net ¼ m G k d (11.24)

Thermal death is more sensitive to temperature changes than microbial growth. Temperature
also affects product formation. However, the temperature optimum for growth and product
formation may be different. The yield factor is also affected by temperature due to the change
in the endogenous metabolism requirements as shown in Eqn (11.24) . An increase in temper-
ature at high temperatures results in a decrease in yield factor is expected. In some cases,
such as single-cell protein production, temperature optimization to maximize the yield factor
(YF X/S ) is critical. When temperature is increased above the optimum temperature, the main-
tenance or endogenous metabolism requirements of cells increase. That is, the endogenous
metabolism increases with increasing temperature with an activation energy of 60 e 85 kJ/
mol, resulting in a decrease in the apparent yield factor.
Temperature also may affect the rate-limiting step in a fermentation process. At high
temperatures, the rate of bioreaction might become higher than the diffusion rate, and diffu-
sion would then become the rate-limiting step (for example, in an immobilized cell system).
The activation energy of molecular diffusion is about 25 kJ/mol. The activation energy for
most bioreactions is more than 48 kJ/mol, so diffusional limitations must be carefully
considered at high temperatures. Figure 11.7 depicts a typical variation of the net maximum
growth rate with temperature. One can observe that there is a growth rate maximum at

3

39°C

2

42°C 37°C

36°C
33°C

45°C

30°C

1

46°C 37°C

0.9

28°C

0.8

47°C

0.7

30°C

23°C
21°C

0.6

0.5

0.4

19°C
17°C

23°C

0.3

48°C

0.2

15°C
13.5°C

0.1

3.1 3.2 3.3 3.4 3.5

10 3 /T, K -1

FIGURE 11.7 Variation of net specific maximum growth rate with temperature for Escherichia coli B/r. Circles
are growth in a glucose-rich medium and triangles are growth in a glucose-minimal medium. Data source: S.L.
Herendeen et al. 1979 J. Bacteriology, 139:185 e 194.