How Cells Grow - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 585

In aerobic fermentations, the rate of metabolic heat evolution can roughly be correlated to
the rate of oxygen uptake, since oxygen is the final electron acceptor.

V L m O 2 X
M O 2

DH G ¼ 4DH g (11.54)

where is the heat of combustion for one unit of degree of reduction, see Eqn (3.94).

Metabolic heat released during fermentation can be removed by circulating cooling water
through a cooling coil or cooling jacket in the fermenter. Often, temperature control (adequate
heat removal) is an important limitation on reactor design (see Chapter 10). The ability to esti-
mate heat-removal requirements is essential to proper reactor design.

D H

g

11.14. O VERVIEW OF MICROBIAL GROWTH KINETIC MODELS

We have described key concepts and kinetics in the growth of cultures based on cell
metabolism and cell generation. To bring about a mathematical description of cell growth,
we often resort to approximations, as illustrated in Fig. 11.10 . A successful mathematical
description is using the least amount of computation to capture the maximum amount of
physical behaviors. To this extent, the Monod equation was commonly used.
Cellular composition and biosynthetic capabilities change in response to new growth
conditions (unbalanced growth), although a constant cellular composition and balanced growth
can predominate in steady and pseudosteady growth (e.g. the maximum growth during batch
growth) phase. If the decelerating growth phase (or growth rate is lower than maximum) is
due to substrate limitation or depletion rather than inhibition by toxins, the growth rate
decreases in relation to decreasing substrate concentrations. In the stationary and death
phases, the distribution of properties among individuals is important (e.g. cryptic death).
Although these kinetic ideas are evident in batch culture, they are equally evident and
important in other modes of culture (e.g. continuous culture). Clearly, the complete descrip-
tion of the growth kinetics of a culture would involve recognition of the complete metabolic
pathway and genetic construction of cell. However, the whole metabolic pathway involves

Apparent Single Enzyme
Description

Single Enzyme
plus age description

Full Metabolic/Genetic
Description

Full Metabolic/Genetic
and Physiological Description

Physiological description

FIGURE 11.10 Simplifications in microbial kinetic description.