How Cells Grow - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 559

extracellular product conversion explains only one part of the biomass yield relation. The
other part is the biomass change. Cells die when environmental conditions become less
favorable or simply due to aging. Because of the complexity in microbial metabolic activities,
it is very difficult to separate the different effects on growth yield.
Yield factors based on other substrates or product formation may be defined; for
example,

dX j Growth

YF X=O 2 ¼ (11.13)

d ½O 2

dP

YF P=S ¼ (11.14)

dS

For organisms growing aerobically on glucose, YF X/S is typically 0.4 e 0.6 g/g for most
yeast and bacteria, while YF X=O 2 is 0.9 e 1.4 g/g. Anaerobic growth is less efficient, and
the yield factor is reduced substantially (see apparent biomass growth yield illustration
on Fig. 11.2 ). With substrates that are more or less reduced than glucose, the value of the
apparent yield will change. For methane, YF X/S would assume values of 0.6 e 1.0 g/g,
with the corresponding YF X=O 2 decreasing to about 0.2 g/g. In most cases, the yield of
biomass on a carbon-energy source is 1.0 0.4 g biomass per gram of carbon consumed.

Table 11.1 lists some examples of YF X/S and YF X=O 2 for a variety of substrates and

organisms.

0.8

0.7

0.6

0.5

0.4

Aerobic growth, Y X/S = 0.303 g-X/g-S

0.3

0.2

Anaerobic growth, Y X/S = 0.114 g-X/g-S

0.1

0.0

0.0 0.5 1.0 1.5 2.0 2.5

Substrate concentration, g/L

FIGURE 11.2 Aerobic and anaerobic growth yields of Streptococcus faecalis in glucose-limited media. The slope
of the biomass growth versus substrate concentration line is the biomass growth yield, based on Eqn (11.11) . Data
source: B. Atkinson and F. Mavituna, Biochemical Engineering and Biotechnology Handbook, Macmillan Inc.:
New York, 1983.