Biomedical Engineering Reference
In-Depth Information
11.11. EFFECT OF REDOX POTENTIAL
The redox potential is an important parameter that affects the rate and extent of many
oxidative
e
reductive reactions. In a fermentation medium, the redox potential is a complex
function of DO, pH, and other ion concentrations, such as reducing and oxidizing agents.
The electrochemical potential of a fermentation medium can be expressed by the following
equation:
RT
4F
RT
F
RT
4F
RT
F
H
þ
¼E
0
þ
E
h
¼ E
0
þ
ln p
O
2
þ
ln
½
ln p
O
2
pH ln10
(11.49)
where F is the Farady constant, the electrochemical potential is measured in millivolts by
a pH/voltmeter, and
p
O
2
is in atmospheres.
The redox potential of a fermentation media can be reduced by passing nitrogen gas or by
the addition of reducing agents such as cysteine, HCl, or Na
2
S. Oxygen gas can be passed or
some oxidizing agents can be added to the fermentation media to increase the redox
potential.
Dissolved carbon dioxide (DCO
2
) concentration may have a profound effect on perfor-
mance of organisms. Very high DCO
2
concentrations may be toxic to some cells. On the
other hand, cells require a certain DCO
2
level for proper metabolic functions. The DCO
2
concentration can be controlled by changing the CO
2
content of the air supply and the agita-
tion speed.
11.12. EFFECT OF ELECTROLYTES AND SUBSTRATE
CONCENTRATION
The ionic strength of the fermentation media affects the transport of certain nutrients in
and out of cells, the metabolic functions of cells, and the solubility of certain nutrients,
such as DO. The ionic strength is given by the following equation:
N
j
2
X
1
C
j
Z
2
I ¼
(11.50)
j
j ¼
1
where C
j
is the molar concentration of ionic species j, Z
j
is the charge of ionic species j, and I is
the ionic strength of the medium.
High substrate concentrations that are significantly above stoichiometric requirements are
inhibitory to cellular functions. Inhibitory levels of substrates vary depending on the type of
cells and substrate. Glucose may be inhibitory at concentrations above 200 g/L (e.g. ethanol
fermentation by yeast), probably due to a reduction in water activity. Certain salts such as
NaCl may be inhibitory at concentrations above 40 g/L due to high osmotic pressure.
Some refractory compounds, such as phenol, toluene, and methanol, are inhibitory at
much lower concentrations (e.g. 1 g/L). Typical maximum noninhibitory concentrations of
some nutrients are glucose, 100 g/L; ethanol, 50 g/L for yeast, much less for most organisms;
ammonium, 5 g/L; phosphate, 10 g/L; and nitrate, 5 g/L. Substrate inhibition can be over-
come by intermittent addition of the substrate to the medium.
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