72
Chapter 3
-4-(-4) • 0.11875 -(-8) ■
0.03625 + 0.3625 . 4.20 + 0.5375 . 3+0.1
= 0
(7)
=> KPi= 1.0
which must be oxalic acid (COOH)2 (or COOH on a C-mole basis).
The presence of even small amounts of the toxic oxalic acid in the citric acid will prohibit its use as a
preservative in e.g. marmalades. Consequently, the producers make sure that pH never increases above 4
where the following side reaction occurs in parallel with the normal synthesis of citric acid in the TCA
cycle (Fig. 2.5).
Pyruvate
+
C 02
oxaloacetate
—2—»
acetate+oxalate
(8)
The enzymes in this pathway are: (1) pyruvate carboxylase and (2) oxaloacetate hydrolase. Pedersen
et
al.
(2000) solved the problem in an elegant way: An oxalic acid non-producing strain of
A. niger
was
constructed by deletion of the gene encoding for oxaloacetate hydrolase. The citric acid yield was the
same as with the wild type strain, and the deletion strain had the same maximum specific growth rate P =
0.20 h'1
as the wild type strain.______________ ___________________________________ _____________
Example 3.7. Growth with external electron acceptors other than oxygen
In the previous examples the electron acceptor was either oxygen or an intermediate in catabolism. There
are, however, a number of external electron acceptors that can replace oxygen in some microorganisms
(one group is the so-called obligate anaerobes). Examples of other electron acceptors are
1.
Carbon dioxide, which is reduced to methane - see Example 4.5
2.
Sulphate, which is reduced to sulfide
3.
Nitrate, which is reduced to nitrite or molecular nitrogen
The concept of a degree of reduction balance may also be used to treat this type of growth. Consider the
anaerobic growth of a microorganism with the composition CH,.gOo.jN
0.2
on a minimal medium
containing glucose and ammonia. The electrons are donated to nitrate, which is reduced to molecular
nitrogen. The overall stoichiometry is
Y ^ C H ^ N ^ + a N ,
+ (1 -
Ya )C 0 2
+
Ysy/H 20 - C H 20 - 0.2Ya N H 3 -la H N O ,
= 0
(1)
The degree of reduction balance gives
Ysxk x
- 6 a - 4 + 16a = 0
<=>
^
4 - r „ « r ,
(2)
10
If the yield of biomass from glucose is taken to be 0.5 C-moles of biomass per C-mole of glucose then a
= 0.19 and 0.38 moles of nitrate is used per C-mole of glucose metabolized. This process is important for
removal of nitrate in biological waste-water treatment.___________________________________________
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