Biochemical Reactions - A First Look
71
formula weight of
P\
is calculated by
M ^
= -yy- ■
30 = 26.7 g (C-mole)'1
This does not contradict the strong indication from the redox balance that
P\
is ethanol but the loss in wt
% ought to decrease with increasing
D
just as it has been argued that the loss in C-mole % should
decrease with increasing
D
as was also confirmed in our analysis.__________________________________
Example 3.6. Citric acid produced by
Aspergillus niger
Citric acid CfeHgCb is produced by fermentation of glucose with
Aspergillus niger
in an aerated tank at
pH between 1.8 and 2. The biomass composition is CH|.gOo,iN0,2. The nitrogen source is NH
4
NO
3
and
practically no CO
2
is formed when the production runs properly.
Typically 68 g citric acid is produced per 100 g glucose. The stoichiometry is
-CH 20 -r„ O t -Y„NH,NO, +y„X * ïv CH„,0ll6
=0
(
1
)
^ = ■ ^ „ • 0 . 2
(2)
The two remaining yield coefficients can be calculated in terms of the known value of
Yv.
v
68/32
.
Ysp ~
|QQy-jQ = "-&375 C-mole citric acid (C-mole glucose)
(3)
A carbon balance gives:
-1 + 0.6375 + TB = 0, and consequently T„ = 0.3625. From the nitrogen
balance one immediately finds T„ = 0.03625. A degree of reduction balance gives:
-4.1- (-4) .
Ym -
0.03625 - K + 0.3625 . 4.20 + 0.6375 .3 = 0
(4)
where Kn = 2 ■
(-3) + 4 . 1+3 (-2) = -8. Thus
=
i
H
- V
, ) * °-06875
mole 0 2
C - mole glucose
(5)
If the experimental value of
Y„
(which is quite easily found) does not match the calculated
Y„
the set of
experimental data is inconsistent. Thus, in an experiment conducted at a higher pH = 4-5 one obtains a
lower citric acid yield 0.5375 C-mole (C-mole glucose)1
but the same biomass yield
= 0.3625. The
oxygen demand is higher than expected:
= 0.11875 mole 0 2 (C-mole glucose)1. Clearly a new
metabolic product is found besides citric acid, and from the carbon balance we find:
Ym
= 1 -0 .3 6 2 5 -0 .5 3 7 5 = 0.1
(6)
and we can use the degree of reduction balance in an attempt to identify the compound: