Biochemical Reactions - A First Look
65
3 and
Y3Q
-
0.1 mole oxygen per C-mole glucose we find:
ysi = 0.333 C-mole biomass per C-mole glucose
7sp = 0.367 C-mole ethanol per C-mole glucose
For RQ = 1 and
-
1 one obtains T1X
=
= 0, which corresponds to complete combustion of glucose
to carbon dioxide according to:
-CH,0 - O, + CO, + H ,0 = 0
(9)
At these conditions neither biomass nor ethanol is produced.
Example 3.3 Anaerobic growth of yeast with ammonia as nitrogen source and ethanol as the only
product
The stoichiometry of the overall reaction for anaerobic growth of
S. cerevisiae
with ammonia as the sole
nitrogen source was given in Eq. (3.23). The degree of reduction balance yields
-4 + 4.20 ^ + 6 ^ = 0
(1)
for a standard biomass with K* = 4.20. Typically for yeast fermentation
Y„
= 0.12 g (g glucose)'1
or 0.15
C mole biomass (C mole glucose)'1. Consequently the maximum expectable yield of ethanol on glucose
is 0.56 C mole (C mole glucose)'1
even if no glycerol is produced as was the case in Example 3.1. The
bacterium
Zymomonas mobilis
also produces ethanol, and the biomass yield is lower than for yeast
fermentation. Typically T„ is 0.05 g (g glucose)"1
or 0.06 C mole (C mole glucose)'1. With the lower
biomass yield a higher ethanol yield of Tsp = 0.624 C mole ethanol (C mole glucose)"1
or 94% of the
theoretical yield is obtained. The bacterium is consequently a more efficient producer of ethanol than
yeast - but yeast
(S. cerevisiae)
has other advantages, e.g. its extreme tolerance for ethanol.____________
Example 3.4. Biomass production from natural gas
The bacterium
Methyiococcus capsulatus
grows aerobically with methane or methanol as the sole carbon
and energy source. The catabolic pathways are different from those shown in Chapter 2 - in fact few
organism are able to grow on C-l carbon compounds, see Goldberg and Rokem (1991). The resulting
biomass is an excellent protein source, which can be used directly as feed for domestic animals and as
feed for fish, e.g. salmon. The protein content of the biomass can amount to 70% of the dry weight
(DW). In hydrolyzed form the protein has considerable potential for use in human diets, as a daily
supplement in school meals in poor countries or in aid packages sent to catastrophe stricken countries.
The nucleic acids are broken down much faster than the proteins in a short, high temperature treatment of
the spray-dried biomass, and consequently long term negative effects caused by the nucleic acids are
avoided when the protein is used for human consumption.
Near Trondheim in Norway a 10,000 tons per year plant for production of biomass, so called single cell
protein (SCP) from natural gas has been operating since 1999. The natural gas is a by-product from oil
exploration in the North Sea. It is converted on land to methanol (nearly 900,000 tons per year), but a
small fraction of the natural gas purified to 99% CH4 is used as substrate in the SCP factory. Methanol
could also be used as substrate for SCP production as was the case in an ICI plant (Billingham, GB),
which operated for a short period in the 1970’s, but was closed down partly as a result of sky rocketing
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