Fermentation - Food and Industrial Bioproducts and Bioprocessing - page 184

carbonyl branch. The methyl branch involves the reduction of carbon dioxide by six electrons
and the consumption of one ATP to form a methyl group (CH 3 ), which is bound to
tetrahydrofolate (THF) (Henstra et al ., 2007). The methyl group is transferred from THF to
a cobalt ion that is part of a corrinoid-iron-sulfur protein, which is derived from vitamin
B12, by methyltransferase (Ragsdale, 2004). The carbonyl branch involves the reduction of
carbon dioxide to carbon monoxide with two electrons by carbon monoxide dehydrogenase.
Finally, the enzyme acetyl-CoA synthase joins the methyl group, carbon monoxide and
coenzyme-A to form acetyl-CoA (Ragsdale, 2004).
In the absence of oxygen, microorganisms convert pyruvate and/or acetyl-CoA to a
number of different organic acids or alcohols, which are then excreted from the cell as a waste
product (Figures 7.1 and 7.3). The most common products formed are acetone, acetic acid,
n-butanol, butyric acid, 2,3 butanediol, ethanol, isopropanol, lactic acid, propionic acid and
succinic acid (Shuler and Kargi, 2002). All of these compounds have industrial importance
as bulk chemicals, precursors for other products, or fuels. The determination of which of
these compounds is formed is dependent on a number of factors, including the species of
organism, the redox balance, media pH, nutrient composition and temperature of the reactor.
Under anaerobic conditions, cells divert pyruvate to fermentative pathways, as shown in
Figure 7.1, in order to oxidize NADH produced in glycolysis. The most common products
produced in these pathways are lactate, ethanol or acetyl-CoA. Acetyl-CoA can then be
further utilized to form other products, which are described later (Figure 7.3). Lactate is
formed by a reduction of pyruvate with NADH. Ethanol is formed by first reducing pyruvate
with NADH to form acetylaldehyde and carbon dioxide, then reducing acetylaldehyde with
NADH to form ethanol.
In heterotrophic anaerobic bacteria, pyruvate is combined with Coenzyme-A to produce
acetyl-CoA and one carbon dioxide (Figure 7.1). In autotrophic anaerobic bacteria, acetyl-
CoA is formed by the Wood-Ljungdahl pathway as previously described (Figure 7.2).
A number of industrially important chemicals can be formed from acetyl-CoA by various
bacteria (Figure 7.3). Firstly, acetyl-CoA can be (1) converted to acetic acid through a
phosphorylation step with phosphate followed by dephosphorylation with ADP resulting in
one ATP and one acetic acid, (2) reduced to acetylaldehyde followed by further reduction to
ethanol using two NADH, or (3) combined with another acetyl-CoA to form acetoacetyl-
CoA or (4) used by the cell to produce cell material. Acetoacetyl-CoA can then be converted
to acetone with the emission of one carbon dioxide, or it can be reduced by two NADH to
form butyryl-CoA. Some bacteria can reduce acetone with two NADH to form isopropanol.
Butyryl-CoA can then be converted either to butyric acid in a manner similar to the
conversion of acetyl-CoA to acetic acid, or it can be reduced by two NADH to form n-butanol
(hereafter referred to as butanol). Butanol and ethanol can also be produced by cells reducing
butyric acid and acetic acid to their respective aldehydes followed by reduction of the
aldehydes to alcohols.
7.3 MICROBIAL GROWTH
Microorganisms generally follow a common pattern of growth in batch cultures, as shown
in Figure 7.4. As cells become acclimated to the media and other environmental conditions
in the reactor, a lag phase occurs. During this time, cells are producing enzymes that they
will need to survive in the reactor. Lag time can be reduced or eliminated by selecting an
inoculum that has been actively growing in an environment similar to that in the reactor.