Butyric acid fermentation is characteristic of several obligate anaerobic bacteria that mainly belong to the genus Clostridium; by means of glycolysis, these are able to oxidize sugar, and occasionally amylose and pectin, to pyruvate. Pyruvate is in turn oxidized to acetylCoA by the pyruvate-ferredoxin oxidoreductase enzyme system, with the production of CO2 and H2. Part of the acetylCoA is converted into acetic acid, with ATP production. The other part generates acetoacetylCoA, which is reduced to butyrylCoA through the production of ^-oxybutyrylCoA and crotonylCoA. The transformation of butyrylCoA into butyrate leads to further ATP production. Thus, this fermentation process produces a relatively high yield of energy, with 3 mol of ATP for each mole of glucose. Small amounts of ethanol and isopropanol can also be produced (Figure 7). Butyric fermentation is quite common in silage when the pH is not low enough to ensure the exclusive activity of lactic acid bacteria. The carbon dioxide produced during butyric fermentation also causes an increase in the pH of the silage, thus enhancing further butyric fermentation. Some bacteria, such as Clostridium acetobutylicum, produce less acids and more neutral products, thus carrying out acetone butanol fermentation. This fermentation had great importance during World War I due to the need for acetone for the production of munitions.
The main fermentation processes are summarized in Table 3 in terms of energy yield.
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