<Li> If propionic acid, butyric acid, and longer monocarboxylic acids are produced (see mixed acid fermentation), the amount of acidity produced per glucose consumed will decrease, as with ethanol, allowing faster growth . </Li> <P> Hydrogen gas is produced in many types of fermentation (mixed acid fermentation, butyric acid fermentation, caproate fermentation, butanol fermentation, glyoxylate fermentation), as a way to regenerate NAD from NADH . Electrons are transferred to ferredoxin, which in turn is oxidized by hydrogenase, producing H. Hydrogen gas is a substrate for methanogens and sulfate reducers, which keep the concentration of hydrogen low and favor the production of such an energy - rich compound, but hydrogen gas at a fairly high concentration can nevertheless be formed, as in flatus . </P> <P> As an example of mixed acid fermentation, bacteria such as Clostridium pasteurianum ferment glucose producing butyrate, acetate, carbon dioxide and hydrogen gas: The reaction leading to acetate is: </P> <Dl> <Dd> C H O + 4 H O → 2 CH COO + 2 HCO + 4 H + 4 H </Dd> </Dl>

Two types of fermentation and where they take place