The acetic acid pathway used by Acetobacter, Gluconobacter and other acetic acid bacteria is an example of what is referred to as an incomplete oxidation.Whereas in most oxidative pathways (e.g., the Krebs or citric acid cycle), organic substrates are ordinarily oxidized all the way to CO2 and H2O, in the vinegar fermentation, acetic acid bacteria usually oxidize the substrate, ethanol, only to acetic acid. However, as described below, exceptions exist where complete oxidation to CO2 can occur.
The actual pathway consists of just two main steps (Figure 11-2). First, ethanol is oxidized to acetaldehyde and then the acetalde-hyde is oxidized to acetic acid. An intermediate step, in which acetaldehyde hydrate is formed, may also occur. Oxygen is required as the terminal electron acceptor (see below). It should also be noted that the conversion of ethanol to acetic acid occurs on an equimo-lar basis, such that after the reactions are complete, the final concentration of acetic acid will be equal to that of the ethanol in the starting material (assuming negligible loss from evaporation). Only a minor amount of the carbon from ethanol is used for biomass or converted to other products.
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