Wine turns sour primarily due to the action of aerobic bacteria, specifically species from the genus Acetobacter (such as Acetobacter aceti). When wine is exposed to air, these bacteria utilize the oxygen to oxidize the ethanol (alcohol) in the wine into acetic acid (ethanoic acid), which is the main component of vinegar and possesses a characteristic sour taste. This process is known as acetification or aerobic fermentation.

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