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An opened bottle of wine developed into a sour taste and smell of vinegar. Why?

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The ethanol (alcohol content) of the wine, is oxidised, by the oxygen in the air into ethanoic acid. Vinegar is dilute acetic acid which is where it gets its smell.

The reaction happens in to stages,

  1. The ethanol is partially oxidised to ethanal
  2. The ethanal is further oxidised to ethanoic acid
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    $\begingroup$ Ethanol will not just get oxidized by air, not commonly. It's typically digested by bacteria in aerobic fermentation, either by contamination or unfinished fermentation. You can make red wine vinegar at homme by just leaving wine in a wide bowl or pan and adding some live vinegar. $\endgroup$ – Stagg C. Apr 26 '15 at 8:05
  • $\begingroup$ Or oxidation involves first other compounds present in the wine. Saying that ethanol is oxidised by atmospheric oxygen, though practically true, is a rather poor answer in chemistry. Ethanol let to air evaporates without being oxidised. OP should rather refer to the comment by Stagg C. and this one of mine. I don't know the details, but the formation of some peroxides should happen first, than these oxidize the alcohol. $\endgroup$ – Alchimista Nov 29 '19 at 10:12

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