After you open a bottle of wine, the clock is ticking and you usually have around 2 days to drink it before it starts to taste like rotten tomatoes. So, [oxidation] is bad for wine. But, I imagine during the [winemaking] process, when wine is transferred from vats to barrels, and stored in the barrels, there must be a lot of oxygen contact? Especially in the barrel, I've been to wineries where they take samples out of the barrel...so how is it that the wine is not oxidizing a ton during [barrel-aging]?
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In a sealed container, any microbes will rapidly use up the available oxygen, creating a sort of surface barrier of concentrated CO2 in the headspace, at least during fermentation. While a barrel is "sealed", very low levels of oxyegenation do indeed occur through the porous walls of the barrel and are part of the maturing process. This is sometimes called oxidative-aging, in contrast to the reductive aging that takes place in a more tightly sealed glass bottle.
An open vessel would absolutely be subject to oxidation as you describe. However, many wine producers will also carefully control oxidation, adding anti-oxidants like sulfur oxide, performing closed transfers, CO2 flushing of lines, the use micro-oxidation techniques, etc. Cooler fermentation temperatures also slow this process. The phenolic compounds and tannins, some produced by the microbes, some from the oak itself, also mellow the effects.1. In fact, much of the character of an aged wine comes from this expected oxidation of tannins and other molecules into more complex flavor and aroma compounds. They key is to limit and control that rate of oxidation.
Another piece of the "spoilage" you describe comes from acetification--that is, turning to vinegar. Additional bacteria that come into contact with the wine will specifically oxidize the alcohol in the wine into acetic acid or vinegar. I would assume this is the greater part of the spoilage you are describing. Non-acetic oxidation tends to leave flavors dulled and cardboardy, flavors likely overtaken after a couple of days worth of acidification by acetic microbes.
1 Echave J, Barral M, Fraga-Corral M, Prieto MA, Simal-Gandara J. Bottle Aging and Storage of Wines: A Review. Molecules. 2021 Jan 29;26(3):713. http://doi.org//10.3390/molecules26030713. PMID: 33573099; PMCID: PMC7866556.