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Oxidation in mead varies considerably depending on the mead. Some melomels are quite prone to oxidation. Traditional meads seem to much less prone to oxidation than most beers or wines. I have kept open bottles of mead in the fridge for up to a month and still found them nice and drinkable. I've not yet had a beer or wine I could say that about.
This is not to say that mead cannot be oxidized. Let me assure you that it is possible. When it occurs, it may create a sherry-like aroma with a nutty character. If you like sherry or Madeira wines, it could be desirable. If it becomes severe enough, there will be a flattening of aroma and the wet cardboard smell may develop along with loss of flavor. Oxidation also promotes development of acetic acid and leads to vinegar and nail polish remover odor.
During fermentation, mead is protected from oxidation due to the production of CO2, its natural resistance to oxidation, and the reductive capacity of yeast/lees. After active fermentation is complete is when the big risk of oxidation develops, and mead should be kept in containers that are fully topped up or such that they are otherwise protected from air exposure.
I am a firm believer in the use of sulfites to protect against oxidation as well, especially with my light fruit melomels. I use them in traditional meads as well. Ascorbic acid may be a useful adjunct, though I have rarely used it. Tannins and phenolic elements from such things as dark berries may provide strong protection against oxidation. With melomels, I am currently experimenting with the use of glutathione which also can help protect fruity aromas/flavors from oxidation - I'm not certain if it will extend the lives of these melomels or not. Time will tell.
If a batch does get oxidized, fining with casein or powdered milk may help remove the oxidized character if it is not too severe. Charcoal fining is another option. In meadmaking, these options are almost never needed.
With making mead the risk of oxidation is low during fermentation and the risk of incomplete fermentation is high if the yeast doesn't get enough oxygen to produce necessary sterols and lipids for the cell membrane. This is why we recommend aeration of the must during the early part of fermentation. It sounds like heresy to a brewer, I know, but it is necessity for a mazer.
Lanne pase toujou pi bon
(Past years are always better)