Degassing CO2 during early fermentation is important for wine and mead. But why not for beer?

I'm getting back into mead making and see plenty of references to degassing the must during early fermentation, or not using an airlock. But little mention of this in beer brewing. Is CO2 not toxic to beer yeast?

  • 1
    Similar question at homebrew.stackexchange.com/questions/2114/…
    – tallie
    Aug 8, 2012 at 2:12
  • Thanks, I've read that answer but it doesn't completely answer the question. It says that because of the lower amounts of sugar, the yeast can ferment this even with the co2 present. Meads and wines start around 1.100 - so if I brew a big barleywine of similar OG would that need degassing during fermentation also?
    – mdma
    Aug 9, 2012 at 20:54

2 Answers 2


In a recent BeerSmith podcast Michael Fairbrother from Moonlight Meadery talks about degassing mead during fermentation (at about 17:45 in the YouTube video), and alludes that unlike beer, mead and wine can handle additional oxygen after fermentation begins.

As we know, depending on the stage of fermentation, oxygen in beer can be detrimental. So I guess it's a trade-off between stressing the yeast with CO2 and having oxidised beer.

Edit: I also suspect that as most beers have relatively lower alcohol levels to wine and mead, there are less overall yeast toxins present in the fermenting beer, so degassing the CO2 isn't as critical to yeast health. Obviously this doesn't apply to the stronger beers, so the oxidation trade-off would kick in there.


I think Tallie has it right. It's likely a trade-off between stressing the yeast with CO2 and having oxidized beer. I would add that degassing or adding O2 after fermentation has started also introduces a mostly unnecessary step, creating more work for the brewer and increasing the risk of contamination.

Depending on how early in fermentation we're talking about, O2 can actually be added to beer. From Yeast, by Chris White and Jamil Zainasheff, p 83:

If it is a very high-gravity wort, more than 1.092 (22 ºP), you must aerate with pure oxygen, as air will not pvodie a high enough level of dissolved oxygen. Unfortunately, that still might not be enough for beers higher than 1.083 (20 ºP) For high-gravity beers, adding a second dose of oxygen between 12 and 18 hours can help fermentation speed and attenuation. (O'Conner-Cox and Ingledew, 1990)

Here, we're not talking specifically about degassing, but it should be noted that oxygen is sometimes added to beer after fermentation has started.

  • Good point about the oxygenation and the stage of fermentation - I've edited my answer accordingly.
    – tallie
    Sep 12, 2012 at 4:28

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