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Brewed a 1.100 SG stout, fermentation has been steady but slowed noticeably about 24-36 hours in. Wondering if aerating again 2.5 days in could help, or is that too late?

5.25 gallon batch, 3 rehydrated packs of US 05 pitched initially after 30 minutes of aeration via aquarium pump and stone.

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    What's the gravity at the moment? More oxygen could help, but you want to make sure that it will all get consumed by the yeast. Personally, I'd be leery of adding any O2 if you've passed the 50% attenuation mark. – Frazbro Oct 19 '18 at 2:21
  • Good info, thx. I’ll check it in the morning to see but assuming it will be past the 1/2way point by then – ts_watson Oct 19 '18 at 2:24
  • Additionally, what temperature is it at? If you're worried, you could also try rousing the yeast a bit, just by gently shaking the fermenter. – Frazbro Oct 19 '18 at 2:27
  • Pitched at 67 F, currently at 72 F – ts_watson Oct 19 '18 at 2:30
  • (22.2C) That shouldn't be an issue then. Check the gravity tomorrow, and if it's past 50%, just let it ride (with maybe a little rouse). If you add oxygen, don't overdo it. – Frazbro Oct 19 '18 at 2:33
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Late oxygenation is never a good idea. Firstly it should not be necessary, and if your fermentation stalls that is indicative of a problem that aeration won't solve. Secondly, high levels of oxygen and low levels of fermentable sugars may push your yeast into switching from fermentation to respiration, which produces acetic acid (vinegar). Thirdly, oxydation reactions will almost certainly cause off-flavours which can't be corrected.

I would suggest a more regular approach to try and restart a stuck fermentation, such as the one outlined here. Post-pitching oxygenation should be considered only as a desperate last-ditch alternative to throwing the batch out (i.e. if throwing it away is your only other option you may as well go for broke and try it).

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  • Thank you Frank. I’m glad I asked! I definitely will not aerate again on this one. I’ve heard about people doing it after 18 hours just to boost the process but maybe I misunderstood the reasoning – ts_watson Oct 19 '18 at 11:28
  • The key is whether your lag time has ended or not when you re-oxygenate the wort. If your lag time hasn't ended after 18 hours (a sign that the yeast is in trouble) you can try oxygenating; if that gets your yeast going that means that lack of oxygen was indeed the problem, and this oxygen will then be absorbed by the yeast before the start of the high growth phase. If that doesn't help, you pitch fresh yeast which then should absorb the oxygen. However, once the lag time has ended and the high growth phase has set in (as in your case) oxygenation should always be avoided. – Frank van Wensveen Oct 21 '18 at 12:15

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