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To the best of my understanding of making beer, wort must first be properly aerated to provide oxygen for the yeast to multiply when first added to the wort. After all the oxygen is consumed, the yeast switch from being aerobic to anaerobic and begin producing CO2 and alcohol. Having a lot of healthy yeast is important, but the goal is to get them all into an anaerobic state to produce alcohol.

With that in mind, does opening a fermentation bucket and exposing the wort to oxygen cause the yeast to go back into an aerobic state? I'd like to periodically check the gravity of the beer (while being very careful with sanitation) to how much and how quickly my beer is fermenting, but I'm afraid to risk hurting the yeast with early exposure to oxygen.

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Opening the fermentation bucket will not add sufficient oxygen to affect the work of the yeast. When the liquid is sitting still, very little oxygen will move into solution. (FWIW, I push my bottling wand through the hole for the airlock and withdraw my sample that way)

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I wouldn't recommend removing a brew bucket lid at all once fermentation is underway.

Always use the small ports for samples, racking, even with dry hopping. Though it's not as easy as just tossing in a hop bag.

There is a great experiment that shows just how fast CO2 mixes with air once given the chance. By floating a balloon of air in a sealed container of CO2, it sinks very quickly once the container has an opening, even though CO2 is heavier than air.

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I always leave the lid of the fermentor on loose, covering the bin so that no bacteria, mold etc will fall in but allowing the air to circulate for the first 24 hours until a nice kraussen has formed then seal it up.

I used to seal it up tight on day 0 for the fear of the air getting in, but after reading Yeast by Chris White I changed my tune entirely and have not looked back. I get far better attenuation now, far faster brews and have not had one go bad since I started doing this about 2 years back... famous last words ;)

In the first couple of days the yeast is undergoing rapid replication, and the little bit of circulation, reduces the CO2 pressure in the liquid and can help provide the oxygen require by the yeasts to produce sterols in their cell walls.

The higher the gravity of your starting wort the more critical extra oxygen is in these early stages. Sometimes brewers will bubble O2 or sterile air through the brew after the first 4--5 days in very high gravity worts, to ensure the yeast has enough to replicate to the required extend and full attenuate the batch.

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