Hot answers tagged oxygenation
You've already understood the general practice. Air (really oxygen) is needed in the wort prior to pitching so the yeast can grow and replicate. After pitching, a sealed environment is desired. I think the reason its OK to aerate with air (vs, say, pure O2) is that the yeast, if healthy, will usually overwhelm anything else trying grow in the fresh wort. ...
Any way you take a sample (unless it's from a pressurized vessel with an outlet) will draw air in. As you suspect, it should be a small amount, and given that your beer A) may still be fermenting (which CO2 will help strip any introduced oxygen out of the beer) and B) definitely still has yeast in it (which will scavenge oxygen, as long as it's still alive), ...
The yeast need the oxygen to grow and reproduce, which is important for the first stage of primary fermentation when the yeast is multiplying and inhabiting your wort, which you want to happen as quickly as possible to avoid risk of infection when the wort is cool, exposed to air and does not yet have a protective yeast head. Unless you're brewing a high ...
Once fermentation has started it is usually not recommended to add oxygen. Exceptions: When brewing a high alcohol beer you may add oxygen up to 12 hours after pitching, but not afterwords. Yeast will consume oxygen during the initial fermentation phase. After that the oxygen stays around to stale your beer.
My experience as as professional brewer and home brewer leads me to suggest that oxygen is not detrimental to beer flavor unless yeast activity is in decline (i.e., after vigorous fermentation has subsided). So if you have forgotten to oxygenate, go ahead and do so as long as your beer has yet to achieve high krausen.
The factors determining oxygenation are yeast health vs taste of the beer. Ever tasted a starter culture, or a bottle that was only bottled a couple days ago? They will be full of acetaldehyde, the cidery/green apple flavor of oxidized beer. It is correct that the yeast needs some oxygen for replication and general healthy-ness, but at the end of the ...
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