Oxygen - good or evil?

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neuron555

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I've been homebrewing now for about a year and a half, and have been working towards more challenging recipes, so the information in these threads has been fantastic (especially liked the thread on late additions of ME). One thing has me puzzled (hah - only one?): there are alot of recommendations to aerate the wort before adding the yeast to get it started up well, but then I keep seeing cautions to not get oxygen into the fermenter later.

Why is oxygen good at one point, but bad later?
 
Fermentation consists of multiple stages. During the first stage, yeast are multiplying rapidly, and this is an aerobic process - meaning it uses oxygen. With lots of oxygen in there during this stage, the yeast can grow rapidly, outcompeting any other organisms that may be in there, and also ensuring a large, healthy yeast population which will carry through the rest of the fermentation smoothly. The fermentation process itself is anaerobic (does not require oxygen) so once the yeast has finished growing, you don't want oxygen in there as the yeast won't be using it up like they were initially, leaving it to oxidize the beer. This is especially true once the vigorous fermentation process has subsided and the yeast are mostly dormant.
 
I'll add that for very high gravity brews, multiple aerations can be useful. Aerating a second or third time during the fermentation will allow the cells that are still functioning to reproduce. Once the desired final gravity is reached, oxygen is 'evil'.
 
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