Noob question about aeration

[humpadilo]

This might be a noob question but from what I understand, it would be beneficial to the yeast to aerate it throught the fermentation. For instance make a large stir plate and turn it on for a minute or two every few days. Am I correct in making this assumption or is it better to not aggitate the yeast after it is doing it's job?

 

[gcdowd]

You want to aerate the wort before pitching the yeast. Once fermentation starts, oxygen is not something you want to introduce (some exceptions). But generally, aerate, pitch yeast, let the beer ferment.

Edit: you are not talking about a starter are you?

 

[MonsterBrew]

Like gcdowd said, it's really not something you would want to do after fermentation begins, rather in preparation for pitching. This can be aerating a yeast starter by using a stir plate etc, or maybe as simple as aerating the cooled wort as you move it from a kettle to the fermenting vessel (check out the venturi approach).

 

[humpadilo]

Edit: you are not talking about a starter are you?

No, I am asking about the Primary Fermenter.

That is what I thought, but what is the danger of introducing O2 to the fermenter? Is it just infections, or something else?

 

[feinbera]

In general, free oxygen will bind to various other compounds in your beer and create off-flavors -- not sure I've ever had it happen to my beers, but it's commonly described as wet cardboard, or less frequently, sherry, rotting garbage, and many other things that your beer is not supposed to taste like. This is called oxidation.

Very early in fermentation, in the "lag phase" (before you start getting visible krausen/airlock activity) the yeast will quickly absorb any free oxygen -- they need it in order to reproduce up to the full number of cells you want fermenting your beer. This has the obvious benefit of giving you enough yeast for a good fermentation, but it has the happy side-effect of pulling all that oxygen out of the wort before it has a chance to oxidize anything else.

After the yeast are done reproducing and are just gobbling up sugars or have finished up the fermentation and dropped out of suspension, they don't need the oxygen, so, it stays in the beer and oxidizes it.

 

[gcdowd]

Once beer is fermented, oxygen will stale the beer fairly quickly

 

[Spintab]

Yeast ferment in two different ways. Aerobic and anaerobic. With and without o2. To build yeast populations and vitality we want aerobic. To make beer we want anaerobic fermentation. Simple as that.