question about applying "Pasteur effect"

[rhythmiccycle]

I was just reading about yeast, and I read that when oxygen is present the yeast will reproduce, and when oxygen is not present alcohol will form.

First of all its that true?

Second, after first pitching the yeast, its it a good idea to allow oxygen into the tank so the yeast can grow, then later seal the tank to allow fermentation to begin?

 

[Airborneguy]

Yes, it's true.

You should oxygenate your wort prior to pitching, then seal your fermentation vessel to allow fermentation to create and anerobic environment for the yeast to operate in.

 

[Hermit]

Chris White, of White Labs, suggests placing sanitized aluminum foil over the air lock until the krausen falls. I just started using bottled O2 so I've been airlocking from the beginning since the head space is filled has some of that O2 trapped inside.

 

[rhythmiccycle]

Thanks, I have two follow questions.

1, is stirring the wort vigorously enough to form bubbles sufficient to "oxygenate"?

2. Is it ever a good idea to open the fermentation tank to allow oxygen in ?

 

[Airborneguy]

1. Probably not unless you stir VERY vigorously. I pour my wort back and forth between my sanitized fermentor and my brew pot about 4-5 times until there is a thick layer of foam on top. Most would say that my method isn't even sufficient, but I've read that most oxygenating stones don't actually achieve much higher level of dissolved oxygen.

2. If you pitch more yeast to deal with a stuck ferment, you might want to re-introduce some oxygen, but I've never done this. I avoid introducing any oxygen after fermentation has begun. The presence of oxygen at that point can induce the yeast to produce by-products that you don't want in your beer.