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Poindexter 10-30-2007 04:05 AM

dumb noob yeast v- oxygen question
 
I am poking around looking for Orfy's 2l starter, I am sure there will be a bunch of good incidental info in there when I find it.

Please tocheck an ass/u/me of mine.

It seems to me if my wort is highly aerated (from straining out the hops) I want to pitch yeast in the growth phase (pre bubbles),

see: http://www.beer-brewing.com/apex/bre...east_cycle.htm

so that my yeast pitch can get all the oxygen out of the wort first, and then start fermentation.



OTOH, if there is zero or very little oxygen in my wort, then I think I want to pitch yeast in krausen or actively fermenting yeast.

Good ass/u/me or bad ass/u/me?

thanks

mrkristofo 10-30-2007 04:15 AM

Your yeast will get all of the oxygen out of the wort first regardless. Anaerobic conditions are necessary for the mechanism of fermentation to begin. It's a "back-up" if you will, that we exploit the hell out of.

The idea with oxygenated wort, is that the yeast is actively reproducing until the oxygen is used up. At that point, when it can't reproduce anymore, it starts fermenting. There's a few biochemical mechanisms behind this, but we'll leave those out.

As brewers, one reason we oxygenate after we pitch to help maximize our yeast numbers. Gives us a quicker fermentation, less off-flavors, less chance of a stuck fermentation, etc.

So, when to pitch the yeast from the starter? Right at high kraeusen.

Is that what you were looking for?

Poindexter 10-30-2007 04:31 AM

Quote:

Originally Posted by mrkristofo

Is that what you were looking for?

that is exactly what I am not understanding. If the yeast is in ferment/krausen and then they get pitched into an environement with oxygen they are going to stop fermenting and go back to reproducing until the O2 is used up and then go back to fermenting.

I think.

But if my yeast is still in oxygen consuming reproduction when I pitch, then they don't have to stop and change direction, they just keep going, and once all the oxygen in the wort is used up, they start fermenting.

I think.

So it seems like I would get a quicker ferment of my highly oxygenated wort (I strain my hops routinely) if my yeast pitch hasn't entered krausen yet.

I think.

See?

Yooper 10-30-2007 04:58 PM

Well, just straining your hops isn't going to give you "highly oxygenated wort" anyway. When I did extracts, I topped off with water with an aerator and strained my wort to try to add all the oxygen I could. Still, room air is about 21% oxygen, so you're not going to have it "highly oxygenated" unless you add o2 with a diffuser stone. You will never have more oxygen in your wort than there is in the air, so no need to worry one way or another.

The reason you want to aerate your wort is for good yeast health. Reproduction phase or not, if you stress the yeast you'll be more likely to have off-flavors. You don't have to over think this- it really is pretty straightforward!

Donasay 10-30-2007 06:56 PM

The highest concentration of O2 you can get in your wort varies depending on the gravity of the wort, but by shaking or using an aquarium pump, the best you can hope for is about 8ppm, if you get an O2 tank and an aeration stone, you can get that up as hight as 20 to 30 ppm with just a minute or two of pure O2. That is highly oxygenated wort, anything you get by shaking or using an aquarium pump is just regularly oxygenated wort.


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