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Old 10-30-2009, 06:26 PM   #1
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Default Why don't we agitate during fermentation?

So when making a yeast starter, a stir plate is standard equipment. Why then wouldn't we use a stir bar to agitate the wort during fermentation? at least, during the colony growth phase?
I understand the desire to have solids settle out of suspension toward the end of fermentation, but wouldn't agitation provide active yeast with maximum exposure to fermentables?

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Old 10-30-2009, 06:30 PM   #2
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It would also provide your beer maximum exposure to O2. If you aerate your wort before pitching, there should be plenty of O2 for reproduction and after that point you want to minimize exposure anyway.

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Old 10-30-2009, 06:33 PM   #3
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This is like the third person today who has asked this....weird.

Besides keeping the yeast in suspension, you use a stirbar to, in a sense, whip oxygen into the stater, because oxygen is important to good yeast growth, or reproduction. But once the yeast has reproduced, oxygen is actually bad for beer. That's why you aerate wort intitially (and maybe again before the 12th hour for extremely high gravity beers- search for those discussions) but unless you like the taste of wet cardboard, you don't want to whip any more oxygen into the beer.

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Old 10-30-2009, 06:33 PM   #4
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Oxidized beer is not yummy, that's the scientific explanation.

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Old 10-30-2009, 06:35 PM   #5
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But a fermenter is (usually) a closed system with a one-way airlock, so the quantity of oxygen in the system is finite. As the yeast utilize the dissolved oxygen in growth, doesn't more O2 dissolve into the solution in equilibrium with the O2 partial pressure?

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Old 10-30-2009, 06:38 PM   #6
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Further, with a given headspace volume, the initial volume of o2 is quantifiable (roughly). Also, couldn't you CO2 purge the fermenter to eliminate the gaseous oxygen anyway?

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Old 10-30-2009, 06:43 PM   #7
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Quote:
Originally Posted by Bonysplicer View Post
But a fermenter is (usually) a closed system with a one-way airlock, so the quantity of oxygen in the system is finite. As the yeast utilize the dissolved oxygen in growth, doesn't more O2 dissolve into the solution in equilibrium with the O2 partial pressure?
Quote:
Originally Posted by Bonysplicer View Post
Further, with a given headspace volume, the initial volume of o2 is quantifiable (roughly). Also, couldn't you CO2 purge the fermenter to eliminate the gaseous oxygen anyway?
If you don't like our explanation, and have your own theory then why don't you try it and prove us wrong? Me personally, I don't want to risk 5 gallons of beer to know if I am wrong or not.

My fermentations work fine without needing to do it. I pitch enough healthy yeast, and aerate my wort with an o2 stone, so I have never had any issues with fermentation. The yeasties seem to swim around eating sugar and peeing alcohol and farting co2 good enough on their own without needing an amusement park ride to do so.
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Old 10-30-2009, 06:47 PM   #8
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Quote:
Originally Posted by Revvy View Post
The yeasties seem to swim around eating sugar and peeing alcohol and farting co2 good enough on their own without needing an amusement park ride to do so.

Classic!
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Old 10-30-2009, 06:47 PM   #9
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Quote:
Originally Posted by Bonysplicer View Post
But a fermenter is (usually) a closed system with a one-way airlock, so the quantity of oxygen in the system is finite. As the yeast utilize the dissolved oxygen in growth, doesn't more O2 dissolve into the solution in equilibrium with the O2 partial pressure?
yes it is and the O2 does get displaced


Why would you need to agitate an active fermentation ? The yeast do a damn good job of churning the wort as they work . Agitation is used to promote growth with O2 like a starter or gentle swirls to get the yeast back up into suspension if its stalled. There is no reason to agitate after fermentation begins

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. The yeasties seem to swim around eating sugar and peeing alcohol and farting co2 good enough on their own without needing an amusement park ride to do so.
I like Revvy's explanation better
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Last edited by springer; 10-30-2009 at 06:51 PM.
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Old 10-30-2009, 06:50 PM   #10
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Ah! It's not a matter of disliking your explanation! I certainly did not mean to offend. I was just looking for a bit more of a scientific explanation than "Oxidized beer is not yummy" I was merely conjecturing and offering my thoughts on the situation

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