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Old 01-31-2008, 11:26 PM   #1
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Default Yeast starter/stir plate theory confusion

Ok, stupid question because I am sure the answer will come in the first post....

I have been reading about yeast starters and the use of stir plates. The stir plate is used to keep the wort aerated to enhance the growth and multiplication of the yeast cells. Heres my problem:

1)CO2 is a byproduct of yeast metabolism, in both aerobic and anaerobic metabolism
2)CO2 is heavier than air
3) Stirring the yeast in a narrow necked Erlenmeyer flask would just be stirring it under a blanket of CO2... Where is the aeration?

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Old 02-01-2008, 12:38 AM   #2
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CO2 is heavier than air, but that doesn't mean much if it's not sitting still. Obviously since there's plenty of CO2 in the air we breathe, it CAN mix with air, it just doesn't happen instantaneously. This is why open fermentation can be okay, but becomes a problem once primary fermentation subsides - after CO2 stops being generated faster than it diffuses from the 'blanket' into the air, it's no longer a blanket any longer. My guess would be that the agitation due to the movement of the liquid surface would be enough to speed this process.

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Old 02-01-2008, 12:43 AM   #3
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I think that the stir plates are usually used in conjunction with breathable stoppers. The vortexing pulls air into the flask and into solution, actively displacing the CO2.

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Old 02-01-2008, 01:27 AM   #4
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Right, I get what you both are saying, and I understand the whole, "well it works doesn't it?" concept. I am just having trouble visualizing the fact that a CO2 blanket, even over a vortexing mass of liquid (which in itself will be vortexing due to the friction between liquid and gas) will up itself out of a narrow necked Erlenmeyer flask. I could see it in a wide mouthed jar, but the narrow aperture of the flask would act to retain the gas inside the flask, minimizing mixture with outside air.

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Old 02-01-2008, 02:18 AM   #5
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In a starter, I don't think the yeast will go through more oxygen than will have been dissolved in the water beforehand. I could be completely wrong.

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Old 02-01-2008, 02:41 AM   #6
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I don't exactly have any real scientific evidence to back me up, but clearly your intuitive 'feel' for the predicted behavior of the gases is different from mine - can't really say who's wrong or right without knowing more I guess. My thinking is that CO2 is not THAT different in density from air, so I doubt that it will take very much of a disturbance to make it mix rapidly enough that a moderate amount of air remains in the flask while stirring.

Perhaps a logical test would be to take a thin wooden strip (like a coffee stirrer), light it on fire, and plunge it down into the headspace in the flask while the starter is being stirred and is actively fermenting away, and see if it goes out instantly like it's in pure CO2, or if it remains burning (even for just a few seconds)

It would be interesting to find out for sure, because lately I have been more lazy and haven't been using my O2 stone to oxygenate my starters, just relying on the stir plate to do the job. However, I suppose that during the period before fermentation picks up, it'd have enough time to get air in there anyway...

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Old 02-01-2008, 02:48 AM   #7
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Wouldn't regular osmosis take care of that? I mean, like, the concentration of O2 in the flask would be going down, but it'd still be a mix...

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Old 02-01-2008, 06:04 AM   #8
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The stir plate is not used to keep the starter wort aerated. It is used to keep the yeast in suspension, and therefore in better contact with the fermentables, resulting in a faster and more complete fermentation.
You do not want your stir plate to spin so rapidly that it aerates the wort, any more than you would want your beer to be aerated once fermentation has begun. Just keep the yeasties in suspension so they can do their job quickly and completely.

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Old 02-01-2008, 07:04 AM   #9
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Old 02-01-2008, 12:09 PM   #10
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Ok, that makes MUCH more sense than what I was reading....... I kept seeing "keep it aerated because yeast only multiply under aerobic conditions, so use a stir plate" It just didn't make sense...

Grant, there would be some diffusion of gasses (not osmosis, that requires a semi-permeable membrane, but I got what you were saying) but that depends on the thickness of the blanket, mixing ratio/ concentration gradient etc. etc.

So if the stir plate is only to keep yeast in suspension, there is no reason not to use an airlock over a breathable cork/loose aluminum foil, as you will only be relying on the O2 already dissolved in the wort.

Hummm, still seems like I am missing some blatant concept or something....
I should stop thinking and start drinking

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