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Old 10-27-2007, 02:49 AM   #1
Bearcat Brewmeister
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Feb 2006
Cincinnati, OH
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I've read a bunch about comparisons done with starters where they either just sat, were swirled on occasion, or used a stir plate. The advantage of a stir plate and swirling is, from what I read, keeping the yeast in suspension and mixing in more oxygen. I have also seen the many posts by people stating that they use a loose foil cover so that oxygen can get in. This is where I have a question - how does the oxygen get in? CO2 is heavier than O2 and I would think it would keep the O2 away from the wort. People that have open fermenters or that knock an airlock off for a moment say that that the CO2 being heavier than air is what keeps a protective layer over the wort. Also, the continuous production of CO2 by the starter is constantly displacing gas nearest the opening - out flow with no room for in flow.

Has anyone done any or read about any studies on the amount of oxygen that is flowing into starters by swirling or using a stir plate? I am starting to wonder if the oxygenation we are giving starters is all coming from the initial blast we give it at the beginning (by injection or shaking) and that the benefit gained from stiring/mixing during fermentation of the starter is strictly from the yeast staying in solution and not being allowed to flocculate.
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Old 10-27-2007, 01:46 PM   #2
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Jan 2007
Calgary, Alberta
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I think if you were to let it sit without swirling, the CO2 in the flask might inhibit the O2 from getting into the fermenting beer in the starter. But the swirling probably does enough mixing that some CO2 escapes and some O2 gets drawn in. There will always be some diffusion of O2 into the vessel filled with only CO2 following the partial pressure gradient, but the swirling of the gas should facilitate this process. Further, the swirling of the starter helps knock CO2 out of solution in the beer, which means that any O2 that is present in the vessel can be dissolved into the beer more easily and taken up by O2.

I don't think the action of stirring alone benefits the yeast/starter much -- the swirling would definitely prevent flocculation, but I think this effect would be small. The gas exchange is the more important benefit, and I guess this is why yeast manufacturers recommend foil to cover your flask instead of an airlock (i.e., the airlock is a one-way cap that only lets CO2 out, and keeps O2 from entering).

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