I know this must be a dumb question: How does the stirring action of a stirplate keep the wort oxygenated? It seems to me that the oxygen in the air initially within the head space of the beaker would rapidly be exhausted and then for the stirring to do any further good would require the inflow of outside air in under the cover to provide fresh oxygen, and if so wouldn't the incoming air carry with it contaminants?
Dumb question about stir plate oxygenation
- Thread starter gkeusch
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flars
Well-Known Member
The air in your house is fairly free of contaminants that affect wort unless you have mold growth on the floors or walls and you mill grains in the same area as your starter is working. The starter wort will finish close to 3.5% ABV will will further protect from contamination as long as your procedures were sanitary.
Thank the gas laws for why it works: as the yeast consume oxygen, more oxygen will be pulled into the flask to maintain its partial pressure.
The stirring is simply to continuously expose the yeast to the head space.
Contaminants are a whole other problem, but, fwiw, I stick open cell foam plugs in my e-flasks which at least filter dust...
Cheers!
Plus, I put a rubber banded paper towel over my flasks. I had a starter get infected from gnats once.
Gentlemans_Ale
Well-Known Member
Never mind
Some interviews I have heard with Chris White he says that stir plate starters are under-oxygenated unless you are adding it. It's better than a still starter, but not significantly. The main benefit is keeping the yeast in suspension.
No idea how accurate that is, but it comes from yeast man himself. He still seemed to be a proponent of the stir plate starter, but more concerned with oxygenation of the wort that the starter is going to be pitched in to than the starter itself.
No idea how accurate that is, but it comes from yeast man himself. He still seemed to be a proponent of the stir plate starter, but more concerned with oxygenation of the wort that the starter is going to be pitched in to than the starter itself.
Use a foam stopper or similar. Gasses will go through, but microbial ne'er-do-wells won't. DON'T USE AN AIRLOCK!
As oxygen is consumed inside the flask, it will be replaced due to the nature of gasses (partial pressure law).
http://www.northernbrewer.com/foam-stopper-46-50mm
Asking/discussing out of ignorance here, not arguing:
My understanding is that the oxygen is consumed very early during cell division then the yeast turns to all its attention to fermenting sugars, which is basically anaerobic. At this point the fermentation process is putting off a ton of CO2. I understand that oxygen "wants" to get back in the head space as the O2 is depleted, but how much actually gets back in against the pressure of the CO2 coming off the sugars? With the stir plate, way more than in a still environment I'm sure, but I'm pretty ignorant of the practical applications of gas laws on something like this. Also, at this point do we want more O2 in the starter? I wouldn't think there would be much of a point once cell density is reached. You would definitely want to start out with a high amount of dissolved oxygen in the starter, but once the yeast starts on the sugar, does additional make any difference?
There's plenty of atmospherical pressure to ensure the Gas Laws apply. Doesn't matter that evolving CO2 is rushing out.
And the stirring function, again, is to continually expose the full yeast population to the surface.
Yeast do propagate in cycles - iirc, in a good environment (all the needed elements for replication) they double every six hours until they decide their population is right to take on the task "at hand". Then they go to work, and when they're done they nap.
I've read Yeast (a few times) and it's clear that White believes stir-plating provides significant performance over a static method - many calculators have his math built in. He does mention for extreme propagation rates supplemental O2 is necessary, but for more pedestrian needs the stir plate clearly is a win...
Cheers!
And the stirring function, again, is to continually expose the full yeast population to the surface.
Yeast do propagate in cycles - iirc, in a good environment (all the needed elements for replication) they double every six hours until they decide their population is right to take on the task "at hand". Then they go to work, and when they're done they nap.
I've read Yeast (a few times) and it's clear that White believes stir-plating provides significant performance over a static method - many calculators have his math built in. He does mention for extreme propagation rates supplemental O2 is necessary, but for more pedestrian needs the stir plate clearly is a win...
Cheers!
I just re-listened to the interview to make sure I understood correctly.
Here is the link to the Basic Brewing podcast in which Chris White was the keynote at a New Zealand Homebrewers conference and most of his presentation was on oxygenation. May 12th, 2016 episode.
He is a big proponent of the stir plate starter. His point on starters was that they do not use much oxygen, but rather keep the yeast in suspension and that they are very good for creating new cells and increasing yeast population. Because they do this with low oxygen, they are good for pitching straight into an oxygenated wort, but not good for going into storage. The "low" 02 starter doesn't leave the yeast with energy reserves to withstand storage very well. This part started around the 42 minute mark of the link above.
I'm not saying it's the bible or anything, just sharing since I happened to listen to 4 podcasts with him recently. I just got my copy of "yeast" a couple days ago and I'm not too far into it. Interesting that in all the podcasts I listened to with him this week he gives basically the same spiel that has also been what I read in the first couple chapters...hoping I find some new intel in these pages I paid for.
Here is the link to the Basic Brewing podcast in which Chris White was the keynote at a New Zealand Homebrewers conference and most of his presentation was on oxygenation. May 12th, 2016 episode.
He is a big proponent of the stir plate starter. His point on starters was that they do not use much oxygen, but rather keep the yeast in suspension and that they are very good for creating new cells and increasing yeast population. Because they do this with low oxygen, they are good for pitching straight into an oxygenated wort, but not good for going into storage. The "low" 02 starter doesn't leave the yeast with energy reserves to withstand storage very well. This part started around the 42 minute mark of the link above.
I'm not saying it's the bible or anything, just sharing since I happened to listen to 4 podcasts with him recently. I just got my copy of "yeast" a couple days ago and I'm not too far into it. Interesting that in all the podcasts I listened to with him this week he gives basically the same spiel that has also been what I read in the first couple chapters...hoping I find some new intel in these pages I paid for.
This article by Woodland Brews seems to come to the conclusion that stir plates accelerate the propagation of yeast cells but that unstirred but occasionally shaken starters end up with virtually the same number of yeast cells. If you are in a hurry, use a stir plate. If you have time to wait (it isn't real long) you can save the money and buy ingredients instead of the stir plate. If you don't need to use a stir plate, a cheap mason jar works the same as an expensive Erlenmeyer flask.
http://www.woodlandbrew.com/2015/02/yeast-starters-stirred-vs-not.html
http://www.woodlandbrew.com/2015/02/yeast-starters-stirred-vs-not.html
