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Home Brew Forums > Home Brewing Beer > Fermentation & Yeast > Stir plate effectiveness concern-maybe too small of a stir bar?

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Old 03-11-2011, 02:05 PM   #11
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Does anyone see an issues with using one like this?

http://www.coleparmer.com/catalog/product_view.asp?sku=0476518

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Old 03-11-2011, 06:17 PM   #12
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I use 2L starters and I bought my stir bar from e-bay for around $5.00 each. It's 50mm with that ring in the middle of it. Good vortex, but noisy. A good noisy though. It lets me know it is still spinning.

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Old 03-11-2011, 07:49 PM   #13
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Someone explain to me how you get gas exchange in a starter after fermentation begins. It sounds like utter nonsense to me.

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Old 03-11-2011, 07:55 PM   #14
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Unless your starter is airtight (and it shouldn't be) then gas exchange is going to happen. CO2 will leak out and air will leak in trying to equalize the gasses inside of your starter container.

That and the yeast being forced to stay in suspension are what make a stirplate great for growing massive amounts of yeast.

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Old 03-11-2011, 08:10 PM   #15
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Unless your starter is airtight (and it shouldn't be) then gas exchange is going to happen. CO2 will leak out and air will leak in trying to equalize the gasses inside of your starter container.

That and the yeast being forced to stay in suspension are what make a stirplate great for growing massive amounts of yeast.
I do believe that forcing the yeast to stay in suspension works, I do not believe that appreciable amounts of oxygen can diffuse into the headspace over the starter while CO2 is billowing out of it.
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Old 03-15-2011, 02:01 PM   #16
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Regarding the air exchange...you would know by the amount of yeast you grow. There is a substantial difference in yeast growth with the addition of O2. But, if your aluminum foil so loosely attached to the neck of the flask, you will indeed get that exchange.

However, if you want to turbo charge the yeast growth - place your flask on a stir plate and feed air via an aquarium pump and sterile filter. BIG DIFFERENCE. In fact, make sure you add a few drops of anti-foam or you risk the yeast foaming over, even with a strong vortex! And, if you step up the starter for a second time (I use a 4L Erlenmeyer) the ferment will be over in 18 hours or less (double your anti-foam or keep the paper towels handy.)

A friend turned me on to this and geez was he right.

If you need a stir bar, check out www.stirbars.com

I bought a 3 inch one and if your stir plate can handle it, you can push anything.

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Old 03-15-2011, 04:54 PM   #17
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Originally Posted by Pivovar_Koucky View Post
Someone explain to me how you get gas exchange in a starter after fermentation begins. It sounds like utter nonsense to me.
If there is a vortex then it would be "utter nonsense" that there wouldn't be gas exchange. The vortex pulls air in. If you have a stir plate try creating the vortex and then put your hand over the flask making it air tight. There will be a slight suction and then the vortex with dissipate because it can not longer pull any air in. The yeast don't immediately start blowing off CO2. While they are in their growth phase they need O2 but aren't putting of much if any CO2 so you are getting O2 pulled in by the vortex.
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Old 03-15-2011, 05:27 PM   #18
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Quote:
Regarding the air exchange...you would know by the amount of yeast you grow. There is a substantial difference in yeast growth with the addition of O2. But, if your aluminum foil so loosely attached to the neck of the flask, you will indeed get that exchange.
I think that there are more factors at work in the growth of yeast than dissolved O2 content. As I said earlier, I do believe that the stir plate can increase the amount of yeast you grow by stirring up yeast, increasing the concentration of sugars near the yeast, etc. But I don't believe that O2 can diffuse against the convection caused by CO2 production during fermentation.

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The vortex pulls air in
Not if you believe in conservation of mass it doesn't.
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Old 03-15-2011, 05:33 PM   #19
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Originally Posted by Pivovar_Koucky View Post
I think that there are more factors at work in the growth of yeast than dissolved O2 content. As I said earlier, I do believe that the stir plate can increase the amount of yeast you grow by stirring up yeast, increasing the concentration of sugars near the yeast, etc. But I don't believe that O2 can diffuse against the convection caused by CO2 production during fermentation.



Not if you believe in conservation of mass it doesn't.
Can you elaborate on how the conservation of mass proves that no suction is created by a vortex in an open Erlenmeyer flask?
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Old 03-15-2011, 05:49 PM   #20
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Can you elaborate on how the conservation of mass proves that no suction is created by a vortex in an open Erlenmeyer flask?
Yes indeed. So let's assume your yeast starter is completely lacking in dissolved gas prior to turning on the stir plate. When you turn on the plate the gas will dissolve until it becomes saturated, at which point no more gas will enter the solution. So unless you have some way of increasing the pressure in the flask, the total amount of gas in the flask becomes consant after the solution becomes saturated.

Next, the yeast start producing a lot of CO2 from fermentation, probably several hundred times the headspace volume of your flask. This will force out the lighter gases (O2, N2, H2O, etc.) and push CO2 out into the environment. At this stage you would certainly have to agree that no net gas is entering the flask. The only way that O2 could enter the solution under these circumstances is by diffusing against the flow of CO2 out of the flask and becoming a significant portion of the gas immediately above the solution.

I didn't do the calculation, but it seems unlikely to me that this would happen. If anyone wants to crush Navier-Stokes and prove me wrong I'm all ears.
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