Stir plate effectiveness concern-maybe too small of a stir bar?

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crlova2

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When I make starters lately they are usually 1.5 liters minimum up to 3.2 liters. My concern is gas exchange (which I understand is a huge key in growing healthy yeasts during a starter) is not occurring as well in my larger starters. I used to get a visible whirlpool/vortex thing on the top level of the wort in the smaller starters on my stir plate which I would assume pulls air in. However, now that I have larger starters and the same stir bar I don't get that visible vortex anymore. Does this mean air isn't being pulled in and thus rendering my stirplate useless? Would a larger stir bar remedy this issue?
 
You don't need a swirling tornado of a vortex to have effective gas exchange, just the smallest 1/2" dimple in the surface of the wort is perfect.
 
You don't need a swirling tornado of a vortex to have effective gas exchange, just the smallest 1/2" dimple in the surface of the wort is perfect.

The problem is that I don't get a dimple either the top of the wort in my erylenmeyer flask is flat...
 
A bigger bar helps. I once purchased on ebay for about $20 three each of 9 or so sizes of stir bar. Only about 3 of the sizes are useful for starters but $20 for 9 bars I can use was a steal.
 
I've seen the stir bars with the little ridge in the middle, they're supposed to spin a bit easier, might get more going for you. Really all you need is a convection current where wort at the top of the flask gets rotated to the bottom (and vice versa) and the yeast stays in suspension. As long as you've got the stir plate going and there's no airlock, you'll get plenty of aeration.
 
I would recommend getting a stir bar with a square profile, the square profile creates more turbulence and a lower rpm...the size will help somewhat, but the profile will make the biggest change. I believe Brewershardware.com carries these.
 
SD-SLIM said:
I would recommend getting a stir bar with a square profile, the square profile creates more turbulence and a lower rpm...the size will help somewhat, but the profile will make the biggest change. I believe Brewershardware.com carries these.

Brewers has them so I replaced my round one with a square one and the sq on seems to work much better! The sq one also cured a noise issue I was having with the round one clanking around in my flask.
 
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.
 
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.
 
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.
 
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.
 
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.
 
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.

The vortex pulls air in

Not if you believe in conservation of mass it doesn't.
 
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?
 
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.
 
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.


I agree with most of what you are saying. However, I think you are forgetting or ignoring that the O2 is used in synthesis of sterols. Using the O2 in synthesis would reduce the amount of molecular O2 in solution, in turn creating an unsaturated (in respect to O2 ) wort thus allowing more to be absorbed. I don't claim any of this fact but I am just inferring it from the logic of what I know thus far about yeast growth and sterol synthesis. I agree that once CO2 is starting to be produced in an appreciable amount that no O2 is probably being taken up but at that point the yeast have for the most part past their growth phase which is what the O2 is needed for anyways. So once the beer starts fermenting hard (producing a lot of CO2) I don't know that the O2 is really all that necessary anymore.
 
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