What is the actual purpose of a stir plate?

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stratslinger

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Well, I'm kind of embarassed to be asking this, considering I've been using one for over a year now, but I've recently read (and been given) a few pieces of conflicting advice regarding stir plates, and I'd like to get a real understanding here. I've heard at least three different purposes for stir plates, with a few counterpoints, and a couple cases where those purposes appear to run counter to one another. I'll outline those here:

1) The stirplate is meant to constantly aerate the starter, drawing in more oxygen to keep the yeast in the aerobic stage of their lifecycle. On first blush, this is the purpose I had assumed was the actual purpose of a stirplate. However, on reflection, isn't the yeast, even in this phase, still constantly giving off CO2? If the starter is off-gassing CO2, creating a sort of positive pressure, is it even going to be possible for it to then draw in any "fresh" O2? So I'm beginning to question this as a purpose.

2) The stirplate is meant to "drive off" CO2. I've read this one several times recently, and it just seems silly to me. We've all looked at the airlock on our fermenter once fermentation is under way, and seen them bubbling away furiously, so it's pretty clear that the CO2 drives itself off on its own. I'm pretty sure this one can be dismissed out of hand. And in this case, the driving off CO2 seems to run directly counter to drawing air into or aerating the starter...

3) The stirplate is meant only to keep the yeast from floculating in the starter, so they'll stay active and reproducing. This one makes a little more sense to me, but then if this is the case it doesn't seem to jive with the general advice to not put an airlock on a starter, but to instead use a foam stopper or simply a piece of sanitized foil...

My main reason for trying to understand this is to try to figure out if I'm running my stirplate too fast, or not fast enough, or, hopefully, just right. I'm hoping for some answers backed up with some sources I can do some follow-up reading on my own.
 
1. Oxygen can be drawn into beer if the CO2 layer is disturbed. Hence the idea of not opening up the fermenter more than necessary.

2. Dissolved CO2 can be pushed out of solution via agitation. Hence why degassing with a whip works for wine. The less dissolved CO2 the less stress on the yeast according to the book Yeast.

3. Agitation does keep the yeast from flocculating and this does help them get at sugars they might otherwise leave behind. Really only a perk if you are using a highly flocculant strain.

In answer to the speed, I believe getting a vortex is key. You want to spin it fast enough to do the gas transfer most efficiently but not so fast as to cause you to throw the bar. Hope this helps, it pretty much all came from the book Yeast.
 
Well, I'm kind of embarassed to be asking this, considering I've been using one for over a year now, but I've recently read (and been given) a few pieces of conflicting advice regarding stir plates, and I'd like to get a real understanding here. I've heard at least three different purposes for stir plates, with a few counterpoints, and a couple cases where those purposes appear to run counter to one another. I'll outline those here:

1) The stirplate is meant to constantly aerate the starter, drawing in more oxygen to keep the yeast in the aerobic stage of their lifecycle. On first blush, this is the purpose I had assumed was the actual purpose of a stirplate. However, on reflection, isn't the yeast, even in this phase, still constantly giving off CO2? If the starter is off-gassing CO2, creating a sort of positive pressure, is it even going to be possible for it to then draw in any "fresh" O2? So I'm beginning to question this as a purpose.

Well it's sort of doing both at the same time, so in reality you're pulling a mixture of O2 and CO2 into the starter. But if you weren't using the stirplate there would be NO O2 coming in.

2) The stirplate is meant to "drive off" CO2. I've read this one several times recently, and it just seems silly to me. We've all looked at the airlock on our fermenter once fermentation is under way, and seen them bubbling away furiously, so it's pretty clear that the CO2 drives itself off on its own. I'm pretty sure this one can be dismissed out of hand. And in this case, the driving off CO2 seems to run directly counter to drawing air into or aerating the starter...

You're correct that normal fermentation drives yeast (EDIT correction: drives CO2 off) off due to the amount produced, but there is still CO2 that stays in solution during fermentation. The stirplate is meant to drive all of the CO2 out as CO2 can inhibit yeast growth. This isn't really an issue during fermentation as it's assumed you're pitching enough healthy yeast to begin with, whereas with a starter your goal is to grow as much healthy yeast as you can.

3) The stirplate is meant only to keep the yeast from floculating in the starter, so they'll stay active and reproducing. This one makes a little more sense to me, but then if this is the case it doesn't seem to jive with the general advice to not put an airlock on a starter, but to instead use a foam stopper or simply a piece of sanitized foil...


Sorry, I'm not really following. Can you explain why the starter or foil might be linked to flocculation?

You mentioned sources, so I can go and find some good ones to include here when I have time and get home tonight. Jamil's "Yeast" book is a great start, though.
 
Im pretty sure you hit it with all three of your points. It gets the wort moving which keeps the yeast in suspension. The motion/swirling of the wort also creates friction at the liquid/air interface which if strong enough to create a turbulent (not laminar) flow will cause a mixture of the available gases (CO2/oxygen/others in atmosphere). The off gassing of CO2 doesnt seem to be advantageous at all, more of a byproduct.

Edit: Thanks to the posts above I stand corrected in RE: C02 off-gassing
 
I've always been told it was to promote the exchange of gasses, ie. Co2 out, O2 in, as well as keeping the yeast in suspension, all accomplished by agitating the liquid.
 
tre9er said:
I've always been told it was to promote the exchange of gasses, ie. Co2 out, O2 in, as well as keeping the yeast in suspension, all accomplished by agitating the liquid.

I agree with this as well but I'll add in regards to speed, you want just enough to create a good vortex but not so much that you over produce heat as that is possible if the vortex is too strong.
 
When you make a starter it's the same as making your beer so you want 02 at first because you just boiled it all out and the yeast need it at first. Once they start producing alcohol and C02 the C02 blanket will keep you from getting any more 02 which is a good thing. At this point the stir plate is just keeping the yeast in suspension.
 
Yeah, how about that. The heavy CO2 supposedly shoots into space with enough force to somehow put the lighter air on the bottom. It’s a level three reverse stratification spell. Same way Harry Potter flies. Magic.

Then again there’s the persistent myth of the CO2 ‘blanket.’ In this scenario, the CO2 makes an impermeable layer that resists other gases pretty much forever.

The truth is somewhere in-between. The CO2 will form a stratification layer, even as the other gases are restoring their equilibrium in solution. It’s a race. The CO2 floating off the lighter gases happens pretty quickly. The gases in air seek to re-establish their partial pressures in the solution, but that is a slow process.

So, basically when the CO2 is outgassing at a fierce rate, the oxygen (and nitrogen and stuff that boiled off) is essentially zero. As the fermentation slows, the equilibrium shifts, until ultimately atmospheric partial pressures will be restored. Even with an airlock.
 
1. Oxygen can be drawn into beer if the CO2 layer is disturbed. Hence the idea of not opening up the fermenter more than necessary.

2. Dissolved CO2 can be pushed out of solution via agitation. Hence why degassing with a whip works for wine. The less dissolved CO2 the less stress on the yeast according to the book Yeast.

3. Agitation does keep the yeast from flocculating and this does help them get at sugars they might otherwise leave behind. Really only a perk if you are using a highly flocculant strain.

In answer to the speed, I believe getting a vortex is key. You want to spin it fast enough to do the gas transfer most efficiently but not so fast as to cause you to throw the bar. Hope this helps, it pretty much all came from the book Yeast.

kaconga nailed it. Or, at least, my understanding of it. But I don't really believe a stir plate will noticeably aerate a starter while in use, or that there's a huge gas exchange at the opening of the flask with CO2 out and O2 in. It just doesn't seem logical if you really try to think it through. If shaking a fermenter doesn't adequately aerate wort to ideal conditions, I have a hard time believing a tiny vortex will really be sucking *that* much O2 in. Especially since the yeast is pushing out CO2 into the wort/beer at the same time.

But degassing the wort (Le Chatelier's principle at work) and keeping the yeast in suspension are the two reasons I believe they work to produce more cells than a simple starter.

Ideally, in my eyes, if you want to aerate your starter, do it with an O2 stone at pitching.
 
There is a difference in growth between having an airlock and simply having a foil cover. I recently did an experiment where I had 2 stirred yeast cultures next to each other. Same yeast, same wort, same innoculation rate. Difference was only foil cover vs. airlock.

The foil covered starter achieved a growth rate of 1.7 Billion cells per gram of wort extract and the airlock covered one only got 1.0 Billion cells per gram of wort extract. I haven't published this data yet b/c it was just one data point and a simple experiment I did.
So something is going on here and I think it is increased O2 uptake by the yeast w/o the airlock. The O2 uptake can happen when the fermenation is not in "full swing" and the CO2 blanket has not been formed yet. Or the O2 diffuses into the CO2.

The major benefit of agitated starters, stir plate or shaker, is that all yeast cells have equal access to the nutrients. That has been pointed out already. It has little to do with flocculation. Even pour flocculators don't grow well when not stirred.

Driving off CO2 also helps. The difference between a stirred stater and a fermenting beer is that the fermenting beer gets super saturated with CO2. That means it holds more CO2 than the equilibrium between the head space CO2 pressure allows. That is not the case for agitated starters.

Kai
 
It will help maintain your 8ppm during the early stages of CO2 production, increasing the time spent in the reproductive stage, and yielding more yeast at the expense of ethanol. During active ferm, it won't add any O2. The active migration of the CO2 front will swamp any diffusion of air through a foam stopper or around the edges of foil.

As a side point, I don't see any reason to have the foil fitting loosely. There's WAY more O2 in the headspace than will be utilized by the extended reproductive phase. Similarly, no reason not to use an airlock. I use foil because it's easier.

The starter beer will be saturated with CO2 whether or not you stir it (during active ferm). The beer will be in equilibrium with the gas (100% CO2) at the interface. With wine degassing, the wine is no longer fermenting and you're attempting to drive off the residual CO2 with (ideally) some nearly inert gas.

Stirring certainly helps homogenize the yeast in the medium, and I'm sure this helps.
 
In the yeast book it is suggested to use foil instead of an airlock until the krausen falls. Since I have a fermentation chamber I tried a coffee filter on my last batch. I caught it as the krausen had just started to subside. My IPA was fairly opaque with yeast. Kai, have you tried the foam stoppers to see if they work any better than the foil?

Typically coffee filters are made up of filaments approximately 20 micrometres wide, which allow particles through that are less than approximately 10 to 15 micrometres.
 
There is a difference in growth between having an airlock and simply having a foil cover. I recently did an experiment where I had 2 stirred yeast cultures next to each other. Same yeast, same wort, same innoculation rate. Difference was only foil cover vs. airlock.

The foil covered starter achieved a growth rate of 1.7 Billion cells per gram of wort extract and the airlock covered one only got 1.0 Billion cells per gram of wort extract. I haven't published this data yet b/c it was just one data point and a simple experiment I did.
So something is going on here and I think it is increased O2 uptake by the yeast w/o the airlock. The O2 uptake can happen when the fermenation is not in "full swing" and the CO2 blanket has not been formed yet. Or the O2 diffuses into the CO2.

The major benefit of agitated starters, stir plate or shaker, is that all yeast cells have equal access to the nutrients. That has been pointed out already. It has little to do with flocculation. Even pour flocculators don't grow well when not stirred.

Driving off CO2 also helps. The difference between a stirred stater and a fermenting beer is that the fermenting beer gets super saturated with CO2. That means it holds more CO2 than the equilibrium between the head space CO2 pressure allows. That is not the case for agitated starters.

Kai

Great points Kai. I imagine that the composition of the air in the headspace changes over time given the consumption of oxygen and the production of CO2. This would in theory make your dissolved oxygen decrease over time. I haven't run any experiments but I think it makes sense.
 
There is a TV show called "Who wants to be a Millionaire" The contestant has the opportunity to ask the audience for help to a question. I'm always amazed that people will press the button with an incorrect answer. I always figured that if I was in the audience and didn't know the answer 100% I wouldn't press any button.
 
Per the yeast book, make the wort, oxygenate, add the yeast, then put on stirplate- plate allows good gas exchange, keeps yeast in suspension, drives off co2. Loose cover faclitates gas exchange.

From my recollection, oxygen is only needed early in yeast propagation, to build strong yeast walls. After that, it is anaerobic.

If you're that curious, the yeast book is a great resource. Read it 3x, still absorbing. Brewers make wort, yeast makes beer.
 
Thanks folks - lots of good info here. And I think my club has a copy of Yeast floating around. Might be time I borrowed it.
 
Similarly, no reason not to use an airlock. I use foil because it's easier.

I think our posts crossed each other. I mentioned an experiment were I saw 70% more growth simply from having a foil over and not an airlock. That experiment was done with 250 ml in a 500 ml flask. So there was lots of headspace in both cases.
 
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