Why don't we agitate during fermentation?

So when making a yeast starter, a stir plate is standard equipment. Why then wouldn't we use a stir bar to agitate the wort during fermentation? at least, during the colony growth phase?
I understand the desire to have solids settle out of suspension toward the end of fermentation, but wouldn't agitation provide active yeast with maximum exposure to fermentables?

It would also provide your beer maximum exposure to O2. If you aerate your wort before pitching, there should be plenty of O2 for reproduction and after that point you want to minimize exposure anyway.

This is like the third person today who has asked this....weird.

Besides keeping the yeast in suspension, you use a stirbar to, in a sense, whip oxygen into the stater, because oxygen is important to good yeast growth, or reproduction. But once the yeast has reproduced, oxygen is actually bad for beer. That's why you aerate wort intitially (and maybe again before the 12th hour for extremely high gravity beers- search for those discussions) but unless you like the taste of wet cardboard, you don't want to whip any more oxygen into the beer.

Oxidized beer is not yummy, that's the scientific explanation.

But a fermenter is (usually) a closed system with a one-way airlock, so the quantity of oxygen in the system is finite. As the yeast utilize the dissolved oxygen in growth, doesn't more O2 dissolve into the solution in equilibrium with the O2 partial pressure?

Further, with a given headspace volume, the initial volume of o2 is quantifiable (roughly). Also, couldn't you CO2 purge the fermenter to eliminate the gaseous oxygen anyway?

Bonysplicer said:

But a fermenter is (usually) a closed system with a one-way airlock, so the quantity of oxygen in the system is finite. As the yeast utilize the dissolved oxygen in growth, doesn't more O2 dissolve into the solution in equilibrium with the O2 partial pressure?

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Bonysplicer said:

Further, with a given headspace volume, the initial volume of o2 is quantifiable (roughly). Also, couldn't you CO2 purge the fermenter to eliminate the gaseous oxygen anyway?

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If you don't like our explanation, and have your own theory then why don't you try it and prove us wrong? Me personally, I don't want to risk 5 gallons of beer to know if I am wrong or not.

My fermentations work fine without needing to do it. I pitch enough healthy yeast, and aerate my wort with an o2 stone, so I have never had any issues with fermentation. The yeasties seem to swim around eating sugar and peeing alcohol and farting co2 good enough on their own without needing an amusement park ride to do so.

Revvy said:

The yeasties seem to swim around eating sugar and peeing alcohol and farting co2 good enough on their own without needing an amusement park ride to do so.

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Classic!

Bonysplicer said:

But a fermenter is (usually) a closed system with a one-way airlock, so the quantity of oxygen in the system is finite. As the yeast utilize the dissolved oxygen in growth, doesn't more O2 dissolve into the solution in equilibrium with the O2 partial pressure?

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yes it is and the O2 does get displaced


Why would you need to agitate an active fermentation ? The yeast do a damn good job of churning the wort as they work . Agitation is used to promote growth with O2 like a starter or gentle swirls to get the yeast back up into suspension if its stalled. There is no reason to agitate after fermentation begins

Revvy said:

. The yeasties seem to swim around eating sugar and peeing alcohol and farting co2 good enough on their own without needing an amusement park ride to do so.

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I like Revvy's explanation better

Ah! It's not a matter of disliking your explanation! I certainly did not mean to offend. I was just looking for a bit more of a scientific explanation than "Oxidized beer is not yummy" I was merely conjecturing and offering my thoughts on the situation

I know everybody says that a stirbar helps exchange oxygen, but without real evidence (anyone have a dissolved oxygen meter?) I'm not convinced.

IMO all a stir bar does is drive off the co2 that is in solution and keep the yeast in suspension.

Even if the top of a flask is left completely open (rarely the case) how is the oxygen getting in there once it is full of co2?

Revvy said:

...The yeasties seem to swim around eating sugar and peeing alcohol and farting co2 good enough on their own without needing an amusement park ride to do so.

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Great!.... Now I have coffee coming out my nose and down my shirt from laughing so hard when I read this!

Somehow I could just see an animated video on homebrewing with the "yeastie boys" doing their thing in a carboy!

Thanks for the chuckle!

Revvy said:

This is like the third person today who has asked this....weird.

Besides keeping the yeast in suspension, you use a stirbar to, in a sense, whip oxygen into the stater, because oxygen is important to good yeast growth, or reproduction. But once the yeast has reproduced, oxygen is actually bad for beer. That's why you aerate wort intitially (and maybe again before the 12th hour for extremely high gravity beers- search for those discussions) but unless you like the taste of wet cardboard, you don't want to whip any more oxygen into the beer.

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mmm.... wet cardboard

Bonysplicer said:

Ah! It's not a matter of disliking your explanation! I certainly did not mean to offend. I was just looking for a bit more of a scientific explanation than "Oxidized beer is not yummy" I was merely conjecturing and offering my thoughts on the situation

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basically there is no reason to agitate the yeast ... I sometimes will call them little pansy bastards if the FG is a little to high . But I don't physically abuse them I just do it mentally

Bokonon said:

I know everybody says that a stirbar helps exchange oxygen, but without real evidence (anyone have a dissolved oxygen meter?) I'm not convinced.

IMO all a stir bar does is drive off the co2 that is in solution and keep the yeast in suspension.

Even if the top of a flask is left completely open (rarely the case) how is the oxygen getting in there once it is full of co2?

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Then YOU do it...make up a 10 gallon batch of beer....split it into two fermenters, pitch EXACTLY one packet of dry yeast into each batch....Spin one with a stirbar, leave the other alone. Bottle and come back in 6 weeks and see if half the batch tastes like cardboard.

Otherwise, ANYTHING we say is conjecture on our part...except for many of us, are conjecture is based on brewing experience and an understand of the role and risk of 02 in brewing.

springer said:

Why would you need to agitate an active fermentation ?

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Assuming a closed fermentation (no exposure to O2 after fermentation begins), I can think of a couple of reasons:

1. For high OG beers, to avoid having to "rouse" the yeast (e.g., walk the barrel).
   
2. To hasten conditioning. Keeping the yeast in suspension would expose more by-products to a larger quantity of yeast in a shorter time, this reducing the conditioning period. Budweiser does this with beechwood chips (increases yeast exposure surface area).

Some of us plan to experiment with a "stir plate" approach for larger batches (5 - 10 gallon). The theory is sound and the equipment cost to test the idea is negligible.

Both sides present good argument. However, I am guilty of swirling my fermenter in the final days in between gravity readings to ensure that all of the sugar (or whatever desired amount) has fermented out. I think that I haven't had issues with taste because I've done it so far in, that the fermentation is complete or near complete. I've never had to shake the fermenter to get signs of life in the first days of the fermentation because it's going crazy on it's own. It kinda sounds like the "pro shake the fermenter" side is worried about stalled fermentations. You must remember to rely on your hydrometer to check your fermentations and not the air lock/blow off tube. If my last statement was wrong, then disregard. The only time that I think a stir bar would do anything useful is possibly for high gravity batches in short cycles to ensure the yeast doesn't settle.

ImperialLover said:

It kinda sounds like the "pro shake the fermenter" side is worried about stalled fermentations. You must remember to rely on your hydrometer to check your fermentations and not the air lock/blow off tube. If my last statement was wrong, then disregard. The only time that I think a stir bar would do anything useful is possibly for high gravity batches in short cycles to ensure the yeast doesn't settle.

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I tend to agree, and the thing to remember is that most of the so-called stuck fermentation threads, weren't stuck fermentations at all. They were people who used the airlock and not their hydrometer as a gauge of fermentation. Once they actually took a reading the found their beer was perfectly fine.

Or they were underpitching or using a yeast with a tendency to stuck at certain gravities, OR they produced a lot of unfermentables in their allgrain batches...

or they were just impatient,, and wanted their beer to be someplace it simply wasn't at when they wanted it.

and honestly except for the rare actual 1.030 stuck, or the impatient brewer....all the swirling in the world is not going to get a yeast to eat a fermentable that isn't there anymore.

I'd like to see Lamarguy's experiment..but I will only believe it if it is done similar to how I mentioned...you cant do two different beers, and two different yeasts, you have to be using the exact same base wort, from the same brew session (not two seperate brewing of the same recipe), and a carefully measured out equal amount of yeast....that's why I suggested two dry packets...and weighed out to be exactly the same amount. Because even with splitting a half gallon or gallon starter into two equal batches there is no gaurentee that the same numbers of cells got into each batch. And the stirplates should be commercially made and NOT home made ones so there can be do variance in rate of rotation either.

But I am wondering.....in doing it with a 5 gallon batchdoes anyone think the stirbar and stirplate needs to be sacled up proportionately, of will the little 1" baby on the satndard plate be enough.

I actually like a bit of oxidization to my barleywines and I have had to shake a saison like a baby mid-fermentation and it turned out just fine. It's all relative. I would never put a fermenter on a stirplate, but I am with ImperialLover in that sometimes yeast need to roused prior to a diacetyl rest. I know it sounds oxymoronic, but I love the complexity of specialty grains and I love a dry beer. So I find myself getting the last few points dropped out with a gentle (or sometimes not so gentle) swirl at the end of a single fermenter fermentation. No one has ever accused me of serving wet cardboard.

I was just reading the Porter section of Designing Great Beers. Daniels says before gigantic porter vats were built, brewers used to "take the fermenter for a walk" to rouse the yeast. This essentially consisted of rolling the barrels around the yard.

Oxidation aside, I don't see the harm in doing it. If you aren't getting the proper attenuation, you should probably look at your pitching rates, and pre ferment aeration.

Bonysplicer said:

But a fermenter is (usually) a closed system with a one-way airlock, so the quantity of oxygen in the system is finite. As the yeast utilize the dissolved oxygen in growth, doesn't more O2 dissolve into the solution in equilibrium with the O2 partial pressure?

Click to expand...

A fermenter is not a closed system, it out-gases.

Also, O2 CAN come back the other way if the pressure on the outside exceeds the pressure on the inside. This happens most often when you put an airlock on while the wort is still warm.

Revvy said:

If you don't like our explanation, and have your own theory then why don't you try it and prove us wrong? Me personally, I don't want to risk 5 gallons of beer to know if I am wrong or not.

My fermentations work fine without needing to do it. I pitch enough healthy yeast, and aerate my wort with an o2 stone, so I have never had any issues with fermentation. The yeasties seem to swim around eating sugar and peeing alcohol and farting co2 good enough on their own without needing an amusement park ride to do so.

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...and finally I find a quote that is signature worthy!!!

I need to invest in an Airstone for pre-fermentation aeration of the wort. I think once your yeast is pitched( at a high rate of coarse), the only time you have to worry about O2 level in your fermentation vessel is the the lag time phase; the shorter the less chance you have of o2 being able to exist. once the fermentation starts, I think the yeast create enough co2 to shove out what ever o2 exists in the car boy. I have yet to make any serious OG beers, so I dont think I will need to agitate mid fermentation any time soon. plus I like the set it and forget it method- I just come take gravity readings once I noticed the airlock has settled out a little bit, but I always rely on gravity readings.

the question I have related to this subject is- how much does a airstone/pump help fermentation? I assume it would help with lag time , and maybe the yeast replicate faster due to higher levels of O2? once their going their going, but how will 02 help with aeration of wort?

lamarguy said:

Assuming a closed fermentation (no exposure to O2 after fermentation begins), I can think of a couple of reasons:

1. For high OG beers, to avoid having to "rouse" the yeast (e.g., walk the barrel).
   
2. To hasten conditioning. Keeping the yeast in suspension would expose more by-products to a larger quantity of yeast in a shorter time, this reducing the conditioning period. Budweiser does this with beechwood chips (increases yeast exposure surface area).

Some of us plan to experiment with a "stir plate" approach for larger batches (5 - 10 gallon). The theory is sound and the equipment cost to test the idea is negligible.

Click to expand...

wasn't walking the barrel used towards the end of fermentation? Just like when I swirl the carboy a few days after active fermentation is complete just to rouse the yeast a little .

And conditioning is done after the active fermentation is completed.

I asked why do it during an active fermentation . Because the yeast are doing a pretty good job of it on their own.

Blinebuddy said:

the question I have related to this subject is- how much does a airstone/pump help fermentation? I assume it would help with lag time , and maybe the yeast replicate faster due to higher levels of O2? once their going their going, but how will 02 help with aeration of wort?

Click to expand...

If you pitch the proper amount of yeast aeration/oxygenation isn't that important the yeast will go right to work on the sugar. O2 is important during reproduction not fermentation.

Revvy said:

I'd like to see Lamarguy's experiment..but I will only believe it if it is done similar to how I mentioned...

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The original large scale stir bar idea was discussed here.

I plan to use a closed, positive pressure vessel to conduct the fermentation test. I'll likely split a 5.5 gallon batch and ferment at the same temperature.

Revvy said:

But I am wondering.....in doing it with a 5 gallon batchdoes anyone think the stirbar and stirplate needs to be sacled up proportionately, of will the little 1" baby on the satndard plate be enough.

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Yes, the size of the stir bar and centrifugal force is certainly a design concern. The goal is to achieve minimal agitation at the bottom of the fermenter, not a vigorous whirlpool. I believe that can be achieved with a 3" - 4" stir bar and a large 6" computer fan.

Only real world testing will tell. Once I get my repaired sanke fermentation kit back next week, I should have an opportunity to test the idea.

lamarguy said:

The original large scale stir bar idea was discussed here.

I plan to use a closed, positive pressure vessel to conduct the fermentation test. I'll likely split a 5.5 gallon batch and ferment at the same temperature.

Yes, the size of the stir bar and centrifugal force is certainly a design concern. The goal is to achieve minimal agitation at the bottom of the fermenter, not a vigorous whirlpool. I believe that can be achieved with a 3" - 4" stir bar and a large 6" computer fan.

Only real world testing will tell. Once I get my repaired sanke fermentation kit back next week, I should have an opportunity to test the idea.

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I'd like to see it done both on a closed setup like yours, but also done in a typical carboy/airlock setup that most folks have. I have a feeling the results would be quite different. That perhaps in a closed environment, purged of o2 it would work great, and their would be no oxydation worries...but in a "real world" or typical hb'ers situation it would be a setup for disaster.

springer said:

wasn't walking the barrel used towards the end of fermentation? Just like when I swirl the carboy a few days after active fermentation is complete just to rouse the yeast a little.

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I suppose it depends on your definition of active fermentation. I consider the fermentation "active" until terminal gravity is reached, the beer is removed from the yeast, or the yeast are inhibited in some way (cold, sodium metabisulphite, etc.).

springer said:

I asked why do it during an active fermentation. Because the yeast are doing a pretty good job of it on their own.

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Agreed, I just elaborated on the potential benefits for having a stir bar in a fermenter post active fermentation.

I most likely should have worded it as during vigorous fermentation not active

I would like to see your results also.. How is the conversion unit that Derrin sells working with the closed system ? I have yet to use the one I bought.

springer said:

How is the conversion unit that Derrin sells working with the closed system ? I have yet to use the one I bought.

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With the pressure relief valve and MFL fittings, it integrates well into my setup. That is until I realized it wouldn't hold more than ~1 psi after a few minutes. Turns out the unit was damaged during shipping and I didn't realize it until I went to use it (gas out port weld had a hairline crack).

Derrin repaired it quickly and I should have it back early next week. Put it this way, I like it so much I ordered a second unit from him.

I have to say the quality of his products is top notch. If it works out for me I will have to buy a few more . I have two 1/2 barrels and three 1/4 that I want to use as fermenters . I already used the 1/2 a few times but with a stopper and airstop

springer said:

wasn't walking the barrel used towards the end of fermentation? Just like when I swirl the carboy a few days after active fermentation is complete just to rouse the yeast a little .

And conditioning is done after the active fermentation is completed.

I asked why do it during an active fermentation . Because the yeast are doing a pretty good job of it on their own.

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Oh, I thought you were talking about rousing at the end of fermentation. I don't claim to be an expert, but it seems to me if you pitch enough yeast, using a stir plate for a whole batch is probably useless. That is unless you are trying to achieve a very specific goal, like a really dry beer with no adjuncts.

I also know we can't compare everything we do to commercial brewers, but you don't see them mixing up thier fermenting wort. I guess I'm trying to say that I don't fully understand the point of this. Why would I want to constantly stir my fermenting wort when I can get desired attenuation through proper aeration and pitch rates?

Even if the top of a flask is left completely open (rarely the case) how is the oxygen getting in there once it is full of co2?

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Two reasons: 1. air-entrainment due to friction 2. yeast give off very little CO2 during their growth.

Revvy said:

I'd like to see it done both on a closed setup like yours, but also done in a typical carboy/airlock setup that most folks have. I have a feeling the results would be quite different. That perhaps in a closed environment, purged of o2 it would work great, and their would be no oxydation worries...but in a "real world" or typical hb'ers situation it would be a setup for disaster.

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Your statement shows a lack of understanding of physical chemistry. Check out references to Henry's Law. In any fermenter situation, carboy or otherwise, gases exist in equilibrium between two phases, gaseous (headspace) and dissolved gas in solution. the rate of transfer in this equilibrium is a surface area limited function.

For example, when you use an oxygen stone or fish tank bubbler to pump o2 through your wort, the wort picks us a high level of o2 quickly due to surface area to volume ratios of tiny bubbles and the high partial pressure of o2. Similarly, boiled, degased starter wort picks up oxygen because whirlpool agitation increases surface area exposure, but still the wort will only dissolve enough o2 gas to be in equilibrium with the partial pressure of o2 in the headspace. So relative to the gas/liquid interface stirring merely hastens the balance of equilibrium.

BUT in a fermenter yeast use o2 and "fart out" co2 into solution So the partial pressure of dissolved o2 decreases over time and the partial pressure of co2 increases over time. Additionally, the total pressure in the headspace is regulated by the airlock/blow off. So as the co2 builds up in solution it increases the partial pressure of gaseous co2 (equilibrium). But since the total pressure is fixed, gas is driven off through the airlock. That gas is a mixture of co2 and o2. Over time the faction o2 in the headspace limits toward zero through being driven off through airlock and being used by the yeast.

My understanding is that "oxidation flavors" are caused by excess dissolved o2. From this it would seem prudent to drive off excess dissolved oxygen as quickly as possible, prehaps through agitation in an oxygen defficient environment.

Edcculus said:

Why would I want to constantly stir my fermenting wort when I can get desired attenuation through proper aeration and pitch rates?

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The objective is maximize the movement of the yeast throughout the wort. A stir plate is an effective means to achieve constant yeast movement. The secondary objective is to speed the conditioning process.

For example, studies have shown leaving cold break in the fermenter increases the nucleation sites for CO2 during active fermentation, which allows the CO2 to be released more readily from solution. This induces stronger currents (swirling) within the wort, a "natural" method for yeast movement. We're just talking about augmenting that process with mechanical motion.

Bonysplicer said:

Ah! It's not a matter of disliking your explanation! I certainly did not mean to offend. I was just looking for a bit more of a scientific explanation than "Oxidized beer is not yummy" I was merely conjecturing and offering my thoughts on the situation

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Then go talk to a scientist, this board is to give advice, not provide you with "scientific" explanations of everything. I've brewed enough beer to know you don't want to put your carboy on a stir plate.

But please, feel free to do so and report back with the results.

JVD_X said:

A fermenter is not a closed system, it out-gases.

Also, O2 CAN come back the other way if the pressure on the outside exceeds the pressure on the inside. This happens most often when you put an airlock on while the wort is still warm.

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Maybe slightly..but not for long. The wort will start to outgas and form a CO2 blanket over the beer since it is heavier than the O2. I respect the Rev's opinions but sometimes i wish someone would say "I'm not sure, I have not tried that sort of thing. You should try it and educate all of us." I once posted a thread basically saying that, "here is what i did, these are my results" and almost everyone told me that i had to be wrong.

oldschool said:

I respect the Rev's opinions but sometimes i wish someone would say "I'm not sure, I have not tried that sort of thing. You should try it and educate all of us."

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Uh, I think I said that, TWICE in this thread....

Revvy said:

If you don't like our explanation, and have your own theory then why don't you try it and prove us wrong? Me personally, I don't want to risk 5 gallons of beer to know if I am wrong or not.

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Revvy said:

Then YOU do it...make up a 10 gallon batch of beer....split it into two fermenters, pitch EXACTLY one packet of dry yeast into each batch....Spin one with a stirbar, leave the other alone. Bottle and come back in 6 weeks and see if half the batch tastes like cardboard.

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Actually THREE times, I also discussed Larmarguy's experiment and offer some ideas as to that as well.

I've been thinking of adding agitation to my next fermenter. I don't see any reason that it would have anything other than a positive affect on the fermentation. The agitation doesn't need to be terribly vigorous and probably not even continuous to get maximum attenuation out of the yeast. You'd likely have to adjust your brewing process a bit for slightly higher attenuation, but in the long run it's probably an extremely consistent fermentation method.

Oxidation shouldn't be a concern at all. I'm not sure why it would be, there's no oxygen in there.

wildwest450 said:

I've brewed enough beer to know you don't want to put your carboy on a stir plate.

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How can you know something when you lack anecdotal observation or even a rudimentary understanding of why it would be true?

Bonysplicer has a point in that the oxidation argument, that is stated as a matter of fact, is laughable. It blows me away that somebody can think that agitating the beer would change the composition, apparently via magic, of the gas that is in contact with the surface. It certain would change the surface area exposed to gas, but that doesn't matter if all of the gas is inert (as it was right before the magic happened).

Also laughable, that two HBT posters with more street cred than Bonysplicer are doing the same damn thing and nobody is calling them stupid.

My 2 cents, I think recirculating the beer with a pump is a more promising approach (and one practiced by a number of commercial breweries).

My other 2 cents, this is the wrong message board to use the word "why" on.