Why don't we agitate during fermentation?

[Bonysplicer]

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?

 

[ChshreCat]

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.

 

[Revvy]

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.

 

[wildwest450]

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

 

[Bonysplicer]

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?

 

[Bonysplicer]

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?

 

[Revvy]

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?

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.

 

[dunnright00]

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.

Classic!

 

[springer]

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?

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

. 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.

I like Revvy's explanation better

 

[Bonysplicer]

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

 

[Bokonon]

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?

 

[bentonre]

...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.

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!

 

[humann_brewing]

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.

mmm.... wet cardboard

 

[springer]

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

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

 

[Revvy]

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?

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.

 

[lamarguy]

Why would you need to agitate an active fermentation ?

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.

 

[ImperialLover]

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.

 

[Revvy]

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.

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.

 

[weremichael]

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.

 

[Edcculus]

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.

 

[JVD_X]

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?

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.

 

[Shooter]

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.

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

 

[Blinebuddy]

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?

 

[springer]

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.

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.

 

[springer]

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?

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.

 

[lamarguy]

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

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.

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.

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.

 

[Revvy]

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.

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.

 

[lamarguy]

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.

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.).

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

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

 

[springer]

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.

 

[lamarguy]

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

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.