What makes fermenting beer churn?

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NorCalAngler

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Yeast are tiny little creatures, but they create quite a commotion while they're fermenting. I searched, but I couldn't find any information on the physics explaining how these tiny yeast cells move the wort as if it is boiling. My buddy said it's probably the amount of CO2 being released. Does anybody know how they manage to stir up the wort so much?
 
That ^^^ NorCalAngler, is an extremely GOOD question. In fact, it is about the BEST question I've seen on the board for quite some time. It is a question that I'd like an answer to, as well! Sure do hope someone steps up with a good answer! [I'm a musician and teacher, NOT a chemist!]

glenn514:mug:
 
WAG here, but what about heat currents caused by fermentation's exothermic reactions?

EDIT: I'm thinking it's mostly the CO2 production as well, but heat production might cause a secondary reaction.
 
+1 on the buddy's theory

+1 on einstein's theory, who's buddy?

Temp changes would be my guess. Heat is energy. Most convection occurs as a result of heat (or heated liquid or air or whatever) rising to the top, where it cools and falls only to be released by more heated material.
 
Fermentation creates CO2. The wort at some point becomes saturated with CO2 at which point no more can go into solution. The 'extra' CO2 formed after that remains in gaseous form and the CO2 gas bubbles rise to the top. All those bazillion CO2 bubbles create upwards currents and churn the wort.
 
What is interesting to me is the churning appears to be more convection type motion, but it doesn't seem like enough heat is generated to move the liquid with the velocity I see in the carboy. I will email a professor at UC Davis brewing school to see if they can provide some insight into the biology/chemistry/physics going on.
 
What is interesting to me is the churning appears to be more convection type motion, but it doesn't seem like enough heat is generated to move the liquid with the velocity I see in the carboy. I will email a professor at UC Davis brewing school to see if they can provide some insight into the biology/chemistry/physics going on.

+1 on einstein's theory, who's buddy?

Temp changes would be my guess. Heat is energy. Most convection occurs as a result of heat (or heated liquid or air or whatever) rising to the top, where it cools and falls only to be released by more heated material.

That kinda answers your question. I'm a chemistry major/physics minor and my lil bro is a biology major. I asked him and we both thought that the primary action is due to CO2 being released. However, since it is an exothermic reaction there is significant heat being released.

It is not an equilibrium reaction, so the activation energy is much higher and consequently the energy released is large

ferment1.gif


There is significant ATP released during aerobic fermentation (read: yeasties working) to make this a heat/energy releasing reaction.


Kinda off but if no real answers come up I'll go to my biochem teacher
 
What is interesting to me is the churning appears to be more convection type motion, but it doesn't seem like enough heat is generated to move the liquid with the velocity I see in the carboy. I will email a professor at UC Davis brewing school to see if they can provide some insight into the biology/chemistry/physics going on.
Read my post above, that is what is happening. You're right that it does look convective. Look closely just under the krausen...see all those bubbles rising?

EDIT: It's not the heat. The heat has nothing to do with it, it's 100% the CO2 bubbles.
 
+1 on einstein's theory, who's buddy?

Temp changes would be my guess. Heat is energy. Most convection occurs as a result of heat (or heated liquid or air or whatever) rising to the top, where it cools and falls only to be released by more heated material.

Third sentence, original post :D
My buddy said it's probably the amount of CO2 being released.
 
EDIT: It's not the heat. The heat has nothing to do with it, it's 100% CO2 bubbles.

I don't think you can say the heat has nothing to do with it. It could be 1% or 2% or 5% (or 0.05%) of the reaction, but we know that fermentation causes heat and we know that heat will cause convective currents to form. That's a given.

I do agree, though, even though I through out the heat idea as a wild-ass guess, that the CO2 formation is almost certainly the dominant cause of the churning.
 
Nope, 100% the CO2 bubbles. I'm a bit surprised we're even having this conversation.

I know fermentation creates heat but not that much. For example, I put lagers in my lager fridge and use two thermometers that read exactly the same. Whatever temp is in the little jar of water in there is the EXACT same temp in the carboy...at high krausen and any other time. My ales are submerged in a water bath, whatever temp the water bath is at is the EXACT same temp in the carboy...at high krausen or any other time. I've heard the 'it can be 10* warmer in the carboy' claims but I've just never seen it and I always measure both the wort in the carboy and the 'environment' in every brew I do.
 
Nope, 100% the CO2 bubbles. I'm a bit surprised we're even having this conversation.

I know fermentation creates heat but not that much. For example, I put lagers in my lager fridge and use two thermometers that read exactly the same. Whatever temp is in the little jar of water in there is the EXACT same temp in the carboy...at high krausen and any other time. My ales are submerged in a water bath, whatever temp the water bath is at is the EXACT same temp in the carboy...at high krausen or any other time. I've heard the 'it can be 10* warmer in the carboy' claims but I've just never seen it and I always measure both the wort in the carboy and the 'environment' in every brew I do.

Are you saying fermentation is NOT exothermic? If so, that is just silly!!! Your water bath and ferm chamber are successfully regulating the heat produced, but heat is most definitely being produced. If you let a carboy, bucket, stainless conical, crock, or whatever of fermenting beer go with no temp control the exothermic process of fermentation will skyrocket the temperature. Uncontrolled fermentation can get into the 90's or 100's depending on ambient temp, amount of fermentables, and amount and strain of yeast pitched.

Now, that does not answer the OP's question, but I agree with Bird, heat production cannot just be dismissed so readily. I would even hazard a guess that most movement is created by the release of CO2 but the convective currents are generated by the exothermic reactions pushing the CO2 around.

I look forward to more input from the more scientifically knowledgable members because I think most of us (myself included) are just blowin beer farts out our ass :D
 
Has anyone ever been to Boston Beer Works and tried their blueberry beer? They put a handful of actual blueberries when they serve it, and the blueberries are moved around in the same manner as clumps of yeast in a carboy. It's the CO2 that attaches itself to the berries, makes them rise to the top, and once they get there, the CO2 it released into the air, and the berry falls to the bottom.

M_C
 
Remember that temperature measures average heat energy not total heat energy. A large mass of liquid will have a very large heat capacity and the larger the volume is compared with the surface area then the less heat can escape. So for instance in the whiskey industry where large fermentation vessels are used and very large cakes of yeast are used to move the fermenatation along quickly, its often essential to have glycol cooling jackets on the fermenters such that the yeast don't kill themselves by overheating the ferment.

also remember that convection happens when one part of the container is heated another part is cool. This forms a convection cell as less dense liquid moves to a position where it cools and then falls as its density increases with loss of temperature.

In the case of a 5 gal. container, we are not heating its base, the yeast produce heat almost homogeneously at high krausen as they are dispersed throughout the wort.

So what is causing this "convection cell" effect? Something has to be given up at some location in the container so that the thing giving it up can move back somewhere else to get more.

What is actually moving in the container? Little bits of fluff? yeast?, trub? - all of the above. The solid particles are "picking up" gas bubbles - they act as nucleation sites for the CO2 coming out of solution. The particles travel to the liquid/gas interface at the top of the container where the gas parts company with the particle. The particle has now lost its buoyant "balloon" of gass and falls back into the liquid where, at some point, it will re nucleate another bubble of gas..
 
TANSTAFB, No I'm not saying fermentation is not exothermic (it is) but gilligson mentioned why it's not the reason for the churning. The tiny amount of heat generated by a yeast cell fermenting is quickly transferred to the relatively massive thermal mass of 5 gal of wort. And as gilligson said, there has to be a temerature gradient (i.e. warmer wort below cooler wort) for the convective currents, the whole volume of wort is heated fairly evenly.

You can see the same thing Misplaced Canuck is talking about in the slice of lemon they serve with weizens.
 
I'm a biologist working in a biochemistry setting (vaccine production), and although I've never really thought much about what causes the physical movement of the wort, I'll take a stab at it:

It's true that the exothermic reaction of yeast fermentation at homebrewing scale does produce heat and could in fact have some (small) impact on the motion of the bulk wort, yet consider this example:

When making 5M NaOH (a strong base used for titration) here at work, I add 200g of solid NaOH to only 1L of water. This in turn creates one of the most exothermic reactions I work with on a daily basis. The glassware holding the solution becomes so hot that one could actually burn one's hand on it if not careful. However, the solution itself does not really get all that turbid. The temperature differences between the dissolving NaOH and the bulk water are far greater than we see in fermentation, yet the actual movement of the water is not all that different from fermentation, if not less.

So, that leaves us with the other major product in the catalysis of glucose by yeast: CO2. The production of carbon dioxide is so large compared to the amount of yeast in solution that it is almost definitely the major factor contributing to the movement of the wort. As someone else said on here, once the wort becomes saturated with CO2, the excess must go up and out, pushing the wort around as it does so. Therefore, you have a great deal of CO2 pushing the wort all around, causing it to churn like crazy.

Hope this helps! :mug:
 
Now that we have THAT settled SpanishCastleALe... :D

giligson's explanation makes a lot of sense with particles acting as nucleation sites for CO2 which carries said particle to surface, CO2 released into headspace, particle falls back in but gets knocked around on its downward fall by billions of OTHER particles of yeast, hops, proteins, etc. that creates the appearance of convective currents that are otherwise impossible as the wort/beer is homogeneously heated...

Sorry for the reiteration, I was envisioning the yeastie beastie dance in my head and musing out loud :ban:
 
rising CO2. Fish tanks use exactly the same principle in their lift tubes using an air pump to move water through undergravel filters.

Heat isn't causing any of the motion. It is popular to think that heat rises, but that is only if the temp above it is lower. Zeroth thermo law, actually, heat moves from higher temp to lower. In this case, the entire mass of beer is exothermic, so I don't think the rising thing is happening (much).
 
rising CO2. Fish tanks use exactly the same principle in their lift tubes using an air pump to move water through undergravel filters.
I had a longer/less concise description of this in an earlier post but decided to not post it at the last second. Glad you did. And from what I've read, the vast majority of aeration of the water occurs at the surface not from the bubbles (although the bubbles create the turbulence at the surface).
 
Fantastic discussion! Thank you everybody for chiming in. If I ever hear back from the UC Davis professor I will post back to this thread. The CO2 nucleation points on the yeast/trub makes a lot of sense.
 
I heard back from the professor. His response was short and sweet; It is the CO2 production pushing everything around.
 
Thank you, NorCalAngler, for asking this question in the first place! During early fermentation of some of my brews, the wort is REALLY moving, almost as if someone was stirring it. CO2 release certainly explains what's happening. I have appreciated ALL the answers! This is one of my all-time favorite threads!

glenn514:mug:
 
The motion of convection is due to buoyancy forces in a fluid. The warmer areas are less dense than the cooler areas. Remember, hot air rises and cold air falls. I don't think there is enough of a temperature difference for you to be seeing heat convection. However, the different densities of sugar water, ethanol, and CO2 might be creating a similar effect.
 
I'm not going to brag about my qualifications, other than to state that I understand a bit of thermodynamics, fluid mechanics, and nucleation theory.

Of course temperature plays a role. As long as you have a temperature gradient (difference) across a fluid volume you will get convection. In a fermentor, there is a gradient from top to bottom - low at the bottom and high at the top where most of the yeast is active (in ales), and from center to side since the fermentation is exothermic and the container walls are cooler than the liquid mass. So these gradients are going to form a convective pattern that goes upwards and outwards, which will advect solid particles.

CO2 production is also going to cause churn. The gas is going to rise, pulling the liquid/yeast chunks with it in a toroidal flow pattern, pretty much the same pattern you get with convection.

Someone mentioned Brownian motion - visible yeast and trub chunks have way too low of a diffusion constant to show measurable diffusive motion.
 
The yeast farts co2 so it propels them around. Like blowing up a balloon and then just letting go of the end before tying it. :D That's what I tell everybody that asks me..
 
Interesting coincidence happened to me this morning. I woke up to yeast filling my airlock. Apparently US-05 loves this SN ESB clone I made. Anyway, the airlock was pretty clogged and I noticed there was much less churning happening in my beer. When I popped the bung to clean it out there was a LOT of pressure in there. I was extremely lucky it didn't blow out. I put a piece of sanitized foil on the carboy, cleaned the airlock and put it back on. I continued getting ready for work and by the time I came back downstairs about 1/2 hour later the churning was going full force again. This is additional evidence it's CO2 propelling the beer debris around. Under pressure and without room for CO2 to escape the activity slowed tremendously.
 
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