The great nitrogen bubble debate

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choosybeggar

choosybeggar

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So I checked out the Party Pig website:

The package requires no CO2 tanks or cartridges because it uses a self-inflating pressure pouch. As beer is dispensed from the PET plastic bottle through an attached push-button valve, the internal pressure pouch expands and maintains a constant 15-20 pounds per square inch of pressure. This is accomplished by combining citric acid and bicarbonate of soda in a controlled manner to produce CO2 gas, which remains inside the pressure pouch and does not come in contact with the beer.
Seems like just the ticket. Thanks.
 

Mike37

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I see the party pig working just fine as long as the beer itself can be primed below 1.5 volumes.

Not saying it would be difficult, just that it's the key to this all working.
 

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+1 to marketing for all of the misinformation out there.

At serving pressures (10-12psi for regular taps, 30psi for stout taps), a negligible amount of N is dissolved into the liquid. Beer is and always has been carbed with Co2, regardless of style or carbing method. This is because Co2 is the gas naturally created by yeast when bottling carbing.

The creamy head is a result of smaller Co2 bubbles created by the beer being forced through the restrictor plate of a stout tap. The smaller the surface area of any surface, the more stable it is. There are some physics equations that prove it somewhere, but im not going to get that sciency on you.

Now to push the beer through the restrictor plate you need a psi of around 30 psi (varying depending on the balance of your system) to get an acceptable rate of flow. If did it with just Co2, too much would be absorbed into your beer resulting in over carbed beer. This is corrected by mixing the Co2 with a gas that cannot be dissolved in liquid at those pressures(i.e. argon, nitrogen).

The sole purpose of nitrogen is to prevent over carbonation of your beer at the pressure required to push beer through the stout tap to create the desired mouthfeel and head for that style of beer.

When I get a little time I would be more that happy to post all of the equations and physics principles that support this, butchphr it might be a while before I can get to it. Nitrogen Sounds fancy and has been used as a catase by breweries (who should / do know better) to market their stouts and some porters.

Even if a beer was pushed on pure nitro across a long/thick enough beer line to support the pressures it would require to dissolve in beer, upon leaving the tap all of the nitro would surge out of the beer leaving you with nothing but foam.
 

mattd2

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...Now to push the beer through the restrictor plate you need a psi of around 30 psi (varying depending on the balance of your system) to get an acceptable rate of flow. If did it with just Co2, too much would be absorbed into your beer resulting in over carbed beer. This is corrected by mixing the Co2 with a gas that cannot be dissolved in liquid at those pressures(i.e. argon, nitrogen)...
Is that 30 PSI at the keg or tap? From what I have seen it is at the keg and I have always had a theory the beergas might not even be needed for a "normal" homebrew setup. If the keg is hooked up with a short section of 1/4"/5/8" beerline then the normal ~15psi drop would not be present and you could serv on pure CO2 at a lower pressure and suitable temp to get the required pressure at the stout tap.
 

Mike37

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mattd2 said:
Is that 30 PSI at the keg or tap? From what I have seen it is at the keg and I have always had a theory the beergas might not even be needed for a "normal" homebrew setup. If the keg is hooked up with a short section of 1/4"/5/8" beerline then the normal ~15psi drop would not be present and you could serv on pure CO2 at a lower pressure and suitable temp to get the required pressure at the stout tap.
That's 30 psi for a decently quick pour. 10 psi of co2 (or nitro, beer gas, etc.) will push beer through a short line and a stout faucet just fine, albeit slower than your typical nitro pour at 30 psi.

I have my keg set up exactly this way right now. At my keg's temperature, I can leave the regulator set at 8-10 and not worry about over carbonation.
 

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that is a ballpark psi setting at the regulator. There could be a variance of + - 1-2 psi based on how you have your lines balanced (line diameter, line length, height distance from keg top to tap, carb volume etc.). Beer gas is not needed at all on a normal homebrew setup. It is only needed when you implement a stout tap.

As for your theory, it definitely merits an experiment. either it works or you end up with pure foam. Assuming it worked, you may run in to the problem of not having enough line to reach your taps if you go too short :p.

Most bars run everything on beer gas for the sake of using 1 gas and being able to push all types of beer. for me there is no place in town that fills beer gas, so no stout tap for me :-(
 

Pratzie

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I was lead to believe that the Nitro is just to assist pushing the beer thru that restrictor plate in the faucet. That the pressure has to be greater then the CO2 alone but that the bubbles themselves are still CO2 that has been pushed thru the plate and has that cascading effect.


Guess im jumping on the pile here, sorry for not reading all the posts firsts
 

mattd2

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...Beer gas is not needed at all on a normal homebrew setup. It is only needed when you implement a stout tap.

As for your theory, it definitely merits an experiment. either it works or you end up with pure foam. Assuming it worked, you may run in to the problem of not having enough line to reach your taps if you go too short :p...
Sorry I meant normal homebrew setup with a stout tap. Looks like someone (Mike37) did experiement.
That's 30 psi for a decently quick pour. 10 psi of co2 (or nitro, beer gas, etc.) will push beer through a short line and a stout faucet just fine, albeit slower than your typical nitro pour at 30 psi.

I have my keg set up exactly this way right now. At my keg's temperature, I can leave the regulator set at 8-10 and not worry about over carbonation.
Thanks Mike, I have been wondering this for a while but don't have the cash to invest in a stout tap! now just to get this method out to more people!
 

branCHEs

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So i think its fair to say that the debate is over. Nitro provides the pressure to push the beer through the restricter plate in a reasonable time with out over carbonating the beer. The actual bubbles are 100% CO2.

My only question: how does Guinness manage to achieve this cascading effect from their can? I believe Boddingtons English Pub Ale also manages to achieve this affect. I know the both have a little plastic ball inside of the can. I have been told that the balls are filled with nitrous and releases upon opening can.
 
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The devices generate a turbulent stream of beer on opening the can. In the sealed can, they are beer filled and at the same pressure as the contents of the can (well above atmospheric). Opening the can produces a dramatic pressure drop but the only way to relieve pressure within the widget is for gas and beer to shoot out of it's small hole, generating turbulent flow and CO2 foam.
 

ajdelange

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The 'widget' has evolved appreciably over the years. The last one I looked at was about the size of a ping pong ball with a very small hole at one pole. These are, I assume, filled with nitrogen at atmospheric pressure before being dropped into the can. The can is then filled with beer and a little liquid N2 is squirted in the instant before the lid is clamped on. The can then goes into the pasteurizing tunnel where the heat causes the N2 to evaporate and the pressure becomes quite high so that the beer, at this higher pressure, is forced into the ball. The can is then sent off to the store, put in your refrigerator and eventually taken out. Even though it is now cold the partial pressure of nitrogen in there is a couple of atmospheres. When you open the can the pressure in the head space drops instantly to 1 atm but the pressure in the ball is still much higher than that as it can only bleed off through the tiny hole. In trying to equalize the pressure nitrogen and beer are forced out through the tiny hole. The widget is designed to spin as a result of this jet of beer. This jet of beer agitates the main volume of beer thus causing the CO2 in solution to come out in much the same way it does when the beer is agitated by being forced through a sparkler or restrictor plate.
 

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So I'm reviving this old thread. Did anyone ever go forward with the party pig/bladder testing? I'm going to buy a stout faucet and try running it on CO2 alone. It just doesn't make any sense to me to buy a Nitrogen tank and regulator when I won't be running a "nitrogen" beer that often. Just interested how many people have had success with a stout faucet with a low carbed keg running temporarily at a high pressure during serving. This seems to make sense to me and really shouldn't be a big pain in the butt to change the regulator for serving.
 

ssgross

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So i'm reviving this once revived old thread because I think we can do much better.
I don't think there is a need, yet, for an experiment as this can all be proven mathematically, and any and all propaganda or articles without mathematical justification can be ignored. In particular, the wikipedia article for partial pressure
https://en.wikipedia.org/wiki/Partial_pressure
has most of the story.
The information needed is:
1) The properties by which nitrogen dissolves in a liquid is known (googlable), and quite negligible unless under extreeeeeeemly high pressures. There is a table on the wiki page for Henry's law that shows this. Co2 has medium solubility, which is why we can use it to make fizz, and nitrogen has very poor solubility.
Pure Speculation: diffusion may speed this up, just like using a stone to aerate your beer, or speed up force carbonation. I suspect this is how small amounts of nitrogen are dissolved into guinness at the factory, as claimed by the propaganda. This type of proprietary knowledge could be verified by an experiment. I liked the pH suggestion in a previous post.
2) The properties by which CO2 dissolve in a liquid are known, and most of us use some type of calculator for this.
3) The properties by which CO2 dissolves in nitrogen (and vice versa) are also known. In particular, co2 readily and easily dissolves in nitrogen
4) From the wiki: "Gases dissolve, diffuse, and react according to their partial pressures, and not according to their concentrations in gas mixtures or liquids." This behavior can be calculated using the formulae in the article.

Put it all together and I think we have the following answer. (Disclaimer: This is still speculation. I'm not a chemist..just a mathematician. I'll try to write this up as theorem and proof if I can find the time. Perhaps even a "how to" so the rest of the world can use their pocket calculators as more than a paper weight.)
1) Nitrogen's function in "beer gas" is solely to provide the added pressure to push through the restrictor plate WITHOUT over-carbonating the beer over time. This functionality can be achieved with any other gas that does not dissolve well, as pointed out by the argon experimenter posts.
2) The same results in pour can be achieved (there is a Brew Your Own article about this) by other means. Namely, crank up the co2 pressure, pour, lower the pressure, vent the keg.
3) it was claimed somewhere in this thread (EDIT: https://www.homebrewtalk.com/showthread.php?t=230229 post #5) that you could a) carbonate with co2, the b) hook up pure nitrogen and leave it with a perfect pour every time. This is a farce. Read the wiki again if you don't believe me. co2 readily dissolves in nitrogen. Thus, the pressure of pure nitrogen in the headspace does nothing to keep the co2 in solution. Only the partial pressure of co2 in the headspace does this. The partial pressure of co2 in solution will move to equilibrium with the head space, and the beer will go flat over time as co2 moves to the headspace. This is why beer gas has co2 in it, at the right partial pressure to maintain carbonation over time.
EDIT: or better yet, the first equation in the wiki shows that (in the headspace) as the volume of nitrogen increases with pints poured, the partial pressure of co2 decreases. Since co2 dissolves easily in nitrogen (this is dalton's law in the wiki), the beer goes flat.

In summary, I propose we develop a mathematical solution based on known chemistry, since none of us are privy to any proprietary information from guinness.
 

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An engineer, a physicist and a mathematician walk into a bar....

The essential facts here are
1) CO2 is much more soluble in water/beer than N2
2) The solubility of CO2 is strongly dependent on temperature; the solubility of N2 isn't.
3) The size of the head in a mixed gas pour depends on the CO2 content
4) The bubbles formed with low Paco2 (partial pressure of CO2) are small
5) If PaN2 is appreciable the bubbles are more stable because the N2 diffuses back into solution more slowly than CO2 does
6)The smaller bubbles give less physical 'prick' on the tongue when they burst.
7)A bubble filled partially with nitrogen gives less carbonic acid 'prick' than a pure CO2 bubble.

The equilibrium equation for the mix of CO2 and Nitrogen as applied to the particular problem of interest can be found in Carroll, T. C. N., The effect of dissolved nitrogen on foam and palate, MBAA TQ, Vol 16, No. 3 1979

It is indeed impossible to mimic the performance of a mixed gas system with nitrogen alone as the beer will go flat (PaCO2 in pure N2 is 0) but it is possible to get a pretty good approximation with pure CO2. One keeps the beer under enough pressure to keep it at 1.2 - 1.5 volumes at the storage temperature then raises the pressure to 20 - 25 psig for serving (i.e. high enough to let the sparkle plate do its job) and then reduces it again at the conclusion of serving. This is clearly not possible if you want your stout 'on tap' but is OK if you are only going to serve it say 1 day a week. I, and several other people have done this. It's not exactly the same but it is pretty close.
 

mattd2

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An engineer, a physicist and a mathematician walk into a bar....

The essential facts here are
1) CO2 is much more soluble in water/beer than N2
2) The solubility of CO2 is strongly dependent on temperature; the solubility of N2 isn't.
3) The size of the head in a mixed gas pour depends on the CO2 content
4) The bubbles formed with low Paco2 (partial pressure of CO2) are small
5) If PaN2 is appreciable the bubbles are more stable because the N2 diffuses back into solution more slowly than CO2 does
6)The smaller bubbles give less physical 'prick' on the tongue when they burst.
7)A bubble filled partially with nitrogen gives less carbonic acid 'prick' than a pure CO2 bubble.

The equilibrium equation for the mix of CO2 and Nitrogen as applied to the particular problem of interest can be found in Carroll, T. C. N., The effect of dissolved nitrogen on foam and palate, MBAA TQ, Vol 16, No. 3 1979

It is indeed impossible to mimic the performance of a mixed gas system with nitrogen alone as the beer will go flat (PaCO2 in pure N2 is 0) but it is possible to get a pretty good approximation with pure CO2. One keeps the beer under enough pressure to keep it at 1.2 - 1.5 volumes at the storage temperature then raises the pressure to 20 - 25 psig for serving (i.e. high enough to let the sparkle plate do its job) and then reduces it again at the conclusion of serving. This is clearly not possible if you want your stout 'on tap' but is OK if you are only going to serve it say 1 day a week. I, and several other people have done this. It's not exactly the same but it is pretty close.
A marketer and an accountant join them; The accountant says "Hey guys, I think your on to something - we could charge people extra for this"
And the Marketer adds "Yeah, but I think the general public will probably get confused with all you science talk - so let's just say the nitrogen makes smaller bubbles and that's what gives it the creamy texture"

And I think that has now gone full circle back to the beginning of this thread :D

On a serious scientific note though - would it be possible to overcarb the beer on CO2, then pressurise to 30PSI with N2, and if you calculate it correctly end up with the right mix once it all equilibrulisese (word?)... that is if you have enough time/patience :)
 

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...

On a serious scientific note though - would it be possible to overcarb the beer on CO2, then pressurise to 30PSI with N2, and if you calculate it correctly end up with the right mix once it all equilibrulisese (word?)... that is if you have enough time/patience :)
No. You could do it for the initial keg volume, but then as the keg empties, it will take more and more CO2 in the headspace to stay in equilibrium with the carbonation in the beer. So more and more CO2 will diffuse out of the beer, reducing the carbonation level.

Brew on :mug:
 

ajdelange

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On a serious scientific note though - would it be possible to overcarb the beer on CO2, then pressurise to 30PSI with N2, and if you calculate it correctly end up with the right mix once it all equilibrulisese (word?)... that is if you have enough time/patience :)
Yes and that is actually how Guiness got to the nitrogen thing. In the 1940's they were selling Guiness in an 11 gal keg with 8 gal beer (not completely fermented) and the head space pressurized to 3.5 atm (absolute) with air. They later replaced the air with pure nitrogen (to discourage bacterial growth according to Carroll) "..which led to the discovery of the dramatic effect of small amounts of dissolved nitrogen gas on head creaminess and durability."

As has been observed, the situation will change as the beer is drawn off but evidently the 3:8 initial ratio of gas to beer is adequate to deliver acceptable product from first to last glass.

Thus you don't actually have to calculate it. Guiness has already done that for you. Carbonate to 1.5 vols then pressurize to 3.5 atm absolute with nitrogen and you should be there.
 

55x11

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An engineer, a physicist and a mathematician walk into a bar....

The essential facts here are
1) CO2 is much more soluble in water/beer than N2
2) The solubility of CO2 is strongly dependent on temperature; the solubility of N2 isn't.
3) The size of the head in a mixed gas pour depends on the CO2 content
4) The bubbles formed with low Paco2 (partial pressure of CO2) are small
5) If PaN2 is appreciable the bubbles are more stable because the N2 diffuses back into solution more slowly than CO2 does
6)The smaller bubbles give less physical 'prick' on the tongue when they burst.
7)A bubble filled partially with nitrogen gives less carbonic acid 'prick' than a pure CO2 bubble.

The equilibrium equation for the mix of CO2 and Nitrogen as applied to the particular problem of interest can be found in Carroll, T. C. N., The effect of dissolved nitrogen on foam and palate, MBAA TQ, Vol 16, No. 3 1979

It is indeed impossible to mimic the performance of a mixed gas system with nitrogen alone as the beer will go flat (PaCO2 in pure N2 is 0) but it is possible to get a pretty good approximation with pure CO2. One keeps the beer under enough pressure to keep it at 1.2 - 1.5 volumes at the storage temperature then raises the pressure to 20 - 25 psig for serving (i.e. high enough to let the sparkle plate do its job) and then reduces it again at the conclusion of serving. This is clearly not possible if you want your stout 'on tap' but is OK if you are only going to serve it say 1 day a week. I, and several other people have done this. It's not exactly the same but it is pretty close.
actually both CO2 and N2 solubility depends on temperature similarly - with a drop of 2.5-3 from 0C to 40C. Overall CO2 is about 100 times more soluble than N2 (3g/L for CO2 and 0.03 g/L for N2).

http://www.engineeringtoolbox.com/gases-solubility-water-d_1148.html

This means that bubbles in your beer are almost entirely CO2 (99% or maybe 97% - depending on gas composition and level of saturation reached).

I doubt that 1% Nitrogen content inside the bubbles contributes to significant reduction of carbonic bite.
I believe Nitrogen contribution in bubbles is irrelevant to the taste - you just need to create small bubbles by applying high pressure and forcing lightly carbed beer through an aperture. The "smooth" taste is simply result of those small bubbles. You can push lightly carbed beer with other gases, or mechanical devices and get the precisely same effect.
 

doug293cz

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actually both CO2 and N2 solubility depends on temperature similarly - with a drop of 2.5-3 from 0C to 40C. Overall CO2 is about 100 times more soluble than N2 (3g/L for CO2 and 0.03 g/L for N2).

http://www.engineeringtoolbox.com/gases-solubility-water-d_1148.html

This means that bubbles in your beer are almost entirely CO2 (99% or maybe 97% - depending on gas composition and level of saturation reached).

I doubt that 1% Nitrogen content inside the bubbles contributes to significant reduction of carbonic bite.
I believe Nitrogen contribution in bubbles is irrelevant to the taste - you just need to create small bubbles by applying high pressure and forcing lightly carbed beer through an aperture. The "smooth" taste is simply result of those small bubbles. You can push lightly carbed beer with other gases, or mechanical devices and get the precisely same effect.
Since a typical beer gas is 25% CO2 & 75% N2, the N2 partial pressure will be 3X the CO2 partial pressure. Thus you will get about 3 parts N2 to 100 parts CO2. Still not much nitrogen in those bubbles.

Brew on :mug:
 

ajdelange

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actually both CO2 and N2 solubility depends on temperature similarly - with a drop of 2.5-3 from 0C to 40C. Overall CO2 is about 100 times more soluble than N2 (3g/L for CO2 and 0.03 g/L for N2).
I borrowed that wording from Carroll. In fact in terms of percentage change in Henry coefficient with temperature nitrogen is more sensitive but because nitrogen is so much less soluble than CO2 the change in level of dissolved N2 changes much less which is clearly what he meant as he has a graph illustrating this. Between 0 and 15 °C the dissolved level of CO2 under 1 bar partial pressure changes by 1.307 grams. The level of N2, conversely, under 1 bar partial pressure of nitrogen, changes by only 5.7 mg between 0 and 15 °C.


This means that bubbles in your beer are almost entirely CO2 (99% or maybe 97% - depending on gas composition and level of saturation reached).
So we take some ungassed beer and put it under 3 atm (to make the math easy) beer mix with PaCO2 = 1 atm (abs) and PaN2 = 2 atm (Guiness is served with a 25% mix). Things come to equilibrium at, lets say, 5° C and we have 1*44000*CO2KHy(5) = 3041.54 mg/L dissolved CO2 and 2*28000*N2KHY(5) = 47.8879 mg/L N2. Now we very carefully (reversibly, if you speak the language of thermodynamics), without disturbing it in any way draw some of the equilibrated beer into a syringe. Next we seal the inlet to the syringe and allow the plunger to come back a bit but a very small bit. What happens here? Gas, a very small amount, leaves the beer to push the plunger back thus creating a very small head space. What is the pressure of the gas in the head space? Since it is in equilibrium with beer that was in equilibrium with 3 bar in the keg, the head space pressure will be close to 3 bar in the syringe. And what will be the partial pressure of N2 in that head space? One percent of 3 bar? No, clearly it will be 66% of 3 bar i.e. the original 2 bar. Were it less than this the nitrogen would have had to stay preferentially in the beer but that, as we haven't allowed enough time for the temperature to change much, would require the Henry coefficient to have changed by some other mechanism.

I doubt that 1% Nitrogen content inside the bubbles contributes to significant reduction of carbonic bite.
I'd doubt it too but as the nitrogen content in the bubbles is 66% (in our example) and 75% in an actual Guiness serving set up the picture is far different. But, you may say, the mg of CO2 in a bubble of a given size aren't very different between pure CO2 and CO2 diluted 2:1 or 3:1 with nitrogen and that's true but you must remember that the ability of the CO2 to form carbonic acid (which is responsible for the bite) depends on the fugacity of the gas which is a function of its partial pressure. The partial pressure is reduced to 25% of what it is with pure CO2 and the fugacity will be down to something close to 25%. Less bite.


I believe Nitrogen contribution in bubbles is irrelevant to the taste -
I thought that too for a long time but if you taste beer drawn on nitrogen carefully I think you will change your mind. I did.


you just need to create small bubbles by applying high pressure and forcing lightly carbed beer through an aperture.
I have done this (out of N2 and who wants to run all the way out to Roberts just for a glass of stout) and have suggested people do this for years claiming you will get 95% of the effect. I'm not backing away from that position but I think I might revise the 95% number downward.


The "smooth" taste is simply result of those small bubbles.
The fact of the bubbles being smaller does have an effect as part of the prick is mechanical (with the rest being carbonic acid).


You can push lightly carbed beer with other gases, or mechanical devices and get the precisely same effect.
Guiness originally got into this because they wanted to be able to draw draught beer with relatively uniform head properties from the beginning of the cask to the end and they wanted the head to resemble that from a hand pump. They started with air and then moved to nitrogen which is Carroll's words led to the 'dramatic' difference a small amount of dissolved nitrogen makes. Thus while you can get a similar effect with just a hand pump you cannot get precisely the same effect. From the physics it is clear that any gas which dilutes (lowers the partial pressure of) CO2 should work in the same way. Clearly an inert gas such as argon (but not radon) would seem to be a reasonable choice for experimentation.
 

55x11

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Regarding your example of a plunger and diffusion of gases back into the extra headspace volume - this is what happens in the headspace of the keg.
The beer coming out of the faucet will still have 1:30 ratio of N to CO2 (assuming 1:3 in beer gas ratio) dissolved in it though.

But, the crucial thing that I am missing is the mechanism of foaming / bubble nucleation. I always assumed that as you open the faucet (say stout faucet), the beer (at this point say 3% nitrogen and 97% CO2 dissolved in it) flows through the constrictor plate and turbulence forces for the gases dissolved in beer to come out of the solution, forming the foam. Due to turbulence, its rapid enough process that its precipitation/nucleation driven, and not diffusion limited. In other words, it's highly non-equilibrium process, and I assumed that most if not all gas dissolved will be knocked out of the solution and into he foam - and that both gases will be knocked out at the same relative rate.
 

ajdelange

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Regarding your example of a plunger and diffusion of gases back into the extra headspace volume - this is what happens in the headspace of the keg.
Yes, and what happens anywhere else is the same as long as the pressure and temperature remain the same.
The beer coming out of the faucet will still have 1:30 ratio of N to CO2 (assuming 1:3 in beer gas ratio) dissolved in it though.
Beer that is in equilibrium with 1 bar CO2 at 5 °C contains 3.04 grams of CO2 (aq) per liter. Beer that is in equilibrium with 3 bar N2 contains 23.9 mg/L N2(aq). In the syringe, you'll have the same mix in the beer and in the headspace.

But, the crucial thing that I am missing is the mechanism of foaming / bubble nucleation. I always assumed that as you open the faucet (say stout faucet), the beer (at this point say 3% nitrogen and 97% CO2 dissolved in it)...
Behind the restricter plate an when the valve is closed the beer still contains N2:CO2::3:1

... flows through the constrictor plate and turbulence forces for the gases dissolved in beer to come out of the solution, forming the foam.
Yes and this may start behind the restricter plate as when you open up the valve the pressure behind the restriter plate drops.


Due to turbulence, its rapid enough process that its precipitation/nucleation driven, and not diffusion limited. In other words, it's highly non-equilibrium process, and I assumed that most if not all gas dissolved will be knocked out of the solution and into he foam - and that both gases will be knocked out at the same relative rate.
Yes, I think so. There may be nuances that are well beyond me but by the time the beer is in the glass lots (but not all) of the gas is out of solution. The rate at which this happens is some power of the concentration of the aqueous species (in mols) and of the activation energy. I have no idea what the rates might be. One the beer is in the glass we know it is moving towards 0.0003 atm CO2 and 0.8 atm N2 and we all know it takes hours for the the CO2 in a beer to establish equilibrium. My recall from scuba diving days that N2 moved into and out of blood pretty quickly but took much longer for bone and cartilage isn't helping me much here. What I do have is some data from that same paper that says that the duration of the surge (bubble show) depends on the ratio of Nitrogen to CO2 in the driving gas and that heads stand much longer if N2 is included in the mix.
 

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Beer that is in equilibrium with 1 bar CO2 at 5 °C contains 3.04 grams of CO2 (aq) per liter. Beer that is in equilibrium with 3 bar N2 contains 23.9 mg/L N2(aq). In the syringe, you'll have the same mix in the beer and in the headspace.

Behind the restricter plate an when the valve is closed the beer still contains N2:CO2::3:1
Hmm... I am sorry for being so obtuse, but I am just trying to understand this part.

So let's say that beer in the keg contains 3g/L of CO2 and 24 mg/L of N2, both dissolved in the liquid. That's a ratio of 100 to 1 or so (I think it should be 30:1 but whatever the ratio is).

When I open the tap, and say empty 1L of beer into a container, I will be knocking both those gases out of the solution, correct? (by providing nucleation site in a form of restrictor plate and a lot of turbulence).

So the foam should contain a ratio of CO2 to N2, of about 100:1 (or whatever it was in the dissolved beer, I think more like 30:1). If not, why not? If the ratio in the foam bubbles is predominantly N2, where does this extra N2 come from? Nitrogen from headspace, where it's indeed at 3:1 ratio over CO2, will have no chance to get to the bottom of the keg through all the beer and make it out of the faucet.

Note: I am not super-competent to talk about fine details of the design of the stout valves, but I don't think it's relevant - I could imagine a valve before or after restrictor plate, and the volume of beer contained there is negligible there. Besides, if the valve is closed, that beer should be in thermodynamic equilibrium with the rest of the keg (same pressure and lets assume the same temperature).

By the way, don't forget about "Guiness" gadget that Guiness people themselves use - basically a syringe that sucks up lightly carbed beer from the glass and reinfects it back into the glass creating a creamy Guiness foam of cascading bubbles - this uses no nitrogen whatsoever and creates the same "nitro" effect, purely mechanically? I did some experiments with the same technique at home and the effect is quite dramatically similar to nitro foam.

Someone with access to residual gas analyzer should take a bunch of foam from nitro beer and analyze it for CO2 to N2 ratio.
 

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So let's say that beer in the keg contains 3g/L of CO2 and 24 mg/L of N2, both dissolved in the liquid. That's a ratio of 100 to 1 or so (I think it should be 30:1 but whatever the ratio is).
That's by weight and were we discussing how N2(aq) and CO2(aq) reacted in solution those numbers (the concentrations) would be what we would use. But we are not talking about the solution phase but a gas phase that arises when bubbles form. In gasses the fugacity takes the role of concentration (activity really) and the fugacity of a gas depends on its partial pressure (and an activity coefficient). Thus we are concerned with the partial pressures of CO2 and nitrogen.

When I open the tap, and say empty 1L of beer into a container, I will be knocking both those gases out of the solution, correct? (by providing nucleation site in a form of restrictor plate and a lot of turbulence).
Not all of it perhaps but certainly an appreciable part of it.


So the foam should contain a ratio of CO2 to N2, of about 100:1 (or whatever it was in the dissolved beer, I think more like 30:1). If not, why not?

If the ratio in the foam bubbles is predominantly N2, where does this extra N2 come from?
We have noted that it takes a lot of pressure to force a little nitrogen into beer. The converse is that a little nitrogen dissolved in beer produces a high nitrogen pressure in any gas in equilibrium with it.

There is no extra gas. The gas that dissolves in the beer was at 3:1 and the gas that comes back out is at 3:1.


Nitrogen from headspace, where it's indeed at 3:1 ratio over CO2, will have no chance to get to the bottom of the keg through all the beer and make it out of the faucet.
If nitrogen has no chance to make it to the bottom of the keg neither does CO2 but in fact CO2 does make it to the bottom of the keg as does N2 if the keg has been properly carbonated i.e. left under adequate partial pressures of both gasses for long enough.

Note: I am not super-competent to talk about fine details of the design of the stout valves, but I don't think it's relevant
Carroll's paper shows curves of surge duration for both 5 hole and 9 hole restrictor plates. The differences are dramatic.

I could imagine a valve before or after restrictor plate,
In the traditional Guiness faucet it is before. I don't believe the ones made today have any valve.

It is clear that the bubbles that first break out of beer carbonated with gas in a 3:1 mix are going to be in a 3:1 mix in my syringe model. But those first bubbles are at 4 bar pressure (assuming the beer was 'carbonated' with 3 atm N2 and 1 of CO2. But those are not the bubbles in the foam. The bubbles in the foam have been released into beer at 1 bar absolute pressure and while the ratio of partial pressures for bubbles in equilibrium with 3:1 carbonated beer is 3:1 at 4 bar it is 0.2:1 but then the pressure inside the foam bubbles is higher than 1 bar because of the surface tension (without which the bubbles would not be tiny). If the internal pressure were as much as 2 bar the N2 to CO2 ratio would be about 1:1.

That's about as far as my limited knowledge will allow me to take it. I think we conclude that the nitrogen content of the bubbles is at least 17% and as much as 15% if we ignore diffusion through the bubble film. If we consider that then we would recognize that N2 is trying to get into equilibrium at 0.8 bar and CO2 is heading for 0.0003 bar
 

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That's by weight and were we discussing how N2(aq) and CO2(aq) reacted in solution those numbers (the concentrations) would be what we would use. But we are not talking about the solution phase but a gas phase that arises when bubbles form. In gasses the fugacity takes the role of concentration (activity really) and the fugacity of a gas depends on its partial pressure (and an activity coefficient). Thus we are concerned with the partial pressures of CO2 and nitrogen.

Not all of it perhaps but certainly an appreciable part of it.




We have noted that it takes a lot of pressure to force a little nitrogen into beer. The converse is that a little nitrogen dissolved in beer produces a high nitrogen pressure in any gas in equilibrium with it.

There is no extra gas. The gas that dissolves in the beer was at 3:1 and the gas that comes back out is at 3:1.
...
That's about as far as my limited knowledge will allow me to take it. I think we conclude that the nitrogen content of the bubbles is at least 17% and as much as 15% if we ignore diffusion through the bubble film. If we consider that then we would recognize that N2 is trying to get into equilibrium at 0.8 bar and CO2 is heading for 0.0003 bar

Now I understand you better. And the chemistry says this is just wrong - the pressure ratio of gases coming out of solution should generally NOT be the same as the pressure ratio of gases that got them in there.

If you dilute a beer gas, say with 1:1 ratio of N2 to CO2, you will end up with 100:1 ratio of CO2 to N2 at 0C (and very similar ratio at room temperature).
If you take out some arbitrary volume of liquid (gently) and agitate it till you get gases out of solution, or boil the liquid or freeze it, whatever, and analyze, the amount of gases that will come out will be whatever is "stored" in liquid. So still 100:1 ratio in this scenario. You are correct, this is by mass. By volume it will be 48/28 (atomic weights of CO2 and N2 respectively) smaller, so 1.7 times smaller, or 58:1. And if you use 3:1 N2:CO2 gas ratio pressure, you will end up with about 20:1 ratio - by volume. Still, that's about 95% CO2 and 5% N2 - by volume, not mass - inside the bubbles.

Perhaps 5% Nitrogen inside the bubbles makes such a dramatic difference in taste or bubble formation process, but somehow I really doubt it. I will bet the exact same effect can be obtained with any other inert gases with very low solubility, and the same effect can be obtained by mechanically pushing on the liquid at the right pressure, through small aperture, at low carbonation level, to agitate and precipitate gases (basically CO2) out of solution and into the foam.

Something fro Brulosophers to test.
 

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Now I understand you better. And the chemistry says this is just wrong - the pressure ratio of gases coming out of solution should generally NOT be the same as the pressure ratio of gases that got them in there.
I'm not sure whose chemistry you are using but the rest of the world's chemistry describes what I have been posting.

The chemical potential of a gas dissolved in a liquid is

mul = mul0 + R*T*ln(c)

where R is the gas constant, T the absolute temperature and c the concentration of the species in the liquid. mug0 is a constant representing the potential when the concentration is 1.

The chemical potential of the gas in the gas phase is

mug = mug0 + R*T*ln(P)

where R and T are the same but mug0 represents the potential when the partial pressure P = 1.

When gas and liquid are in equilibrium the chemical potentials are equal and there is no net exchange of gas between solution and head space

mug0 + R*T*ln(P) = mul0 + R*T*ln(c)

so ln(c) - ln(P) = (mul0 - mug0)/(R*T)

and the ratio of concentration to pressure is

c/P = exp((mul0 - mug0)/(R*T))

which is clearly the Henry coefficient.


If you dilute a beer gas, say with 1:1 ratio of N2 to CO2, you will end up with 100:1 ratio of CO2 to N2 at 0C (and very similar ratio at room temperature).

If you equilibrate water or beer with a 1:1 mix of N2 and CO2 the concentration ratio , cCO2/cNO2 , in mg/L will be approximately 100 to 1 but if you draw some of that liquid out of keg with a syringe keeping it under pressure (if we used 1 bar each N2 and CO2 that would be 2 bar) and then reversibly (that is a thermodynamics term which you should really understand the meaning of if you are to grasp this) decrease that pressure a tiny bubble will form. The partial pressures of CO2 and Nitrogen within that bubble will be 1:1 (to within an infinitesimal tolerance) because only an infinitesimal mass of either gas has moved to the bubble and Raoult's law still applies. mul0 and mug0 are still the same constants and so is R. For purposes of this discussion we'll assume T is the same.


If you take out some arbitrary volume of liquid (gently) and agitate it till you get gases out of solution, or boil the liquid or freeze it, whatever, and analyze, the amount of gases that will come out will be whatever is "stored" in liquid. So still 100:1 ratio in this scenario.
What you find in the gas depends on the volume to which you have expanded in the syringe or, invoking the gas law, the pressure within it. I put some numbers in a previous post for a 3:1 N2/CO2 ratio. When the gas (no bubbles here - we assume at this point we've put some hexanol or octanol in the beer) volume is 4% of the liquid volume (I started with 100 cc of beer at 4 atm and eased the plunger back to an extra 4 cc) the pressure (ignoring water vapour) over the liquid would be about 2 atm and the ratio of partial pressures 1:1. If I ease back another 4 cc so the volume of the gas is now 8% of the volume of the beer the pressure in that gas is (sans water vapor again) about 1.5 atm and the ratio of N2 to CO2 pressure is now 0.61. And it continues to drop until the volume of the gas is about 10 times the volume of the liquid at which point PaN2/PaCO2 is clearly leveling off at about 0.04 (not 0.01 ~ 100:1) By weight the ratio is 0.027.

You can do these calculations yourself. To get you started for CO2 the Henry coefficient is
0.03875*exp( 2400*( (1/T) -(1/298.15) ) )

and for N2 it is

0.000625*exp( 1300*( (1/T) -(1/298.15) ) )

with T in Kelvins.

R = 0.08206 // atm•L/mol•K

What you have to do is start with a volume of beer with x mol/L dissolved gas with no head space under some pressure. Now you start to add in head space incrementally. When the head space is Vh the moles of gas in it are P*Vh/(R*T) and the concentration of the gas dissolved in the beer is Vb*P*KHy. The sum of these two is Vb (volume of beer) times the initial concentration. Solve for P. It is then a simple matter to get the partial pressures and the ratio.



You are correct, this is by mass. By volume it will be 48/28 (atomic weights of CO2 and N2 respectively) smaller, so 1.7 times smaller, or 58:1. And if you use 3:1 N2:CO2 gas ratio pressure, you will end up with about 20:1 ratio - by volume. Still, that's about 95% CO2 and 5% N2 - by volume, not mass - inside the bubbles.
What you seem to be failing to take into account is the pressure of the released gas i.e. the volume to which the gas which escapes from the beer expands. What controls that is the surface tension of the beer. The bubbles are tiny and their internal pressure is inversely proportional to the radius. Thus the expansion is to only a small fraction of the beer volume and we are pretty high up on the N2/CO2 curve. I don't know what the radius of the bubbles is nor what the surface tension of Guiness is (but I'll bet the lab guys at Guiness do). It's quite plain from the physics alone that the ratio of N2 to CO2 is pretty high (not as high as 3:1 or maybe not even as high as 1:1). From the physics. Now let's take into consideration that there is a huge drop in CO2 chemical potential across the membrane of the bubble. The atmosphere is at 0.0003 bar CO2. But it as at 0.8 bar N2. I don't know what the diffusion properties of a Guiness bubble are WRT either of these gasses but it's pretty obvious the CO2 wants to get out much more than the N2 does. So I'm guessing that when you take this into account the ratio of N2 to CO2 is pretty high. Given the evidence (that the CO2 prick is reduced noticeably) we have further support for that notion.


I will bet the exact same effect can be obtained with any other inert gases with very low solubility,
I don't see why that shouldn't be the case.[/QUOTE]

and the same effect can be obtained by mechanically pushing on the liquid at the right pressure, through small aperture, at low carbonation level, to agitate and precipitate gases (basically CO2) out of solution and into the foam.
You'd lose that one for reasons given above. I used to say you could get 95% of the nitrogen effect that way but I'm lowering that now that I understand the process better.
 

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I'm putting up a plot for the syringe gedanken experiment. In this experiment we put a beer under 3 atm nitrogen and 1 atm CO2 and wait for equilibrium to be reached. We then equip our lab daemon with a syringe and lower him into the tank through a gas lock. He fills the syringe, caps it and locks the plunger in place, and returns to the lab with the syringe which is full of beer and is at pressure 4 atm (absolute). We now start to release the plunger lock such as to allow the plunger to come back a bit relieving some of the pressure. Bubbles form in the beer and rise to the top. There is no foam as he syringe was wetted with a miniscule drop of octanol. We measure the partial pressures of N2 and CO2 in the head space of the syringe and compute their sum and their ratio. These are plotted in, respectively, red and blue, against the ratio of the gas volume to the beer volume.

It is obvious that when the gas volume is tiny its composition will be the same as the head space gas in the keg: 3 atm N2 and 1 atm CO2 for a total pressure of 4 atm and a 3:1::N2:CO2 partial pressure ratio. It is also obvious when the plunger is drawn way back such that the gas volume is many times the beer sample volume that the total partial pressures will be very low and that their ratio will be ratio of the dissolved gasses molar concentrations in the beer:

3*N2Khy(5)/CO2Khy(5) = 0.0371124

The curves show what happens between these two extremes. Note that the pressure curve does not include the partial pressure of water vapour.

The mistake 55x11 is making is in assuming that the conditions in a stout bubble are represented by this low pressure region.

NitrogenStout.jpg
 

55x11

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I'm not sure whose chemistry you are using but the rest of the world's chemistry describes what I have been posting.

The chemical potential of a gas dissolved in a liquid is

mul = mul0 + R*T*ln(c)

where R is the gas constant, T the absolute temperature and c the concentration of the species in the liquid. mug0 is a constant representing the potential when the concentration is 1.

The chemical potential of the gas in the gas phase is

mug = mug0 + R*T*ln(P)

where R and T are the same but mug0 represents the potential when the partial pressure P = 1.

When gas and liquid are in equilibrium the chemical potentials are equal and there is no net exchange of gas between solution and head space

mug0 + R*T*ln(P) = mul0 + R*T*ln(c)

so ln(c) - ln(P) = (mul0 - mug0)/(R*T)

and the ratio of concentration to pressure is

c/P = exp((mul0 - mug0)/(R*T))

which is clearly the Henry coefficient.


If you dilute a beer gas, say with 1:1 ratio of N2 to CO2, you will end up with 100:1 ratio of CO2 to N2 at 0C (and very similar ratio at room temperature).

If you equilibrate water or beer with a 1:1 mix of N2 and CO2 the concentration ratio , cCO2/cNO2 , in mg/L will be approximately 100 to 1 but if you draw some of that liquid out of keg with a syringe keeping it under pressure (if we used 1 bar each N2 and CO2 that would be 2 bar) and then reversibly (that is a thermodynamics term which you should really understand the meaning of if you are to grasp this) decrease that pressure a tiny bubble will form. The partial pressures of CO2 and Nitrogen within that bubble will be 1:1 (to within an infinitesimal tolerance) because only an infinitesimal mass of either gas has moved to the bubble and Raoult's law still applies. mul0 and mug0 are still the same constants and so is R. For purposes of this discussion we'll assume T is the same.


What you find in the gas depends on the volume to which you have expanded in the syringe or, invoking the gas law, the pressure within it. I put some numbers in a previous post for a 3:1 N2/CO2 ratio. When the gas (no bubbles here - we assume at this point we've put some hexanol or octanol in the beer) volume is 4% of the liquid volume (I started with 100 cc of beer at 4 atm and eased the plunger back to an extra 4 cc) the pressure (ignoring water vapour) over the liquid would be about 2 atm and the ratio of partial pressures 1:1. If I ease back another 4 cc so the volume of the gas is now 8% of the volume of the beer the pressure in that gas is (sans water vapor again) about 1.5 atm and the ratio of N2 to CO2 pressure is now 0.61. And it continues to drop until the volume of the gas is about 10 times the volume of the liquid at which point PaN2/PaCO2 is clearly leveling off at about 0.04 (not 0.01 ~ 100:1) By weight the ratio is 0.027.

You can do these calculations yourself. To get you started for CO2 the Henry coefficient is
0.03875*exp( 2400*( (1/T) -(1/298.15) ) )

and for N2 it is

0.000625*exp( 1300*( (1/T) -(1/298.15) ) )

with T in Kelvins.

R = 0.08206 // atm•L/mol•K

What you have to do is start with a volume of beer with x mol/L dissolved gas with no head space under some pressure. Now you start to add in head space incrementally. When the head space is Vh the moles of gas in it are P*Vh/(R*T) and the concentration of the gas dissolved in the beer is Vb*P*KHy. The sum of these two is Vb (volume of beer) times the initial concentration. Solve for P. It is then a simple matter to get the partial pressures and the ratio.



What you seem to be failing to take into account is the pressure of the released gas i.e. the volume to which the gas which escapes from the beer expands. What controls that is the surface tension of the beer. The bubbles are tiny and their internal pressure is inversely proportional to the radius. Thus the expansion is to only a small fraction of the beer volume and we are pretty high up on the N2/CO2 curve. I don't know what the radius of the bubbles is nor what the surface tension of Guiness is (but I'll bet the lab guys at Guiness do). It's quite plain from the physics alone that the ratio of N2 to CO2 is pretty high (not as high as 3:1 or maybe not even as high as 1:1). From the physics. Now let's take into consideration that there is a huge drop in CO2 chemical potential across the membrane of the bubble. The atmosphere is at 0.0003 bar CO2. But it as at 0.8 bar N2. I don't know what the diffusion properties of a Guiness bubble are WRT either of these gasses but it's pretty obvious the CO2 wants to get out much more than the N2 does. So I'm guessing that when you take this into account the ratio of N2 to CO2 is pretty high. Given the evidence (that the CO2 prick is reduced noticeably) we have further support for that notion.


I don't see why that shouldn't be the case.
You'd lose that one for reasons given above. I used to say you could get 95% of the nitrogen effect that way but I'm lowering that now that I understand the process better.[/QUOTE]

I appreciate the response, and the explanation, I now understand it fully I think. I can double-check calculations but it looks about right.

I wasn't arguing with Henry's Law, I was just saying that ratio of gases won't be 3:1 entire time because liquid will quickly run out of N2 and so there will still be more CO2 in the foam than N2 - at 1 volume of CO2, liquid only has something like 0.05 volumes of N2, assuming beer gas. But you are correct, it's not going to be 95% CO2, assuming it's equilibrium process (I assumed most gases will end up in the foam eventually).

But is it really an equilibrium process? My feeling is that because we are forcing beer through Guiness tap with small aperture, turbulence is "knocking out" or precipitating dissolved gases without much regard for equilibrium.

If foam was all about equilibrium, the glasses of beer (normally carbed only) would end up with the same amount of head, but we know that the pour, length of line, serving pressure, type of faucet, can change the amount of foam dramatically.

Furthermore, you can take any glass of beer where the foam appears to be settling, insert a syringe, pull some liquid back and reinject it back, forcefully - and this will foam the beer more.

So my assumption is that the faucet can precipitate most of N2 and CO2 out in non-equilibrium pathway of some sort, by agitating the liquid.

But let's assume it's equilibrium process that proceeds gradually. Then it would appear first bubbles to form will have highest amount of N2 (3:1), and the last ones will be essentially pure CO2. Is that a factor in foam stability at all? Would the N2 dominated bubbles that formed immediately during pour be on top of the foam and CO2 bubbles at the bottom?

I definitely agree about the disproportionate ratio of partial pressures of CO2 and N2 relative to atmosphere. But could it be that the top N2 layer of bubbles somehow prevents bottom bubbles from bursting, becoming a sort of "membrane" that CO2 must diffuse through?

Anyways, thanks for clarifying the physics/chemistry, I think it's a bit clearer now, even though I am still confused on how equilibrium or non-equilibrium this whole process is.
 

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The mistake 55x11 is making is in assuming that the conditions in a stout bubble are represented by this low pressure region.
I admit it, I was wrong for me to think/assume the composition will be dominated by low-pressure ratio. I owe you a beer for this detailed explanation! :mug:

It's a beautiful (and informative curve) - I would guess most Guinness style stout pours will end up with head/volume ratio of about 0.1. Which seems to correspond to N2:CO2 partial pressure ratio of about 0.5.

Still blows my mind to think about how rapidly the composition of gases within nucleating bubbles shifts from 3:1 for early bubbles to something below 0.1-0.2 probably (so that the average is say 0.5).
 

ajdelange

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You ask lots of reasonable questions about where we are WRT equilibrium none of which I can answer except to say that we aren't at equilibrium but then in brewing we never are. Thermodynamics is great at telling us whether something can happen but not so good at letting us know whether it will nor how long it might take. Nonetheless, its about all we have in many cases and it is clear in most of them which direction we are going in.

It may be helpful to look at the same data in the form of a ratio vs bubble pressure (remember it is not really bubble pressure but rather head pressure in the syringe experiment which we are assuming is a reasonable proxy for bubble pressure). I think it is reasonable to suppose that bubble pressure is somewhat above 1 atm as the bubbles are sitting under 1 atm when in the glass and there is the additional internal pressure from the surface tension. Based on that it looks as if the ratio is going to be 0.2 to 0.4 (Nitrogen/CO2). But then I have made the thermodynamic argument that CO2 wants out more than N2 because of a much larger CO2 chemical potential gradient across the bubble film. This suggests that the bubbles upon reaching the surface and becoming foam bubbles will shrink by preferential loss of CO2. But as it is a thermodynamic argument I can't say if this does happen nor how long it might take if it did.

NitrogenStoutA.jpg
 

55x11

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You ask lots of reasonable questions about where we are WRT equilibrium none of which I can answer except to say that we aren't at equilibrium but then in brewing we never are. Thermodynamics is great at telling us whether something can happen but not so good at letting us know whether it will nor how long it might take. Nonetheless, its about all we have in many cases and it is clear in most of them which direction we are going in.

It may be helpful to look at the same data in the form of a ratio vs bubble pressure (remember it is not really bubble pressure but rather head pressure in the syringe experiment which we are assuming is a reasonable proxy for bubble pressure). I think it is reasonable to suppose that bubble pressure is somewhat above 1 atm as the bubbles are sitting under 1 atm when in the glass and there is the additional internal pressure from the surface tension. Based on that it looks as if the ratio is going to be 0.2 to 0.4 (Nitrogen/CO2). But then I have made the thermodynamic argument that CO2 wants out more than N2 because of a much larger CO2 chemical potential gradient across the bubble film. This suggests that the bubbles upon reaching the surface and becoming foam bubbles will shrink by preferential loss of CO2. But as it is a thermodynamic argument I can't say if this does happen nor how long it might take if it did.
assuming bubble size is about 100 microns (as reported by Guinness themselves) and surface tension of beer is about 40mN/m, the Laplace pressure should be on the order of 4*\gamma/R=1600 Pa, or about 1.6% of atmospheric pressure - small correction.
 

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I never put any numbers in but recall that a 1u bubble in water was a little over an atmosphere (1.4 atm - checked it). Doubling that for two interfaces in a foam bubble and noting that beer's surface tension is about half that of water's gets us back to about 1.4 atmosphere for a 1 u bubble and about 1.4% of an atmosphere for a 100u bubble which is ROM what you got. Now how about the size? A human hair is about 100 u and while it seems that the bubbles in the beer are perhaps that size the bubbles in the foam seem smaller than that based on visual inspection (not that I see that well in this range of sizes). In any case I don't think we could expect more than 0.1 bar (14u bubble).

I set a dial caliper for 100 u and looked at it under a dissecting microscope then drew 100 mL of stout (75% N2, 25% CO2) and looked at the foam. This is very crude because my 'measurement' is by comparison of how wide the caliper gap looks to how big the bubbles appear to be but I'd say there were no bubbles as big as 100u with the average size perhaps 1/2 - 2/3 that and quite a few less than that. Doesn't really change the conclusion but interesting.


EDIT: Wait a minute! That's radius we're talking about. I was looking at the diameter of the bubbles! So radii 50u and less for internal pressures of 2.8 - 10% of an atmosphere but 10% of an atmosphere would correspond to a 28u diameter bubble. Sill not a game changer but might as well have it right.
 

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I'd doubt it too but as the nitrogen content in the bubbles is 66% (in our example) and 75% in an actual Guiness serving set up the picture is far different.
I'm still really confused as to how you can say there is almost no nitrogen dissolved in the beer, yet the bubbles will form mostly full of nitrogen.
 

ajdelange

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Just the laws of physics at work. Nitrogen is not very soluble meaning that a lot of nitrogen pressure in a space over water (or beer) puts very little into solution. But the law works the other way too. It also says that very little nitrogen in solution puts lots of it into the space above the liquid.
 

BitterSipper

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Just the laws of physics at work. Nitrogen is not very soluble meaning that a lot of nitrogen pressure in a space over water (or beer) puts very little into solution. But the law works the other way too. It also says that very little nitrogen in solution puts lots of it into the space above the liquid.
OK, but there's basically no nitrogen to make many bubbles. If you actually try to dissolve pure nitrogen in beer/water etc, you get basically no bubbles, you would barely get any observable head and the beer would taste completely flat. Knowing that, to say the bubbles will be mostly nitrogen goes against basic logic. Maybe the first bubble.... but the main composition of the head will be essentially void of nitrogen and everything that effects your perception should be irrelevant to the nitrogen.
 

ajdelange

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All the physics have been set out with numerical examples in the previous postings in this thread. You just need to understand the physics to see how it works. See esp. the gedenken experiment with the syringe.

There is, in fact, quite a bit of nitrogen in a can of Guiness. Each can gets a couple of drops of liquid nitrogen just before it is sealed. That is enough to raise the pressure high enough in the pasturation tunnel to drive the beer into the widget and, expanded to atomospheric pressure would lead to quite a bit of gas.

Remember that Guiness does not have much head. A quarter inch is about right - at most, perhaps, a half. It is made up of tiny bubbles that are mostly nitrogen because of the laws of physics. Work the numbers given in the examples above and it should become clear. The fact that it is counter to your intuitions doesn't mean it runs counter to everyone elses. Yes, there are some nuances here involving surface tension and the Henry coefficient for the two gasses. You will have to understand those aspects of physical chemistry before you can understand how this works.
 

BitterSipper

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The fact that it is counter to your intuitions doesn't mean it runs counter to everyone elses.
I am not speaking of intuition I am speaking of experimental results. Again, try dissolving nitrogen in beer, you will get effectively zero head, and that would be from beer with a greater content of dissolved nitrogen.

Repeating "physics says" is a response of no value, and a poor rebuttal.

That combined with zero response of the valid logic that since there's many times more CO2 dissolved than nitrogen, that it's impossible for there to always be more nitrogen, makes me wonder if you understand the physics.

"if you can't explain it simply, you don't understand it well enough"
 

ajdelange

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I only repeat that you need to understand the p-chem because clearly you don't as you clearly demonstrate in your penultimate paragraph. I have dissolved nitrogen in beer with the result being just what the science predicts.

I am not so concerned about proper rebuttal as I am about helping you to understand this. You will not be able to do that unless you can follow the rather straightforward postings that precede this one. I can't explain it much more simply than I have already.
 
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