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Old 03-04-2013, 12:29 PM   #1
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Default The great nitrogen bubble debate

Ok, maybe there is no great debate but what the hell. We've all heard and digested the explanation of nitrogen beer's tiny cascading bubbles and contribution to creamy mouthfeel.

I first leaned the "science" of nitrogen bubbles from Cecil Adam's Straight Dope column and have been repeating this notion (which I now believe to be flawed). To wit, according to the column:

"From my online reading I learned that the secret of Guinness's creamy mouthfeel, as the taste experts put it, is a mix of nitrogen and carbon dioxide rather than pure CO2 as the bubblizing ingredient. Nitrogen bubbles are much smaller than CO2 bubbles, a mere 50 microns in diameter, and produce a smoother head. But nitrogen doesn't produce bubbles as spontaneously as CO2. At the corner tap they deal with this by using a special nozzle that aerates the stout with nitrogen as it's poured."

But of course, what is ignored is why the hell is CO2 needed at all in this mix? And why aren't there mixed populations of little nitrogen bubbles and large CO2 bubbles?

Here's what I now believe. CO2 is needed because the foam consists of CO2 bubbles. The bubbles are small because of the manner in which they are formed (by jetting the beer through tiny apertures). The role of Nitrogen is to provide the driving force (in other words, 6 PSI of Co2 would do a piss poor job of driving beer through a stout faucet). I've tried carbonating with pure CO2 then connecting beer gas and the beer pour perfectly right away without time for the equilibration of nitrogen gas in solution?

Your thoughts?

Link to Cecil Adams' Guinness column:http://www.straightdope.com/columns/...out-float-down

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Old 03-04-2013, 12:33 PM   #2
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Oh yeah, I forgot one thing. Has anyone ever tried no Co2? I use beer gas so it's formally impossible. But I'm betting straight nitrogenated beer poured through a stout tap will have minimal foam.

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Old 03-04-2013, 12:46 PM   #3
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Quote:
Originally Posted by choosybeggar View Post
Here's what I now believe. CO2 is needed because the foam consists of CO2 bubbles. The bubbles are small because of the manner in which they are formed (by jetting the beer through tiny apertures). The role of Nitrogen is to provide the driving force (in other words, 6 PSI of Co2 would do a piss poor job of driving beer through a stout faucet). I've tried carbonating with pure CO2 then connecting beer gas and the beer pour perfectly right away without time for the equilibration of nitrogen gas in solution?
What you say above is what I accept as fact regarding the concept. I've seen a lot of supposed "facts" about nitrogen having a stabilizing effect on foam, but the fact is that almost none of it dissolves. Nitrogen is around 100 times less soluble in water than CO2. So, nitrogen simply provides the "oomph" without additional effervescence, while CO2 provides the bubbles.

I've proven the concept to an extent by using argon in place of nitrogen when serving through my stout faucet. The pour is exactly as you'd expect with nitrogen. When I do that, I do not mix the gas. I carbonate to a low level, then push with pure argon.

What I have not done is a side-by-side comparison of the same beer pushed through a stout faucet with CO2, beer gas, nitrogen, and argon. It'd be an expensive test but definitely worth doing in the name of proper science.
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Old 03-04-2013, 04:36 PM   #4
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According to George Fix (Principles of Brewing Science, 2nd edition, p. 172):

CO2 is needed to maintain the equilibrium pressure of the beer (i.e., keep it at the desired level of carbonation). The reason that nitrogen produces a more stable foam is that a 75% nitrogen mixture is nearly in equilibrium with the air around it. Hence, there is a very small partial-pressure differential for nitrogen, and, since the amount of CO2 is reduced by 75%, the partial-pressure differential of CO2 is likewise reduced by 75%. In short, nitrogen-CO2 bubbles are much closer to being in equilibrium with the air than straight CO2 bubbles, because the air is mostly nitrogen.

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Old 03-04-2013, 05:18 PM   #5
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Though more believable, that still sounds a bit hokey to me. The bubbles in the beer's head are produced almost entirely by CO2 coming out of solution. The composition of the gas inside those bubbles is not 75% nitrogen, by virtue of nitrogen's low solubility. Only the gas in the keg's headspace is 75% nitrogen. I would very much like to see further research and experimentation on the topic.

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Old 03-04-2013, 05:27 PM   #6
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Quote:
Originally Posted by Gavagai
According to George Fix (Principles of Brewing Science, 2nd edition, p. 172):

CO2 is needed to maintain the equilibrium pressure of the beer (i.e., keep it at the desired level of carbonation). The reason that nitrogen produces a more stable foam is that a 75% nitrogen mixture is nearly in equilibrium with the air around it. Hence, there is a very small partial-pressure differential for nitrogen, and, since the amount of CO2 is reduced by 75%, the partial-pressure differential of CO2 is likewise reduced by 75%. In short, nitrogen-CO2 bubbles are much closer to being in equilibrium with the air than straight CO2 bubbles, because the air is mostly nitrogen.
I've heard that one and I formerly believed it. Now my belief is that a foam consisting of smaller CO2 pockets (a stout tap foam) is inherently more stable than one comprised of larger bubbles (for example, those forming spontaneously on the asperities of your beer glass).

Two lines of reasoning proffered so far in this thread refute the concept of concentration gradient-dependent foam stability. First, when I lightly carbonate a beer then hook up beer gas, immediately the beer pours perfectly, without an interval for nitrogen gas to equilibrate in the beer. Secondly, and more convincingly, is Yuri_Rage's observation that argon pressurized lightly carbonated beer pours identically to nitrogen-pressurized beer. Given there is virtually no argon in the atmosphere, the pressure gradient is ginormous. Yet, the foam is stable.
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Old 03-04-2013, 06:05 PM   #7
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I just did some non scientific experimenting with a stout faucet on 100% co2. I was indeed able to get the super dense "Guinness head" bubbles that you could scoop up with a spoon like pudding.

Now, it didn't stick around for more than a minute or two, but that's a product of the beer. This was just a blonde ale with zero head retention ingredients and not much head retention capability out of a regular faucet.

The key is low carbonation with sufficient pressure to overcome the stout faucet's restrictor plate. I strongly believe nitrogen contributes very little to nothing except for that required pressure.

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Old 03-04-2013, 06:56 PM   #8
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Couldn't the N2 get 'mixed' in with he beer during the pour? Meaning, couldn't the bubbles have a significant N2 content even though it wasn't initially dissolved in the beer to any significant degree?

I would have to guess that the smaller bubbles are a direct result of the restrictor plate in the faucet. The velocity of the beer knocks the growing bubbles off when they are small. With a standard beer, the size of the bubbles is much larger because they are nucleating on the glass, and are allowed to grow much larger by the time buoyancy frees them from the glass.

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Old 03-04-2013, 07:09 PM   #9
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Quote:
Originally Posted by Gavagai View Post
According to George Fix (Principles of Brewing Science, 2nd edition, p. 172):

CO2 is needed to maintain the equilibrium pressure of the beer (i.e., keep it at the desired level of carbonation). The reason that nitrogen produces a more stable foam is that a 75% nitrogen mixture is nearly in equilibrium with the air around it. Hence, there is a very small partial-pressure differential for nitrogen, and, since the amount of CO2 is reduced by 75%, the partial-pressure differential of CO2 is likewise reduced by 75%. In short, nitrogen-CO2 bubbles are much closer to being in equilibrium with the air than straight CO2 bubbles, because the air is mostly nitrogen.
That is echoed by the lecture series "The Brewmasters Art" which I think is available from audible.com. But the speaker (with endless awards and such, as you can hear from their sample) furthermore emphasizes a key point on why nitro bubbles have a disproportionate effect. Nitro bubbles do not cannibalize each other like co2 ones do. Small co2 bubbles adjacent to a large one get sucked into the large one thru the bubble interface... the small disappear into the large, which eventually pops.

Oh, here it is http://www.learnoutloud.com/Audio-Bo...ters-Art/34208
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Old 03-04-2013, 07:14 PM   #10
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Nitrogen will not get mixed into solution via pour turbulence because it gets forced into solution at the beer surface inside the keg (at a much lower rate than CO2), and the beer is drawn from the bottom of the keg. There is no turbulent interface in a nitrogen rich, pressurized environment.

My hypothesis is that small, dense foam is inherently more stable than large, loosely packed foam, regardless of gas composition. I am all but convinced that the bubbles are comprised almost entirely of CO2 that has been forced out of solution via the restrictor plate (i.e., little to no nitrogen content within the foam). I wish I could accurately test the nitrogen absorption in a beer gas system in order to further prove the theory. These discussions about nitrogen bubbles, while seemingly scientific, also seem to violate physics.

I do agree that the partial pressure of CO2 keeps the beer carbonated, though I have experienced success serving force carbonated beer for up to 3 weeks on pure argon alone without any need for reintroduction of CO2 to keep the pour consistent. I also agree that the lower carbonation and fine foam texture contribute to a different mouthfeel with less carbonic bite. I simply disagree that there is any significant nitrogenation happening.

Perhaps someone could design an experiment wherein beer carbonated to about 1.2 volumes is physically forced through a restrictor plate with no introduction of gas, be it nitrogen or otherwise. I'm suggesting a mechanical serving system for the purpose of showing that CO2 based foam is indeed stable. That pour could become the control in an experiment utilizing pure nitrogen, pure CO2, and beer gas for storage and serving. Pour each beer several times over the course of about 2 weeks, with the temperature and pressure of each keg kept stable throughout (except the CO2 keg, which would have to be dialed back for storage, lest it become overcarbonated). Note any differences.

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