What's the best way to purge oxygen from a keg?

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AiredAle

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There is a generally accepted belief that beers that are drunk a few weeks after kegging or bottling won’t show oxidation-related defects. I generally don’t have beers hanging around for months, but now and then I do age an IPA or a holiday ale for 6 to 12 months. I haven’t noticed any oxidation-related flavors, in any of my beers, so I’m content that my method of purging my corny kegs of oxygen is good enough, but I want data, not beliefs. Also, I don’t have the most sensitive palate, so maybe I’m missing the flavors.

I’m not interested in resuming this debate, so let’s assume for the sake of this thread that oxidation defects in home brewed beer are real, and that one source of oxygen is encountered during kegging.

So I need some help that only you brewing mavens can provide.. There is an on again-off again discussion on this and other boards about the best or right way to purge a corny keg to minimize oxygen content in the purged keg. Some folks pressurize and release the pressure several times (Method 1), some fill with CO2 from the liquid post to push the air out through the pressure release valve (Method 2), and some fill with sanitizer and push it out with CO2 (Method 3). No one seems to have any real data or can offer a diffusion equation that can be used calculate/estimate the amount of O2 that ends up in the CO2 purge gas. I want to find a diffusion rate constant for O2 diffusing into CO2, so I can calculate how much O2 is getting into my purge gas.

My belief is that Method 1, pressurize and release, is the least effective purge method, since it would take five 30 psi purges to get to 0.1 % O2, which I suspect is enough oxygen to cause trouble, even when most of the gas in the keg is pushed out by the incoming beer.

So, here’s my math to back up that statement. 30 psi (2 atmospheres) CO2 dilutes a corny full of air (one atmosphere of air) a dilution of 2:1. So starting with a keg full of air at atmospheric pressure and pressurizing and releasing with 30 psi CO2 reduces the O2 content in the keg like this, assuming good mixing in the keg: First purge cycle: 22% O2 to 7% O2, second purge gives 2.67% O2, third purge leaves 0.9 %, a fourth leaves 0.3 %, a fifth 0.1 %. If the purge gas pressure is lower than 30 psi, then even more oxygen is left in the keg after purging. Also I suspect few of us do more than 3 purge cycles, we leave even more O2 in the keg. Also this method uses a lot of CO2, roughly 88 grams, or about 3 ounces per purge, so three purges at 30 psi is a half pound of CO2.

I use Method 2, purging empty, sanitized cornies through the liquid in line with the cover in place and the pressure release valve open. I want to figure out how much oxygen is diffusing into the CO2 as I purge this way. I purge slowly enough that I think I can ignore gas mixing. It takes about ten minutes for me to get a strong CO2 odor at the pressure release valve, so I know I’m purging at about 2 liters per minute. I know that gases diffuse rapidly into other gases, but I can’t find any web based reference that gives a definite rate constant, and I can’t find my old physical chemistry text, so if you have any references, let’s have them. I’d like to know if my method is useless, or practical.

It seems intuitive to me that the most effective purge is Method 3, fill the keg with sanitizer, and push it out with CO2, since most of the air is already displaced with sanitizer. But, any air left in the keg will still let some O2 diffuse into the purge gas, or mix the air with the CO2 if the purge gas goes into the keg too fast. But how much?

Anyone here who can help? I believe that my method is adequate, because I don’t detect oxidation defects, and no one else has mentioned them, but I still want to be able to compare these methods on a quantitative basis.

Oh, and if you have a different purge method, what is it, and why do you think it effectively gets the O2 out of the keg?
 
Nope. All gases, heavy or light, will mix by diffusion. Heavy ones mix more slowly than light ones, but they all mix over time. They mix fast if the flow into a container is turbulent. Even if you could perfectly layer air on top of say, Xenon (atomic weight 131), a pretty heavy gas, they would end up completely mixed and stay mixed in time.

I want to figure out how fast O2 from the air mixes with CO2, so I can figure out how much ends up in my purged keg, and then in my beer.
 
assuming you have a small amount of head space in your keg when you first fill it, there's a relatively small amount of O2 in there to start with. If you purge multiple times after first connecting the gas, you will force out most of that small amount of oxygen before it has a chance to mix with the CO2. After that all you introduce is CO2, so the percentage of O2 by volume is going to decrease steadily over the consumption time. You will never get all of the oxygen out of the keg. I have never had an issue. So, the question is: what amount is acceptable to you? Personally I say that if there's no detectable change in the beer, then I'm below that threshold.
 
Are you using corny kegs?

I've found that I can fill them completely with liquid by going in the 'out' valve and bleeding sanitizer out the pressure relief valve. Then just force the liquid out with gas and you can be assured that the keg is void of air.

I also feel confident in 'blanketing' vessels by using a sparge stone on the end of the CO2 tubing. There are certain conditions in my cellar that cause the CO2 to become visible as it leaves the stone (picture Elvira), and it's then easy to see that the stone causes the gas to come out gently filling the vessel from the bottom up (assuming the stone is on the bottom, of course) with no mixing of air. This is also an effective method of completely evacuating a vessel of air, but I rarely go there with the size of my tanks. I have no data for you on this but I've seen it with me own eyes, and practice it on a regular basis.

CO2 (or argon) will have a temporary blanketing effect that lasts for at least an hour or so before significant diffusion occurs; a useful fact for transfers.​
 
No offense, this is way over thinking the matter, and is not "brew science". Most everyone on here "burps" their keg to purge, why, because it works.
 
and some fill with sanitizer and push it out with CO2 (Method 3). No one seems to have any real data or can offer a diffusion equation that can be used calculate/estimate the amount of O2 that ends up in the CO2 purge gas. I want to find a diffusion rate constant for O2 diffusing into CO2, so I can calculate how much O2 is getting into my purge gas.

Regardless of the equation, I suspect that the 3rd method will drive o2 down to as low a level as homebrewers are likely to get.
 
Viinc:

CO2 (or argon) will have a temporary blanketing effect that lasts for at least an hour or so before significant diffusion occurs; a useful fact for transfers.

Vinic, you may be right about the one hour, but what do you base the statement on? How do you know it's an hour? I can state that the time to significant diffusion is 1 minute, and neither of us can argue this out on a factual basis. That's why I want quantitative information or a method to estimate diffusion rates.

WildWest:

No offense, this is way over thinking the matter, and is not "brew science". Most everyone on here "burps" their keg to purge, why, because it works.

I know it's overthinking, but I still am interested in understanding the underlying physics that determine the amount of mixing. And if that's not 'brew science" what the devil is it? It certainly meets my definition of brew science.

Sure, burping works for most folks, but how do you know you aren't leaving oxygen behind if you can't quantify it. Taste varies among individuals and is subjective. I looking for a quantitative way to estimate oxygen levels, short of buying an expensive O2meter.
 
Viinc:



Vinic, you may be right about the one hour, but what do you base the statement on? How do you know it's an hour? I can state that the time to significant diffusion is 1 minute, and neither of us can argue this out on a factual basis. That's why I want quantitative information or a method to estimate diffusion rates.

WildWest:



I know it's overthinking, but I still am interested in understanding the underlying physics that determine the amount of mixing. And if that's not 'brew science" what the devil is it? It certainly meets my definition of brew science.

Sure, burping works for most folks, but how do you know you aren't leaving oxygen behind if you can't quantify it. Taste varies among individuals and is subjective. I looking for a quantitative way to estimate oxygen levels, short of buying an expensive O2meter.

It is going to be impossible to calculate the o2 pickup in a practical situation that can only be controlled to a certain extent. You would have to measure this empirically with, you guessed it, an expensive meter.

Why not just sulfite the beer and call it a day?
 
determining an explicit equation would be difficult, but here are a few considerations:

geometry is almost certainly a concern. diffusion can only happen where the gases are in contact.

you'd also probably want to account for the co2 that escapes the beer while it's being transferred.

i don't think you can assume a complete mixing of the two gases inside the keg before it is burped in method one. there will be some mixing, certainly, but i'd be surprised if the gas inside was completely homogeneous.

graham's law states that the relative diffusion rates of two gases is inversely proportional to the square root of their relative molecular masses.

also co2 is typically very cold when it comes out and i'd guess that any complete description would have to take thermal effects into account.

calculation of o2 content for the various methods is an interesting exercise, and you might get lucky and find a (relatively) simple equation that describes diffusion as a function of time, room temperature, co2 pressure, and corny keg cross section. but with so many variables and competing effects i don't think i'd be confident in any sort of model without good experimental data using cornies, a standard co2 tank, real beer, etc. if i were to embark on that sort of undertaking, i'd probably pick a phenomenon that has a greater effect on the flavor of my beer.
 
You left out cask conditioning as a method of removing oxygen. You rack your beer into the corny with some corn sugar solution added (same as you would add to your bottling bucket if bottling). Seal the corny, purge a few time with CO2. Leave at a few psi and let the yeast consume the O2 and generate the CO2. It takes a little longer but you have efficient removal of O2 by the yeast and you have active yeast for sometime in the keg which is known to extend shelf life of bottled (and kegged) beers. No equations needed. :ban:

Dr Malt :mug:
 
BierMuncher,

Why didn't I think of that? I assume you have some of Maxwell's demons assisting you with sorting the O2 from the N2 in the air? Can you send me a breeding pair?

Dr Malt,

Good idea, kegging before complete fermentation would work as well.

John from DC,

I know (well I believe...) oxidation isn't a major issue with my beer; I am more interested in this issue because it is a frequent question/discussion point on this and other boards, and was hoping we could come up with some quantitative basis for deciding what's the best way to purge a keg. Your points are valid; it is a difficult phenomenon to pin down.

I was hoping to get some better idea of how fast O2 from air diffuses through the CO2. Something along the lines of it takes one minute to diffuse 100 ppm of O2 one inch into the CO2 at the air-CO2 interface, just so I have some basis to decide on how I want to purge.

At this point, I think I will switch from purging air filled cornies to sanitizer filled cornies. No math needed, just displace the maximum amount of air with water and then displace the water with CO2.

So, thanks to all for the ideas.
 
I flood the keg with CO2 for one minute with the lid off, rack the beer in to the bottom gently, seal it, flood it, vent it. If there's still O2 in there after that, F it.
I do the exact same thing. I have an 8 tap kegerator and don't drink very much beer, so my beer is usually around for months and I have never had an issue with oxidative staling.
 
but you have efficient removal of O2 by the yeast

Not really, in a bottle the yeast will consume about a third of the o2 in the headspace. Headspace/liquid space in a keg is smaller, but I wouldn't consider this dependable near zero oxygen packaging a priori.
 
Remilard:

In a bottle, air can diffuse under the cap resulting in O2 in the head space. The major brewers use oxygen scavenging caps to absorb this oxygen to extend shelf life. So whether the yeast are consuming all the O2 or it is equalibrium with diffusion is a question. I know that active brewers yeast in a closed container removes O2 to less than 1 ppb.

Dr Malt
 
What I do is seal the keg while empty hit it with a blast of Co2 then purge, repeat 3 times.
Fill the keg from the Out post - purge one or two more times and I am done.
 
I flood the keg with CO2 for one minute with the lid off, rack the beer in to the bottom gently, seal it, flood it, vent it. If there's still O2 in there after that, F it.

For real, I have used this and kept beers on tap for months with no ill effects.

I have even transferred commercial keg beer into cornies using the same method and it has kept for quite awhile at least until it was consumed.
 
Remilard:

In a bottle, air can diffuse under the cap resulting in O2 in the head space. The major brewers use oxygen scavenging caps to absorb this oxygen to extend shelf life. So whether the yeast are consuming all the O2 or it is equalibrium with diffusion is a question. I know that active brewers yeast in a closed container removes O2 to less than 1 ppb.

Dr Malt

Well obviously there is a limit to how much oxygen yeast can scavenge. A fermenting carboy in a closed room will not remove the oxygen in the room.

Various sources, George Fix come to mind, have reported that the yeast in bottle conditioned beers scavenges less than half of the oxygen present in the headspace. I would certainly entertain information to the contrary.
 
I thought we were discussing how to remove O2 from a closed corny keg. Where did removing O2 form an entire room come from??

Yesat in an enclosed container like a corny keg or sealed conical fermenter will lower the O2 in the liquid to less than 1 ppb. I know this as we measure the O2 on 8000 gallon fermenters every day.

Dr Malt
 
Yesat in an enclosed container like a corny keg or sealed conical fermenter will lower the O2 in the liquid to less than 1 ppb.

I'm just beginning to understand this subject, but noticed the highlighted words. This is true, I don't think anyone's arguing it. The yeast will take up oxygen in the beer while they're active, but what about the oxygen absorbed from the headspace after they run out of sugar? They won't actively pull oxygen from the headspace, so it's going to diffuse at a normal rate, which will keep happening long after the yeast go dormant.
 
I thought we were discussing how to remove O2 from a closed corny keg. Where did removing O2 form an entire room come from??

Yesat in an enclosed container like a corny keg or sealed conical fermenter will lower the O2 in the liquid to less than 1 ppb. I know this as we measure the O2 on 8000 gallon fermenters every day.

Dr Malt

I'm just beginning to understand this subject, but noticed the highlighted words. This is true, I don't think anyone's arguing it. The yeast will take up oxygen in the beer while they're active, but what about the oxygen absorbed from the headspace after they run out of sugar? They won't actively pull oxygen from the headspace, so it's going to diffuse at a normal rate, which will keep happening long after the yeast go dormant.


I am interested in the answer to this question. Does priming with sugar help reduce oxygenation vs. force carbing?
 
Thanks for all your answers to this question. Happily, for me at least, I have found the answer about how fast O2 diffuses through CO2. I had a conversation with a colleague of mine who is a PhD material scientist, and she handed me a book written by one of her professors at the University of Minnesota. The book is called “Diffusion, Mass transfer in fluid systems”. Lest you doubt me, the author is E. L.Cussler, and it’s published by Cambridge Press.

Working through the chapters on gas-gas diffusion, not only did I realize why I am not an engineer, but I came across a table of diffusion constants for gas pairs, and more important, an equation to calculate penetration distance of the diffusing gas as a function of time, as room temperature and one atmosphere pressure. So, armed with my calculator, and the table I calculated the distance O2 diffuses into CO2 in one second, one minute and one hour. The answers surprised me.

The equation is quite simple, and the assumptions are too, RT, one atmosphere pressure, no convectional mixing. Distance (Z) is in centimeters, time (t) in seconds, diffusion constant is D. The equation is –

Z = the square root of (D x t)

D for O2 into CO2 is 0.16 cm squared/second

So, in the first second Z is 0.4 cm
First minute Z is 3.0 cm
First 5 minutes Z is 7.0 cm
First hour Z is 24 cm

I was surprised that the diffusion is as slow as it is. Even in an hour, O2 won’t penetrate to the bottom of a corny keg if it’s in the air being pushed out of the keg by a CO2 purge.

Subsequently I looked at gas diffusion into liquids, and it is generally about 10,000 times slower. For example O2 in water, the diffusion constant is 2.1 x 10 to the -5 power. So in one minute O2 diffuses only 0.0126 mm or 0.5 thousandths of an inch. Assuming no mixing…

So, as I thought about this, I realized that the any method we have been using to purge air from corny kegs will be fine, as long as air or O2 don't get much time to diffuse into the beer.

Now, I realize this argument is based on ideal conditions, and the real world is less than ideal with different temperatures of gases and liquids, mixing of gases regardless of how careful we are, and so forth, but this does help me get a grip on this issue. I also realize that in the real world, careful homebrewers rarely experience oxidation related problems in their beer. And this makes sense to me, because the common thead to most of the method folks described don’t give the beer time to pick up appreciable amounts of O2 by purging before or immediately after racking.

This also explains the reason we shake our kegs to carbonate quickly: to expose fresh liquid to the CO2 because leaving it on it own to diffuse through 18 or so inches of beer in a corny with no mixing is dead slow.
 
At this point, I think I will switch from purging air filled cornies to sanitizer filled cornies. No math needed, just displace the maximum amount of air with water and then displace the water with CO2.

So, thanks to all for the ideas.

But when you blow the water out, won't there be little drops
still clinging to the side, and those drops will have 02 dissolved
in them, unless you boil it first and then cool it under an
inert atmosphere? Or maybe you could buy a shed, airtight
it, then put some giant CO2 cylinders outside and purge
the entire building, and do your brewing in a spacesuit?
Jim:D
 
Various sources, George Fix come to mind, have reported that the yeast in bottle conditioned beers scavenges less than half of the oxygen present in the headspace. I would certainly entertain information to the contrary.


The oxygen in the headspace isn't doing anything. Nothing happens
until it dissolves in the liquid and a reaction can take place. So you really
need to know how much of the O2 in solution gets taken up. I would guess
all of it until the yeast dies or becomes dormant.
Jim:mug:
 
The equation is quite simple, and the assumptions are too, RT, one atmosphere pressure, no convectional mixing. Distance (Z) is in centimeters, time (t) in seconds, diffusion constant is D. The equation is –

Z = the square root of (D x t)

D for O2 into CO2 is 0.16 cm squared/second

So, in the first second Z is 0.4 cm
First minute Z is 3.0 cm
First 5 minutes Z is 7.0 cm
First hour Z is 24 cm

Something is wrong here because gases diffuse far more quickly
than that. If you open a bottle of some smelly organic molecule
like benzene thiol it doesn't take minutes to travel to your nose,
and that's a much bigger and slower moving molecule than O2.
Anyway, these numbers would only apply with no flow of CO2,
just a static amount of CO2 in the keg.
Jim:mug:
 
The oxygen in the headspace isn't doing anything. Nothing happens
until it dissolves in the liquid and a reaction can take place. So you really
need to know how much of the O2 in solution gets taken up. I would guess
all of it until the yeast dies or becomes dormant.
Jim:mug:

Right, and according to numerous sources the yeast die or become dormant with around half of the oxygen remaining.
 
Thanks AiredAle. I was about to get out some corny kegs and do some testing with a really expensive O2 meter I was hoping to borrow.

On the practical side, like others here, I have not experienced any oxidation type off flavors in my kegged beer using either the cask conditioning with the 3 purges of CO2 on filling, or the 3 purges on filling followed by forced carbonation. It is not that I consume the product fast either.

Dr Malt
 
What is happening when your thiophenol bottle is opened is that mixing is taking place by both diffusion and convective mixing of the air and the thiophenol. The thiol diffuses out of the bottle and is blown around by the air currents in the room. Plus, the odor threshold for thiophenol is about 1 ppb, so very little will escape the bottle before you smell it and wish you had opened it in a fume hood.

You’re right the numbers only apply in a static situation. My point in my post was that this is a baseline, and that one assumption was no mixing by any method but diffusion.
I also wanted to get some real data regarding this issue, because it is so often debated on this and other forums but no one has any basis in fact to argue from. One only reads, “diffusion is fast”, and “I never have any problems with oxidation with my way of keg purging, so it must be good”.

In fact the numbers I posted overstate the rate of diffusion because the rate is also dependent on the pressure of the gases diffusing into each other. The pressure of oxygen in air is not one atmosphere but 21 % of one atmosphere, since the concentration in air of oxygen is 21%.
 
Dr. Malt,

Please do the measurements, that's exactly what we really need here, some experimental data to answer the question. I'd do the measuring but I don't have access to an O2 meter. Pity you're in the NW, I'd show up and help you do it.
 
Thanks for all your answers to this question. Happily, for me at least, I have found the answer about how fast O2 diffuses through CO2. I had a conversation with a colleague of mine who is a PhD material scientist, and she handed me a book written by one of her professors at the University of Minnesota. The book is called “Diffusion, Mass transfer in fluid systems”. Lest you doubt me, the author is E. L.Cussler, and it’s published by Cambridge Press. < clip>........

Thanks, AiredAle, for sharing that information and for this thread. Ifound it relevent to the hobby and stimulating. I have enjoyed it. In fact, I have looked for one particular physical chemistry book from college days that I thought contained the info, but haven't been able to locate it. thanks again..
 
If you really, really, REALLY, want to get the O2 out, and you have some cash and skills.

Buy one of these:

http://www.pemed.com/physof/madapump01_95.jpg

Its called an aspiration pump. (Used in respiratory)

Hook it up to the out post, turn the keg upside down, and turn it on. After a few minutes, most all air will be pulled from the keg. Once that is accomplished, fill with CO2 on the in post.

Personally, the 'burp' method is sufficient for me.
 
C2H5OH,

Thanks, I'll stick to my slow fill from the "out" post method. I suspect you would either collapse the corny with your vacuum pump, or the lid O-ring will leak like a sieve since it is designed to seal under pressure not under vacuum. Visually a cool experiment to try though, imploding a corny keg. You could also put a pint of water in the keg, seal it up but open the PRV, put it on its side over your turkey fryer burner, boil the hell out of it for ten minutes to purge the air with steam, turn the flame off, close the PRV and see if it impodes on cooling....
 

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