The "protective co2 blanket"

Many times I see advice on this forum, to not worry about oxidation and leaving beer in the primary for months because of this "co2 blanket" that protects the beer.

It's been many years since my school classes, but if I remember the Ideal Gas Law and other physics principles, it's never 100% co2 that is in the fermenter and of course the co2 dissipates. If it didn't, we'd all die in our sleep from c02 poisoning.

But when I pointed that out, I was told that of course the oxygen is gone from the fermenter (that's why it's fine to swirl the fermenter to rouse the yeast) because of this "co2 blanket".

I know that there are other gasses in a fermenter, and I know that co2 will dissipate as it searches for equilibrium (hence the airlock bubbling). But can some of our great brains give a better explanation of this? It really bothers me when other brewers say not to worry about oxidation due to the co2 blanket that seemingly would hang around forever.

I doubt the headspace in a fermenter is 100% CO2, but it is probably pretty close. During active fermentation there is a lot of CO2 produced and obviously pushing a large volume of gas out of the fermenter (bubble bubble). I think that the shear volume of CO2 activity pushes most of the air that was originally in the fermenter out.

So while you will likely have trace amounts of O2 and other gases in the headspace, I doubt it is enough to cause problems. Now, if you transfer to a secondary vessel after fermentation has stopped, your theory would make sense to me. Oxygen would be present, would diffuse into the CO2 and cause some oxidation. That is why I advocate (when using a secondary vessel) to transfer after 2/3-3/4 of the fermentables are gone and the beer is still actively fermenting.

Hopefully someone can come in a drop some actual data or citations from someone smart. It is a very good question.

as I understand it-

fermentation creates co2. as that comes out of suspension, it moves into the headspace. that additional pressure causes gas to move out of the airlock. that gas is a mixture of air (which is a mixture of gases) and CO2 that you've just created.

you had a finite amount of air originally, to which you've been introducing CO2 and expelling the mixture continuously. So, now there is a pretty high concentration of CO2 in your headspace. once fermentation is over, there's nothing to force gas out, so the CO2 stays there. nothing can get in since you have an airlock. so, I'm not quite sure what your comment about CO2 dissipating means, but it can't leave the container.

also, since co2 is denser than o2, it should settle to the bottom of the headspace.

in my case...
when i have brews in the primary fermenter for awhile, i pull the lid/cap off the fermenter and take a whiff and smell instant Co2 (the burns...) and this goes on for quite awhile (i love sniffing as they go). never tested it past a month and a half though...

However, if i was worried about it, i'd just give it a shot of Co2 from my keg tank. OR! put some fermentables in to have the yeast eat and make more Co2 with it to help cover that "blanket" back up
Thats just my thinking and what i would do, but i honestly always get the burns of the nose when i smell my beer thats been in primary for a few months.

Yooper said:

it's never 100% co2 that is in the fermenter and of course the co2 dissipates. If it didn't, we'd all die in our sleep from c02 poisoning.

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If we slept in hermetically rooms indeed we would die. This is how one suffocates.

Yooper said:

It's been many years since my school classes, but if I remember the Ideal Gas Law and other physics principles, it's never 100% co2 that is in the fermenter and of course the co2 dissipates. If it didn't, we'd all die in our sleep from c02 poisoning.

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If you were sleeping inside of an airlocked chamber, you certainly would die of asphyxia.

But we don't - and the air around use is continually being circulated and mixed (even if not very quickly), and the CO2 we exhale is being consumed by other organisms that produce O2 as waste.

There's extremely little to no mixing going on inside a fermentation chamber, and as CO2 is produced, the heavier CO2 displaces the lighter O2 through the airlock (as well as N2 and other lighter trace gases). There may be some degree of mixing going on, but it likely isn't much to worry about - there is very little gas movement to provide for mixing. Once active fermentation has kicked off, there isn't going to be much O2 left inside the airlocked fermenter, and very very little of that will be in contact with your wort.

If we were so unlucky as to live in a universe where molecular oxygen was lighter than the waste gases produced by carbon dioxide, we'd be screwed as homebrewers.

Semi-useless trivia: That "starved for oxygen" feeling you get from holding your breath for a long time underwater (for example) is not from oxygen starvation - it's from excess carbon dioxide in your lungs.

Argon is heavier than the normal N2/O2/trace gas mixture in the air at STP, but if you purge a flask with Ar and put it on a balance, you will see just how quickly diffusion works, as the weight will drop and equilibrate in minutes with a 50L flask. The difference in weight between O2 and CO2 would not enough to counteract diffusion over any period of time based on my experience. If you have a sealed system (such as a sealed fermenter with an airlock, check valve, or other backflow preventer), that is another matter.

Generally, to render an atmosphere "oxygen free" you must perform a series of three pressure (>30psig) and vacuum (<0.2psig) flushes using an inert gas, which will bring the O2 down well below 0.1%.

My conclusions? The headspace in a fermenter would not be completely purged of O2 by the CO2 generation in the fermenter, but that wouldn't make any difference, given the fact you are not processing it (racking, bottling, etc.) under an inert atmosphere anyways. Diffusion should occur through the airlock medium (water or sanitizer) over a prolonged period of time when no positive pressure exists in the fermenter (that is, after primary fermentation is complete), but this should not matter, either, as you are still not processing it under an inert atmosphere.

How would the CO2 dissipate in a closed fermenter? As the co2 builds up and pushes out most (notice I didn't say ALL) of the oxygen and other gasses through the airlock, then you have a situation where the great majority of the gasses inside the fermenter is co2. With the airlock in place, the only way that can change is if one were to remove the airlock and/or lid of said fermenter. Once that airlock stops bubbling, nothing is getting out, but also nothing is getting in. In order for that co2 to dissipate, one would have to release it to open air. Until then it is trapped.

Even if it's just slightly dropping oxygen concentration, the CO2 buildup in the headspace dramatically effects the speed of oxidation. It doesn't have to be anywhere near perfect for the beer to be good, even over a long time. It just needs to be "good enough" to meet the expectations of the recipe and our taste buds.

Reminds me of another fun fact: The average adult human body produces around an ounce of alcohol a day, as a normal metabolic process (this varies with diet, metabolism, and activity), and processes this as a matter of course.

So in other words, "sober as a judge" is still based on the expectation of having about an ounce of alcohol filter through your body throughout the day. This natural level is also taken into account when they measure BAC.

My point is that if we stopped producing alcohol in our own bodies, or indeed we did brew in a completely oxygen free environment, we'd probably notice a big difference from what's currently accepted as normal, and it wouldn't necessarily be a positive change.

Bernie Brewer said:

How would the CO2 dissipate in a closed fermenter? As the co2 builds up and pushes out most (notice I didn't say ALL) of the oxygen and other gasses through the airlock, then you have a situation where the great majority of the gasses inside the fermenter is co2. With the airlock in place, the only way that can change is if one were to remove the airlock and/or lid of said fermenter. Once that airlock stops bubbling, nothing is getting out, but also nothing is getting in. In order for that co2 to dissipate, one would have to release it to open air. Until then it is trapped.

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Diffusion would allow CO2 to carbonate the liquid in the airlock (even without bubbling action), and oxygenate it as well, allowing transfer of each to the other side. The point of the airlock is to block most of this, but it's not completely impermeable.

Damn! I think that dominatrix whip might actually be fishing line.

But without any real science being presented I&#8217;ll go with your equilibrium theory. Always chuckle at the 4 weeks in the primary, don&#8217;t worry if your airlock doesn&#8217;t bubble folks. There&#8217;s only one reason an airlock wouldn&#8217;t bubble. Leaks. If C02 is leaking out, O2 is getting in during an extended primary.

I'd think that in a sealed fermenter, if you don't pop the top and if it doesn't suck back, you&#8217;ve got a relatively O2 free environment. Take a bunch of hydo samples and you&#8217;re at risk.

Figure out how to seal your fermentor and how to hold a constant temperature and bubbles will be your friend.
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It&#8217;s your fault!

Agreed with those the posts above, you won't have oxygen in your headspace unless you remove the airlock. The second you remove the airlock, your co2 blanket is no longer 100% co2. How much o2 gets in is up for debate. You could essentially not have to worry about oxidation in a glass carboy, yeast autolysis would be more of a concern during extended aging.

Plastic buckets are another story, as HDPE is oxygen permeable. So while there might be co2 in the headspace, o2 will make its way in through the surface contact with the bucket. It's a slow process, but it definitely happens.

CthulhuDreaming said:

Semi-useless trivia: That "starved for oxygen" feeling you get from holding your breath for a long time underwater (for example) is not from oxygen starvation - it's from excess carbon dioxide in your lungs.

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Some what true. For most people the "starved for oxygen" feeling making them come up for air would be the fact that they took a deep breath and holding it. If the person is not a avid free diver there body is simply not used to having there lungs so "full"(most likely breathing from the chest so thats only 1/3 of there actual lung volume) and there mammalian reflex has not been used frequently. Also the person probably didn't breathe up correctly but thats getting way to deep into the actual science of it

You are correct that the body does not have a sensor that detects how much O2 is in our blood but how much CO2 there is. The urge is your body complaining about the excess its not used to having. If you hold it for another 45 seconds to a min it will go away. Only to come back a few min later.

Sorry for the book, But free diving has been my favorite hobby most of my life.

grouperdude said:

But free diving has been my favorite hobby most of my life.

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The Big Blue is one of my favorite movies of all times. I've looked at the science of free diving, never thought that it might apply to beer science...

I guess I'm not being really clear. I'm coming at this from a winemaker's perspective, as I was a winemaker first. Long term aging requires very little headspace, to prevent oxidation. The co2 that forms that "blanket" isn't 100% c02, and it does dissipate.

Something Jack Keller recently said was this: "The thing to remember (and part of what was discussed previously) is that CO2 is a gas and follows the laws of all gases, which means it will act in accordance to the natural properties it is endowed with and react to environmental influences, but one gas does not push all other gases from a space just because it is heavier or colder or newly introduced.

In a closed environment, like a carboy with an airlock, CO2 created by fermentation will quickly fill the ullage and "push out" most of the air that was in that space through a process best thought of as pressurized dilution. But even after 5-6 days of vigorous fermentation and continuous bubbling away, if you measured the air in the ullage you'd be surprised to find there is still some oxygen, hydrogen, helium, etc. in there. You wouldn't think so, but there is. It won't be much, but there will be some."

He talks about sparging with co2 or argon at racking to mitigate some of the loss. Sulfites are used, as an antioxidant, to protect the wine as well.

if you have a closed vessel, how is it going to dissipate? where is it going to go, and what is going to replace it?

Yooper said:

if you measured the air in the ullage you'd be surprised to find there is still some oxygen, hydrogen, helium, etc. in there. You wouldn't think so, but there is. It won't be much, but there will be some."

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yeah, this is exactly right. co2 will still be mixed with air. but, again, it won't be much. now as soon as you pop the bung to rack, then you're introducting more air.

Looked at from a thermodynamic perspective the chemical potential of oxygen in the air is about the same as the chemical potential of oxygen in the water in the airlock because it is in contact with it but that chemical potential is higher than that in the headspace because, we hope, the partial pressure of O2 in the head space is much lower than it is in the atmosphere. Thus oxygen will flow from the lock to the headspace entering it at the top. Now there is a potential difference between the top of the headspace, near the air lock and the bottom near the surface of the beer. Oxygen will migrate to relieve that potential difference. Does any of this happen to an appreciable extent? No, probably not. And it wouldn't take place very fast either.

Bubbling an inert gas through a liquid will purge it of other gases. I've used it in the lab to generate oxygen free buffers (what a pain to work with!). So besides the yeast consuming what oxygen is in the wort initially, the CO2 evolution will scrub some O2 out as well. Once fermentation has stopped, we are all familiar with the fact that when we pull a sample for a hydrometer test, it is partially carbonated. The little bit of liquid in our airlocks is enough to "seal" the fermenter to allow some pressure to build up, and keep some of the CO2 in solution. This CO2 in solution, will be in equalibrium with that in the headspace, so as long as you have some bubbles in your beer, the headspace should be fine.

Now, that liquid in the airlock is exposed to the outside air as well as the air in the headspace. I don't have a good idea of how fast it occurs, but 02 from the outside will certainly dissolve into the airlock liquid, and then could diffuse from there into the headspace. During active fermentation, it would be scrubbed out, but once that stops, there could be a slow movement of oxygen into the headspace. Again, I do not know how fast this will occur, but according to the gas laws, it will happen. They way you brew, I don't think it is an issue. Now for others who do long primaries, or in wine making, perhaps this could be an issue.

Perhaps this is why many really big beers take on sherry notes (a good thing). Even though the airlock was on and filled, because they were femented and aged so long before packaging that enough O2 makes its way in so the sherry notes (oxidation) can be produced. Not being a wine maker, don't the pro's actually seal up the wines for aging? Perhaps if one wants to age for a long time, they should replace the airlock with an impermeable seal of some sort.

Maybe I'll brew one of my rare big beers (I do need to finish off some grain bags) and as soon a fermentation is complete, rack it to a keg (purged) and let it age there as opposed to in a carboy as I have done it the past.

Raenon said:

Diffusion would allow CO2 to carbonate the liquid in the airlock (even without bubbling action), and oxygenate it as well, allowing transfer of each to the other side. The point of the airlock is to block most of this, but it's not completely impermeable.

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Agreed it is not 100% impermeable. But it is plenty impermeable enough (if that makes any sense at all) from a homebrewer's standpoint. RDWHAHB!!!

Yooper said:

In a closed environment, like a carboy with an airlock, CO2 created by fermentation will quickly fill the ullage and "push out" most of the air that was in that space through a process best thought of as pressurized dilution. But even after 5-6 days of vigorous fermentation and continuous bubbling away, if you measured the air in the ullage you'd be surprised to find there is still some oxygen, hydrogen, helium, etc. in there. You wouldn't think so, but there is. It won't be much, but there will be some."

.

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Yes there will be some. But not enough to get into a lather about. RDWHAHW!!!!! (Relax, Don't Worry, Have A Homemade Wine!)

Bernie Brewer said:

Agreed it is not 100% impermeable. But it is plenty impermeable enough (if that makes any sense at all) from a homebrewer's standpoint. RDWHAHB!!!


Yes there will be some. But not enough to get into a lather about. RDWHAHW!!!!! (Relax, Don't Worry, Have A Homemade Wine!)

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Oh, yes, I know that it's not really "much". But as a winemaker, we take care to top up and to use antioxidants. And each time you open the fermenter after fermentation is finished, especially with buckets and lids, there is going to be some gas exchange.

What brought all this up was discussing why not to use a bucket for "secondary". The protectiveness of the co2 blanket would be ineffective by then.

As you increase the headspace pressure, you're forcing gasses out of the airlock. The CO2 and O2 are mixed fairly evenly in the headspace. The idea is to constantly push this gas mixture out of the headspace, thereby decreasing the overall amount of oxygen present (you're pushing out a mixture of CO2 & O2). Once you reach the end of primary fermentation, presumably you have a very low overall volume of O2 present, and its concentration is low relative to the CO2.

Unless you're creating a vacuum in your carboy, then purging & filling under CO2 pressure, you won't get rid of O2 in the headspace. Decrease the amount of oxygen present overall, however, and you'll essentially remove the effects of oxidation.

There was a great thread on this from last year as well:

https://www.homebrewtalk.com/f14/blanket-co2-259954/

Dollars to donuts the exchange over the airlock medium is faster than the HDPE, yet many people worry about HDPE being used instead of glass or PETG...

I remain unconvinced it makes much difference given the handling during processing.

RickFinsta said:

...Generally, to render an atmosphere "oxygen free" you must perform a series of three pressure (>30psig) and vacuum (<0.2psig) flushes using an inert gas, which will bring the O2 down well below 0.1%...

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This seems like a lot of trouble to get a low O2 environment. Are we really achieving anything by purging a vessel with CO2 before a transfer?

RickFinsta said:

Dollars to donuts the exchange over the airlock medium is faster than the HDPE, yet many people worry about HDPE being used instead of glass or PETG...

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This is something that's always irked me a little too. The outwards pressure from the liquid on the walls of the vessel are higher than the air pressure from the outside, so how would the gas push through? I guess things work a little different at the molecular level though...

RickFinsta said:

Dollars to donuts the exchange over the airlock medium is faster than the HDPE, yet many people worry about HDPE being used instead of glass or PETG...

I remain unconvinced it makes much difference given the handling during processing.

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Exchange is based on surface area, an airlock with a low surface area has much slower diffusion than a plastic bottle with large surface area, this is why barrel aging oxidation is better replicated by an oak dowel in a carboy neck rather than a small oak barrel.

In regards to the gas system which exists above your beer/wine/coolaid being fermented, really you are looking at a probability model. The probability that oxygen or CO2 are pushed out is relative to the amount of the gas in question in the headspace, thus when fermentation starts: lots of air, lots of O2, lots of O2 pushed out, as CO2 becomes the dominant gas, predominantly CO2 gets pushed out, HOWEVER, on occasion you knock out one of those pesky O2 molecules, until effectively the amount of O2 in the gas above your liquid is negligible (in regards to oxidation/spoilage of beer).
As soon as you open the carboy however, you start to disrupt this equilibrium. You re-introduce air and thus you re-start the probability excercise above, so if you want to regenerate the minimal O2 environment purge you must! Basically, the yeast's CO2 production acts like a constant purge during active fermentation, but once they stop gassin' you need to start gassin' (again assuming this is a big deal to you).
It seems prudent, however, to step back and consider the process of fermenting and creating beverages, lest we all start buying 50lb CO2 tanks and purging needlessly.
Diffusion at the liquid-gas interface is the process which causes spoilage, so we desire to minimize this process. Now, generally, unless you are opening a carboy A LOT once fermentation is complete you shouldnt see too much issue as the liquid is saturated with CO2, which would need to be forced out and replaced by oxygen for the dreaded oxidation to occur. So (IMHO) no need to purge after gravity readings etc. However, when you agitate the liquid by transfering between vessels you are going to cause a shift in concentration of dissolved CO2, introduce a significantly higher partial pressure of O2 (from the room air in the carboy if you dont purge) and throw the whole system out of equillibrium. This is where you might see noticeable gains from a purge before the filling of the container and possibly even a second after the transfer.
To summarize: if you feel you are having identifiable oxidation issues, consider how to minimize oxygen presence when you throw the liquid-gas system out of equilibrium. Realistically, diffusion is a really slow process and natural turbulence from the uneven walls of the tubing you use to siphon probably causes as much oxygenation as the difference between siphoning into a purged vs unpurged carboy (this I have no scientific data to confirm though, so take it with a grain of salt).

denimglen, that is the industry standard practice for handling pyrophorics and other **** that will kill you if it comes in contact with anything but trace levels of oxygen. My point was to illustrate the absurdity of worrying about some of this stuff; we are NOT handling any of this in a sterile environment, an oxygen free environment, etc. My last job included working on a lot of "air sensitive" and "moisture sensitive" compounds and processes that frankly, weren't, so topics like this are a bit of a source of amusement for me.

Reasonable actions taken to prevent continued, long-term introduction of oxygen to our beer are good, anything else is overkill and probably not economically worthwhile.