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Old 02-13-2012, 08:50 PM   #1
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Default 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.

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Old 02-13-2012, 09:01 PM   #2
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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.

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Old 02-13-2012, 09:10 PM   #3
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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.

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Old 02-13-2012, 09:24 PM   #4
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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.

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Old 02-13-2012, 09:24 PM   #5
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Quote:
Originally Posted by Yooper View Post
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.
If we slept in hermetically rooms indeed we would die. This is how one suffocates.
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Old 02-13-2012, 09:48 PM   #6
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Quote:
Originally Posted by Yooper View Post
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.
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.
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Old 02-13-2012, 10:09 PM   #7
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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.

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Old 02-13-2012, 10:35 PM   #8
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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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Old 02-13-2012, 10:45 PM   #9
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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.

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Old 02-13-2012, 10:48 PM   #10
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Quote:
Originally Posted by Bernie Brewer View Post
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.
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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