Airlocks are a MUST (apparently)

[Peteoz77]

So this guy comes into the LHBS yesterday, and he's making some kind of massive beer.. like 30%. He plans on it taking a year or so. We chatted a fair bit and I mentioned that some folks online don't use a bubbler, since they feel it stresses the yeast to have to push through the bubbler, so they go without.

The dude then goes into this massive 10 minute long speach about how oxygen gets into the beer without an airlock. I STUPIDLY mentioned that once there was a layer of C02 on top, that it would stay there, since it's heavier than oxygen and protect the wort. This guy says "oh No, the oxygen can get to the wort after the first three stages of the yeast duplication" Yadadadadada.

So, apprently, once the yeast stops producing C02, the outside oxygen can get in......

Is he high?

[Brewingchick]

I have no scientific evidence to back me up, but I think he's got a point. Oh, not for primary, when there is plenty of co2. But for long term aging.

I'm primarily a winemaker and my experience has given me some oxidized wine in the past. Oh, I had an airlock, but I used a "carboy cap" which seemed to fit tight but was certainly not 100% airtight in retrospect. The wine had very sherry-like notes after about three months. It seemed like it fight tight, but I could move the cap on the carboy. The carboy next to it had a bung and airlock and tasted great. (I actually blended them, and ended up with a phenomenal wine, but that's not important to this story!)

During primary, lots and lots of co2 is expelled and co2 is heavier than air so it does form a "blanket" over the beer. But over time, the co2 can be dispersed (at least in MY carboy it was) and the wine (and I assume beer) can oxidize.

Now, how long would it take? That, I do not know. I know that in my winemaking I always top up, and always use an airlock (or a bung with no hole for the airlock if not using an airlock) to prevent oxidation.

[Evan!]

During primary, lots and lots of co2 is expelled and co2 is heavier than air so it does form a "blanket" over the beer. But over time, the co2 can be dispersed (at least in MY carboy it was) and the wine (and I assume beer) can oxidize.

Good point, but I think that it only gets "dispersed" if you take a lot of samples, rouse/stir it, etc. I use airlocks on secondary (which is rare, but it happens) just to keep fruitflies out, but past that, if you purge the vessel with co2 like you should, and don't take a boatload of thief samples or move it around a lot, the co2 blanket should remain intact.

[Brewingchick]

Evan!, I think you're correct. I do NOT purge my carboys with co2, for a couple of reasons. One, it's usually wine and I want to degas the wine and have the co2 go OUT of the wine, and not add more. Two, I'm not smart enough to figure out how to do it with my gear!

I imagine that Evan! is right- if you purge with co2 (maybe even more than once, "just in case") then the worry would be gone. However, if you don't purge with co2, I'm not sure than the co2 blanket would stay intact indefinitely. I mean, even in our universe, we don't have co2 covering every surface that produces it. Sure, it's heavier than oxygen, but through diffusion I think it would still disperse into the atmosphere. Like I said, though, this could be over two weeks or two months or two years. I have no idea how long an intact co2 blanket would stay put.

[oldwoman]

I think brewingchick's last post is spot on; nothing is static, so the CO2 won't remain indefinitely. Entropy owns us all! However, I would imagine that this would take a long, long time to occur and isn't really a necessary consideration for 95% of the batches we brew.

[Big_John]

Evan!, I think you're correct. I do NOT purge my carboys with co2, for a couple of reasons. One, it's usually wine and I want to degas the wine and have the co2 go OUT of the wine, and not add more. Two, I'm not smart enough to figure out how to do it with my gear!

I imagine that Evan! is right- if you purge with co2 (maybe even more than once, "just in case") then the worry would be gone. However, if you don't purge with co2, I'm not sure than the co2 blanket would stay intact indefinitely. I mean, even in our universe, we don't have co2 covering every surface that produces it. Sure, it's heavier than oxygen, but through diffusion I think it would still disperse into the atmosphere. Like I said, though, this could be over two weeks or two months or two years. I have no idea how long an intact co2 blanket would stay put.

The laws of physics pertaining to gas mixtures indicates that you are correct. Combined gasses, such as our atmosphere naturally mix uniformly. The law of partial pressures applies when a vessel is vented to the atmosphere. The gasses contained in the vented vessel will eventually come to equilibrium with the atmosphere and so will the soluable gasses dissolved in the wort. Atmospheric pressure is about 14.7 psi at sea level. This is a substantial amount of pressure and the atmosphere contains roughly 20% oxygen IIRC. CO2 will slowly migrate out of the vented container and O2 will slowly migrate into it until equilibrium is attained.

[murl]

The laws of physics pertaining to gas mixtures indicates that you are correct. Combined gasses, such as our atmosphere naturally mix uniformly. The law of partial pressures applies when a vessel is vented to the atmosphere. The gasses contained in the vented vessel will eventually come to equilibrium with the atmosphere and so will the soluable gasses dissolved in the wort. Atmospheric pressure is about 14.7 psi at sea level. This is a substantial amount of pressure and the atmosphere contains roughly 20% oxygen IIRC. CO2 will slowly migrate out of the vented container and O2 will slowly migrate into it until equilibrium is attained.

Spot on, good explanation. CO2 is more dense, therefore it moves slower but it is never still, kinetic theory of matter. Gases assume the shape of their container, and therefore mix, so if you don't have something to prevent O2 from entering then you could see some oxidation.

[ben the brewman]

I dont know but its good to see you back pete

[Peteoz77]

I dont know but its good to see you back pete

Thanks ben!

[Evan!]

Short answer, yes, he is high. CO2 is heavier than atmospheric air. Even after the co2 production stops, the layer of co2 stays hovered over the beer. The only way you'll get o2 uptake after that point is if your rouse/stir for some reason...in which case it won't matter if you have an airlock or not.