# Physics question- Gas inside beer bottles

On another message board I belong to, someone asked why some beer manufacturers hand dip the top of the capped bottle in wax. This is part of one reply given by a home brewer:

Can this be correct? Since the pressure inside the bottle is higher than the pressure outside of the bottle, can oxygen from the air actually want to enter the bottle more than other gasses because “oxygen still wants to be in there”? Isn’t it possible for a properly sealed capped bottle to never allow gasses to get in or out?

If the positive pressure inside the bottle counts at all for whether or not stuff is kept out of the bottle, then eventually that carbonation is going to escape the bottle. It may take a long time, but some Belgian ales are designed to age for several years (I just bottled one that had been in the fermenter for two years). So, there’s going to be a window between “good pressure inside the bottle keeping oxygen out” and “bottle pressure has reached exterior air pressure”, also known as “flat”. Once you get to that point, oxygen can get in, and the beer will start to taste not so good (not counting the “flat” part).

Isn’t it possible for a cap to give you a perfect seal so that nothing gets in? If oxgen is going to get in, will wax help? If you still hear “pffft” when opening the bottle, can you assume that no outside gasses have entered?

I just found this on a homebrew web-site:

I’m pretty sure O2 molecules are bigger than CO2 molecules, no?

Is it correct that O2 would get inside the bottle regardless of the pressure inside the bottle? It doesn’t make sense to me that if the air pressure inside the bottle is higher than the air pressure outside the bottle that O2 would be seeping in.

Well diffusion is a statitistical process.

Here is one of my physics profs explaination of diffusion: (imagine this with a thick japanese accent for full effect).

Gas diffusion is similar. The pressure introduces a bias in the random motion, but a few molicules will buck the trend. Basically, if there is any leak path at all, the pressure will, in the long run, determine the ratio of gas concentration inside the bottle vs. outside the bottle. The outside pressure is ~ 15psia, and the pressure inside perhaps double that…so we’d expect about half the concentration inside the bottle.

I think it would be more accurate to think in terms of partial pressures, not absolute pressure. There’s more O2(and N2) outside the bottle than inside it, so O2 will tend to diffuse inward. There’s more CO2 inside than out, so it will want to escape. The dynamics will depend on the partial pressures and the relative diffusivities of the different gasses through the sealing material.

I was going to be flip and say “No,” since, while both molecules have an “O2,” CO2 has an extra “C” which must take up additional volume. However, things don’t always work out that way in chemistry. I’ll check my CRC tomorrow when I go to work. For now, I’ll just say “I don’t know.”

I’m not going to provide a better answer than has already been provided, but here are some similar examples:

IIRC there is a similar phenomenon with rivers that empty into the ocean. Even though fresh water is flowing in one direction, salt will move upstream. For instance, if you take a boat down the Delaware R. and check salinity between Trenton and the Bay, you will see the salinity increase gradually as you get farther downstream.

I have the same problem with my glovebox. I keep it under a nitrogen atmosphere because many of my chemicals will react with oxygen (sometimes violently :eek:.) I keep the box pressurized above atmospheric pressure (I’m not sure how much off the top of my head, since I’m not at work right now even though I should be there.) The box is pretty well-sealed, but it’s not perfect. Despite the overpressure, oxygen still gets into the box and makes me grumpy.

O[sub]2[/sub] bond length, 1.2074 Å
CO[sub]2[/sub] bond lengths, 1.1600 Å.

That’s all I can provide. If you want to look at total volume that contains a certain electron density, you’re on your own :).

So, a tightly compressed vinyl seal which can maintain significant pressure within the bottle lets oxygen in. Yet, a dipped wax seal that isn’t strong enough to hold even a bit of pressure on its own will keep oxygen out? Vinyl compressed between metal and glass is too porous to keep out oxygen, but wax isn’t? I find that difficult to believe.

If somebody had a solid scientific test showing that wax made a significant difference, I could accept it. Without that, though, my intuition says that dipped wax is far more porous than a bottlecap seal, and won’t make a noticeable improvement.

As regards the OP (and speaking totally out my nether orifice), I would share your skepticism. Unless you completely encased the bottle, I don’t see how this would help. The highly non-polar wax won’t bond very well to the very polar glass (which is why it’s so easy to chip/peel off), so there’s plenty of room at the wax/glass interface for gases to diffuse in/out.

Heres a way to test, fill a bottle with overpressurized normal air and cap it. Leave it for a few months and see if it’s gone flat. If it has, that means oxygen can pass through th barrier, otherwise, it cant.

Here’s another interesting but not-so-scientific comparison: I bottled a batch of beer two weeks ago. The beer in bottles with crown caps is still not so carbonated, but the beer I put in Grolsch bottles (with the rubber seal and manual flip-tops) is carbonated and fizzy. So, those rubber caps get a better seal than the plastic-lined crown caps, I would wager.

Then again, they hold 16 ounces rather than 12, so maybe it’s just that there’s 1/3 more beer to form carbonation.

Nope, CO[sub]2[/sub] and O[sub]2[/sub] take up the same amount of space. Remember the rule that 1 mol of any gas at STP takes up… umm… 22.4L? 23.4L? It doesn’t matter. My point is that it’s the same volume.

Re other point with O[sub]2[/sub] seeping in: I don’t think that the beer has to go flat in order for O[sub]2[/sub] to get in. I think that there’d still be enough pressure in the bottle to keep plenty of CO[sub]2[/sub] in solution. The problem comes with having CO[sub]2[/sub] in the headspace at a pressure like that. Just like with any diffusion, things are going to go from low concentration to high, so since we’ve assumed 0 N[sub]2[/sub] and O[sub]2[/sub] inside the bottle, the net equillibrium will be into the bottle. Since there is a greater pressure of CO[sub]2[/sub] inside the bottle, the net shift will be out of the bottle.

It makes sense to me, guys, but I don’t think that I transferred it to words very well.

When you bottle your beer, the headspace is full of air at 1atm pressure. That oxygen isn’t going away when you condition the beer. The suggestion is that so much more oxygen is going to seep in while it’s sealed that the beer will be ruined, when the oxygen that’s already there (20% of the initial headspace) is not ruinous.

Is that possible? Think Snapple. They use vacuum sealed safety caps using similar seal technology as beer. Those bottles hold vacuum in the headspace for months or years without the safety lid popping, how much air could have possibly make its way in there? I’d say, almost none, and that’s with vacuum in the headspace instead of higher than atmospheric pressure. There’s no way you get enough diffusion to make a difference when you are already bottling the beer with plain air in the headspace.

This, I think, is the underlying question from the OP: why does the concentration of various gas molecules matter? I mean, I can wave my hands and invoke the statistical mechanics notion of entropy, but that’s hardly a mechanism. Is there really such a thing as “partial pressure”, or is it a statistical artifact? That is, if molecules from inside and outside are exchanged at random it’s more likely for a CO[sub]2[/sub] from inside to be replaced by an O[sub]2[/sub] from outside than the other way around.

Remember that there are two bonds of 1.1600 Angstroms each in a CO2 molecule, and only one O-O bond (that is slightly longer) in an O2 molecule.

22.4L for 1mol of an ideal gas (unless they lied to me back in highschool), which doesn’t exist of course. My mistake though, I was thinking of the size of individual molecules and if that had anything to do with permeability. I know it does for bigger molecules, but I’m not so sure about these two.

Correct. My apologies if anyone was confused by my use of an “s” to indicate a plural for the linear CO[sub]2[/sub]'s two bonds.

Also, I guess someone should note that these bond lengths are just averages (probably at 25 ºC). Then again, as I mentioned above, I have no idea if this really matters at all.

The technique that is usually used in homebrewing to reduce headspace oxygen is to place a crown cap on each bottle after it is filled with beer, but not crimp it immediately. After several minutes, much of the air in the headspace will be replaced by heavier CO2. Then, all of the caps are crimped in the order in which the bottles were filled.

So, an unsealed cap with a slight overpressure of CO[sub]2[/sub] will result in removing the oxygen, but sealing the cap will somehow allow it back in?

There’s an appreciable flow of CO2 out of the beer when the bottle has just been filled (at least there is the way I homebrew), so much of the air is flushed out. Once the cap is sealed, there’s lots of time for diffusion. It’s the difference between trying to swim upstream in a fast-flowing river and swimming in a lake.