Understanding the Use of Wax Seals on Beer Bottles: A Brewer's Perspective

[x-ray vision]

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:

A brewer’s perspective:
A crown cap is not an absolute seal. Even though the headspace may be entirely CO2 (it rarely is), Oxygen still wants to be in there. It’s a simple law of partial pressures. With little to no O2 in there, there’s an inbalance, and the O2 will equalize eventually. Even if the brewer uses oxycaps, eventually the O2 absorbing properties will be used up, and you’ll be left with a beer that will oxidize. Besides, a brewer using oxycaps is practically admitting their bottling line is ****, and they need more help to get low airs in the headspace.

Now, putting a wax seal on prevents intake of oxygen, but in the case of beers not made for aging, it’s a pain in the ass. You need to get the beer cold, then trim the wax around the cap with a sharp knife in one smooth motion. I’ve only ever done 2 beers with this type of finish - a russian imperial stout, and a belgian dubbel. The stout is aging nicely, and the Dubbel, which was also bottle conditioned, showed no signs of oxidization(when tasted two years later).

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?

 

[NewScientist]

Just quite a stab in the preverbial dark but what about diffusion?

-NewScientist

 

[shyboy]

Yes, why not? CO2 is bigger then O2, so if there is no macroscopic flow, then O2 will diffuse through membrane faster then CO2. O2 has bigger velocity as well.
Anyway, if there is no macroscopic flow (i.e. gas is not pushing another gas out), the molecules will not "see" one another, so for oxygen the bottle will look like empty. Of cource, if there is a relatively big leak, then the outcoming CO2 flow will push all incoming gases away.

 

[suncake]

I personally would think the bigger issue is keeping the CO2 *inside* the bottle. The partial CO2 pressure inside the bottle is much, much greater than the atmosphere...so that's a big issue.

As far as oxygen, here's my guess. I don't know the exact oxidation reaction(s) that so many beer snobs are deathly afraid of, but it's safe to assume that the O2 in bottled air is consumed. As the partial O2 pressure get's low enough, an equilibrium between the consumption of O2 by oxidation and the release of absorbed O2 will be reached.

Now, if you were to consume O2 by oxidation, then allow O2 from outside the bottle to creep in, the equilibrium will change allowing for further oxidation.

Side note-- make sure whenever you're talking about diffusion to speak in terms of partial pressures. The driving force for diffusion isn't total pressure, it's partial pressure (well, actually it's chemical potential (well, actually it's electrochemical potential...)).