Puzzled about bottling from keg and oxygenation

[Banjoblue]

So, I've seen the videos for counter pressure fillers and beer guns which purge the bottle of air and then fill with pressurised beer from the keg with the aim of reducing or eliminating oxygenation. But I'm puzzled by this.

First, when the wand is withdrawn, the beer level drops. In doing so it must draw air from the atmosphere. The beer would have forced out any Co2 as it rose and then when it drops back brings in air, so what's the point in purging?

Secondly, the beer is already carbonated so it has absorbed Co2 which, I think, will want to equalise in pressure once drawn from the pressurised keg into a bottle that is open to the atmosphere. So if it is trying to give up pressure how could it simultaneously be absorbing oxygen from the atmosphere?

How quickly does beer oxygenate? If the beer going into the bottle is going in smoothly and not being agitated, then how much oxygen can it absorb in such a small period of time?

Once you've reached fill level, won't the beer be starting to give off Co2 as the pressure tries to equalise? Won't that have the effect of purging the small space above the beer?

genuinely curious about this as I'm not at all clear how oxygenation of finished beer works anyway.

cheers

BB

 

[steveinUS]

I have bottled many a kegged beer with a beer gun, and they have generally had very good shelf life. The point is to minimize the beer's exposure to oxygen.

Purging the bottle first from the bottom with CO2 theoretically keeps the first bit of agitated beer as it starts filling the bottle, from becoming oxidized. Once the bottle is filled and the beer gun is removed, I immediately set the bottle under my bench capper that is pre-loaded with a crown ready to go. Just before bringing down the lever to apply the cap, I give the bottle a stern tap near the bottom, with a metal wrench or something similar. It causes a spontaneous release of CO2 which very quickly fills the head space of the bottle with foam, which drives out any air in the head space before you apply the crown. This is a trick I learned at our local microbrewery when we were hand-bottling a collaboration beer we had brewed with them. We also used oxygen-absorbing crowns.

From a technical standpoint, I don't know the details of how quickly oxidation can occur to begin a beer's ruination. And some beer styles, like Old Ale, et al, gain some of their outstanding aged character from a bit of oxidation. I do know that oxygen can creep into a bottle over time, across the seal of the crown cap. This ingress can happen even to a pressurized bottle of fully carbonated beer.