Last night I was reading in the Brewing Elements series, “Water”, that the acceptable oxygen levels in commercially packaged beers were about 50 parts per billion. Yes, you read the denominator right, billion. This is to prevent staling of the finished product. While their supply chain is considerably longer than mine, it got me thinking about the difference between kegging with forced carbonation and bottling. If you naturally condition in the keg you should be able to consider it bottling for this purpose.
With forced carbonation, you may or may not be driving off all the oxygen. Henry’s Law states that the partial pressures of gas in solution add up to the pressure on the solution so when forcing carbon dioxide into your beer under pressure, you may not be forcing oxygen out since the pressure is high enough it could allow both gasses to remain. If oxygen is not forced out, you’re exposing your beer to oxidization and all the nasty flavors that brings, sherry and wet cardboard. At Homebrewer’s Night I’ve tasted oxidized beers but have never thought to ask if the brewer filled the bottles from kegs so I’m simply guessing here. But my bottle conditioned beers have never had that distinctive oxidation flavor and I’ve kept some of them upwards of 18 months.
I’m guessing that here, too, yeast is your friend. It’s alive and respiring down there in the bottom of that bottle, scavenging oxygen from its environment every chance it gets. So I’m thinking the yeast keep the oxygen level below that which can cause staling. Of course I don’t have equipment lying around to test this but simply logically, when it comes to oxygen in the beer, the advantage goes to bottle conditioning.