[Joe T]

The seals aren't 100% perfect. The amount would be small I'm sure.

yup, over 50 or 100 years you'll lose all your carbonation too.

According to Charlie P, The Home Brewer's Companion, referring to plastic pet bottle:"although there is pressure in a bottle of beer and it seems that because of that outward pressure, no air would ingress into the beer, it doesn't happen that way. Why? Because the outward pressure is created by only carbon dioxide gas. There is no oxygen gas inside the beer pushing outward ; consequently, it's as though the oxygen sees a void and thus makes it's way in, slowly. There is a very small amount of air ingress through the seal of a bottle cap as well." He cites Boyle's law "which has to do with the observation that gases will seek equilibrium in the environment they are capable of inhabiting."
It seems as though "air tight" doesn't necessarily mean "oxygen impermeable". And PET plastic and crown cap gaskets are oxygen permeable but what about the rubber gasket on Grolsch-type botlles or kegs? Is it oxygen impermeable? I don't know.  What about the oxygen permeability of modern beer bottles which are a glass-plastic blend vs. Old-school recyclable bottles which are 100% glass. So what is truly THE best way to protect beer from oxygen? My guess is a keg with the headspace purged occasionally.
BTW, Charlie P is still the man just as much as anyone whom is more recently "beer-famous".

[tonyccopeland]

I have noticed the quality of my beer seems to go down a noticible amount (color and carb) when I bottle from a keg.  After a few months of reworking my bottling process I think it may be due to temperature swings in my cheap garage fridge (24 to 48 degrees F), but I am still testing and measuring. Has anyone else seen where temperature changes in the glass bittle affect the seal of the bottle over time? Or am I chasing a unicorn?

[majorvices]

It seems that an IPA I've had in bottles for perhaps 2 months or so has become darker in colour and the sediment at the bottom of the bottle that I pour out, seems to be darker and slightly greyish in colour. Does this seem normal? It doesn't taste like it has gone off. It started out quite golden and has gone more towards an amber/red colour.

I wonder if the reason why it is darker is simply because the yeast has dropped out of suspension. When a beer is cloudy it usually doesn't look as dark as when it is clear. All you have to do is check out a fermentation in a  carboy. Even a red beer will look "milkish" during fermentation and will actually appear to darken as the yeast drops out.

As far as the rest of the conversation: I do think oxidation can cause beer to darken. And I do think that active yeast during natural carbonation will help minimize the effects of o2 pick up. But this is only if oxygen pick up is minimal.

As far as the oxygen absorbing caps go, they work pretty well. But some have shown that they also absorb hop aroma. So consider that when using them.

[majorvices]

O2 caps are activated when wetted. I guess it's magic.

They have a limited working time from what I have read, and don't stop further o2 from seeping in any better than other quality caps.

How is it possible for oxygen to seep into a bottle after it is capped and carbonated?

Just curious

I know, it's hard to believe that it happens but it does. It has to do with atmospheric pressure and the exchange of gasses. Cans will oxidize too due to the seal at the top of the can, though some think less fast as bottles (though I have my doubts after seeing a few canning lines in action and seeing a few cans seeping at the seams.)