Cans keep out oxygen, so they say. Bottle tops don't do quite such a good job, so we are told. It's all to do with the seal that can be achieved. Oxygen can sneak through the tiny molecular gap that might be present between glass and the plastic seal on a cap. Metal to metal is a far tighter closure, apparently.
That is all well and good, but is it possible to get beer into the can reliably without oxygen pick-up? As it happens, Stonch has already cast doubt on that. I can't really be sure, except for the fact that the canning lines I have seen seem to not have all that a reliable way of purging with CO2 and there is certainly no pre-evacuation of the can.
However, our bottling line at Hardknott is the double pre-evac counter pressure filling type machine. What this means is that a contraption attaches to the bottle neck sealing hermetically from the atmosphere. Most of the air1 is pumped out. The bottle is then filled with CO2 at a pressure of 1 atmosphere gauge pressure2, i.e. 2 atmospheres absolute. This process is then repeated again, hence the word "double" in the name.
Only then is the beer allowed to flow into the bottle, under a counter pressure of more CO2 at around 2 bar. When the beer goes into the bottle it is extremely oxygen free. When the pressure is released the beer fobs a little, and we set the rate of bottling to get the right fobbing, along with a little squirt of sterile water to help it foam. This fob is all beer and CO2 with only a tiny area exposed to the air.
The canning lines I have seen try to puff out the air with a little tube that goes to the bottom of the can. The beer is filled from the bottom of the can to try and push the CO2/air mixture of unknown purity out of the can. Any fob will have quite a significant area in contact with the air.
In my view to can reliably with low levels of oxygen it would be necessary to do the process in a blanket of CO2, rather than the open systems I've seen.
Cans might keep out oxygen, but I'm fairly sure it is a lot harder to stop it being in there in the first place. The filling process is certainly not as hermetic as with our bottling line.
Because it is difficult to explain in words I did a little animation for our bottling line.
Top from Hardknott Brewery on Vimeo.
Incidentally, I hadn't really thought about the problems of canning compared to bottling until Jon from Stringers pointed it out to me the other day. All the result of a chat over a beer. Isn't beer good.
1Our vacuum pump gets down to about 0.1 atmosphere absolute pressure.
2Gauge pressure is the pressure relative to the atmosphere.