So far I've just run co2 into kegs through the gas in at very low pressure for a minute before racking into them, making sure the syphon tube sits at the floor of the keg. I figure co2 sinks, and my tank is generally cooler than ambient air, and all I really need is a thin layer of co2 to sit on top of the beer to buffer it from the oxygen. Then I seal the keg and run co2 into the headspace, purge, refill, purge, refill. Is this problematic?
I've also never aged in a keg rather than carboy. What pressure should I keep the keg at during lagering? When I cool it, won't it develop negative pressure and suck air (and maybe contaminants) into it if I pull the release valve?
Thanks
Sam
I find that if I move beer with CO2 into a sealed, sanitized, purged keg, it's useful to keep purging the keg from time to time so you don't get negative pressure from the keg to the carboy. It also helps to vent any O2 that I inadvertently left in the keg. When I first started doing this, I couldn't tell how much beer that I was moving into the keg, and flooded my head space, had to re-sterilize the seal, etc. In order to leave enough head space in the keg, you can transfer either by the level of beer in the carboy or by mass.. To transfer by the level of beer in the carboy, simply find out how many centimeters of beer from point x to point y on your carboy equals the appropriate volume to move to your keg, and stop your transfer when the level has dropped from point x to point y. To transfer by mass, fill your keg with water to the level that you want, and then measure the mass of the water. When you're ready to transfer, place your keg on a digital floor scale (you need a good scale) adjust for tare, and fill until you've reached the appropriate mass.
If that's too much trouble, you can also do the following: purge the keg but then open the top and put the seal in a StarSan solution. Fill the keg slowly with beer using CO2. There should be enough residual StarSan in the keg so that the foam that gets pushed out re-sterilizes the top of the keg. Just in case, once you've filled the keg to the appropriate level (I use the top weld seam on the keg as a marker), re-sterilize the top of the keg by spraying with a fine mist of Starsan, and then reattach the seal without touching the opening of the keg with your hands until the seal is set. Pressurize the seal with CO2 and you're good to go.
If the above sounds complicated it's really not. Just a matter of some simple steps that become automatic after a while. The key is sanitation, sanitation, sanitation...
You're not completely purging the keg when you lift the release valve, and it's a closed system when the valve resets. I wouldn't worry about negative pressure but maybe someone more knowledgeable than me can advise you better. But I agree that too much paranoia is a good thing. I typically soak my QR valves before using them to transfer beer, etc.
Edit: What Yooper said, above. lol