I've recently started brewing more lager beers. With lager beers you need to ferment them at 50-55 degrees. That being said fermentation control is necessary in order to get good results from the yeast. The only way I have to control this is through my kegerator. I don't have a huge kegerator and using carboys is out of the question because they are a lot wider than corny kegs. So, I've started to ferment lagers in corny kegs.
Here are some points to consider when fermenting in corny kegs:
1. Since lager yeast is a bottom fermenting yeast, it doesn't normally create the huge krausen that top fermenting ale yeast does. That being said, you don't need a lot of head space when fermenting lagers in a corny keg.
2. With the above in mind I brewed up an extract lager using nothing but Zeus hops and I topped it up to 4.5 gallons . . .just to be safe so that there wasn't any kind of blow off coming out of the keg.
3. Once you pour your cooled wort into the keg and top it up to 4.5 gallons the easiest way to vent it is to either pull up on the release valve on the keg lid and keep it released or just take off the release valve pin by unscrewing it, if it unscrews and either insert an air lock or just cover up the hole with a sanitized mason jar. There's no need to put a blow off tube on the IN post of the keg. So, don't do it. Unless you have one of those release valves that can't be taken apart from the top. If that's the case, then attach a blow off tube to the IN disconnect and attach the blow off tube to it and put the blow off tube into a mason jar of water.
4. To be on the extra safe side, use some fermcap-s to keep control of any kind of krausen on the beer. You can use this if you are fermenting ales or lagers. It will keep the krausen down to about an inch. It works great.
5. You'll also want to strain out any kind of hop debris from the kettle if you are fermenting in the keg. More on this below.
6. Once fermentation is complete you'll want to pull out the keg for 1-3 days at room temp for a diacetyl rest. This will allow the yeast to clean up any kind of by-products it released during fermentation and if you leave it at room temp longer then it takes the "green" away from the lager and allows you to keg it, lager it and drink it a little faster.
7. After your rest then you'll want to "jump" the keg to another keg. Add 1/2 gallon of water to second keg to make 5 gallons before transferring fermented beer to the keg. Simply hook up an Out disconnect to another OUT disconnect with some tubing in between and set your co2 to about 3-5 psi so you can push the beer from one keg to another. Make sure you attach your co2 disconnect to the primary fermenting keg! Attach the lid to the new keg and vent the gas pressure when you see any kind of slowing down in siphoning the keg. This keeps the beer in a state of constant co2 so you don't have to worry about oxidation. You may pick up some yeast sediment from the first keg, but that's normal. This is also why you want to strain off the hops. You don't want hops going through your dip tube and clogging up your poppets when you "jump" the kegs. That's just a pain in the ass!
8. Once transferred to the second keg, "lager" it at 34 degrees for 2-6 weeks, tap it and enjoy.
Here's the recipe I used for the Zeus Pils I made.
Zeus Pilsner Summer Beer, with a bite!
7 lbs extra light liquid malt extract
1/2 lb 20L crystal malt
1 oz zeus 16% a.a 60 min
2 oz zeus 16% a.a 5 min
S-23 German Lager yeast 1 packet
Ferment for 2-3 weeks, jump to next keg and lager for 2-6 weeks, enjoy.
P.S. I've never used 100% Zeus hops in a beer so this should be interesting. Zeus, Tomahawk and Columbus are all the same hop and I've used Columbus before, so it should turn out to be a nice hoppy, crisp beer with some bitterness and great flavor.
Let me know how yours turns out if you try this out! Cheers!