Many homebrewers like fermenting in a keg. With the price of glass carboys going up, not to mention the danger involved in transporting the heavy glass containers around from place to place, it makes sense to buy another corny keg instead.
There are many advantages but also some problems. One problem is that many primary fermentations produce a lot of krausen (or foam) which will plug the small openings in the keg. The only solution I know of is to use
Fermcap-S, a product that reduces the krausen to about 1/2" and prevents the need for some type of blow-off tube. (Purchase Fermcap-S from Northern Brewer)
I have heard of several ways of fitting an airlock on the keg. One is to find an extra lid and drill a hole big enough to use a rubber stopper with an airlock. You can remove the poppet from the gas "IN" plug, heat the end of a small length of 1/2" OD tubing and fit it over the plug. The airlock fits in the other end. I have also seen where the pressure relief valve is removed and a piece of 1/2" tubing is forced in the hole, with the airlock on the other end.
I did some research and found a really great product for fermenting in a keg. It is an American Sanke Keg Fermenter Kit with Thermowell, made and sold by Brewer's Hardware. Their kit fits an American Sanke keg and has a thermowell for fermentation temperature monitoring. (see the picture below) It also has a Tri-Clover compatible clamp with silicone o-ring for attaching to your own keg. It makes better sense anyway. A lot of homebrewers are collecting Sanke kegs for their RIMS or HERMS setups. Plus, you can return the keg to service if you need to. I've seen some positive reviews and a lot of talk on the forums about this kit. Very well made and innovative product.
American Sanke Keg Fermenter Kit with Thermowell from Brewer's Hardware
Regardless of how you plan to attach an airlock to your keg, you won't be able to ferment a full 5 gallons of beer in a 5 gallon corny keg. There will always be some amount of krausen formed and this necessitates having a little head space in the keg.
To transfer from the fermentation keg to another keg for secondary conditioning, you will have to rack under pressure. To do this, you must shorten the long "beer out" tube in the primary keg from 1/2" to 1" using a tubing cutter or hacksaw (be sure to file off the burrs). By shortening the dip tube you will leave the trub and yeast in the bottom of the keg. The keg used for secondary shouldn't need an airlock since it won't generate much pressure while clearing, but you can swap out the airlock fittings if you wish. Otherwise you will need to "burp" the keg by releasing CO2 pressure with the pressure relief valve periodically.
The main advantage of fermenting in a keg is that you can cut out almost any chance of oxygen contamination. You can purge the receiving kegs prior to racking by either leaving the lid unsealed and filling with CO2 for a 20-30 seconds prior to racking or by filling the keg with sanitizer and pushing all the sanitizer out with CO2. If you decide not to purge the entire keg, make sure you purge the head space after filling with beer.
Using your racking cane (or auto siphon) is the same for both a keg and a carboy. Try to keep the end of the hose on the bottom of the receiving keg and below the surface of the beer at all times to prevent splashing and aerating the beer or wort.
For safety reasons, to minimize oxygen contamination, or when all your carboys are full of fermenting beer, try using one of your clean and sanitized corny kegs the next time you need to ferment your beer.
Purchase all of your kegging supplies at Northern Brewer.com
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