Fermenting in a corny keg has been a game-changer in terms of my homebrew in both beer quality and my overall process. If you already have a kegging system, then you can reap some of the major benefits of a pressure-rated conical at a fraction of the cost. There’s something so simple about the process that I absolutely love. Maybe it’s the fact that I’m so accustomed to cleaning and sanitizing kegs and keg parts that it feels like second nature.
I will say the process of doing a closed transfer from fermenting keg to serving keg has never been easier. I can’t say I’ve had the same success with my bucket fermenter. I think this is largely due to my inability to cold crash my bucket fermenter because it’s too large for any fridge. This makes transfers much more susceptible to clogs. The pressure-rated capability of kegs also makes moving beer a lot easier than relying on gravity.
As a quick note, I use ball lock kegs, so all of my suggestions below are geared towards ball lock kegs and fittings. You can of course do the same thing with pin lock kegs and their respective fittings. Assuming you’re using a 5-gallon keg, you will need to scale your batch size back to 4.5 gallons to accommodate the size of the vessel and the necessary headspace required for fermentation.
Why Use a Cornelius Keg as a Fermentation Vessel
- Kegs are easy to use and easy to clean
- Great for pressure transfers from serving keg to fermenting keg
- Have the option to ferment, crash, and serve in the same vessel (best for quickly consumed kegs)
- Kegs are pressure-rated higher than most true conicals/unitanks available to homebrewers
- Easily cold crash in a totally sealed vessel with no risk of oxygen suckback
- You can naturally carbonate with a spunding valve on the tail end of fermentation
- Carbonate as you cold crash
Why Ferment Under Pressure?
Corny kegs give the homebrewer the ability to ferment under pressure. Fermenting under pressure is valuable for some beer styles because it decreases ester production and allows you to ferment at higher temps without the off-flavors that accompany it. This is especially useful if temperature control is not an option for you.
Prepping Your Keg
To no surprise, the liquid dip tube must be either bent or cut so it sits further off the bottom of the keg so you don’t suck up any trub transferring your beer out of the fermenter. The styles of beer you typically brew will dictate the liquid tube length.
Cutting the Dip Tube
I mainly brew hoppy beers with massive dry hop additions so I opted for removing just under 2.5 inches off my curved dip tube. This gives me a little under .5 gallons of dead space at the bottom of the keg. A straight dip tube may need more or less so keep that in mind. My recommendation is to cut a little at a time and test it so you don’t do too much. To cut the tube, you’ll need a small metal tube cutter. They’re easy to use and make a clean cut without burrs. My cutter is meant for brass and aluminum but it still worked.
Floating Dip Tubes
The other option is to swap out your standard dip tube for a floating dip tube. A floating dip tube fits into your keg’s liquid post but uses flexible tubing and a float ball to keep the dip tube end at the very top of the liquid level. This is probably the most risk-averse method and also keeps your keg’s stainless dip tubes intact in case you need to use it for dispensing beer later on. Floating dip tubes are pretty cheap and work exceptionally well (my preferred method). This would be best for varying batches with different trub levels. They also are great for dispensing clear beer in general.
Pressure Release/Blowoff Tube
In terms of pressure release during fermentation, you have a few options. The simplest method is to use a blowoff tube submerged in Star San solution. I made my own using a gas quick disconnect equipped with a hose (don’t forget the swivel barb for 5/16 ID tubing). The other option is to use a spunding valve and leave it FULLY OPEN unless you’re using it to regulate pressure while fermenting under pressure. I built my own spunding valve using this link from Homebrew Finds. I opted for the tube version #3 with brass barb since my fermenting fridge has low clearance. Traditional spunding vales directly clip onto the gas post and add several inches of height.
Fermentation will generate A LOT of pressure. Don’t ever ferment in a totally sealed keg or you could potentially create a dangerous situation.
Kegs obviously don’t come with any form of integrated cooling system so a fridge is really the only option for maintaining fermentation temp. I tape an Inkbird probe securely to the outside of the middle of the keg and place the keg in my fermenting fridge. I use an old T-shirt and tie it tightly around the keg, covering the probe, so the ambient fridge temp will have less of an impact on the temp reading.
I’m sure there is a more sophisticated method of doing this but it seems to work pretty well at maintaining consistent temps. I’ve been using a Tilt hydrometer in my kegs to monitor specific gravity and temperature. I can confirm they work in a pressurized keg and also proved that the internal temp of the keg was within 2-3 degrees that my Inkbird probe reported from the side of the fermenter.
For heating, I use a FermWrap heater taped to the outside of the keg. It fits perfectly and wraps around the majority of the keg. Just be sure to position the temp probe where the heater doesn’t reach. You’ll want to position the heater as low as possible on the keg because the bottom is always going to be the coldest part of the keg. The FermWrap heater will heat quickly and can easily get the keg up to 80°F+.
One of the biggest drawbacks with keg fermenting is of course the lack of options to add hops in a no-oxygen closed system (like a hope doser). Of course, this is the same story for most basic fermenters. Conicals can be equipped with contraptions to easily add hops and eliminate oxygen exposure. That being said, if you work quickly, I think your risk of oxygen exposure is incredibly low, especially if you purge the headspace in the keg with CO2 immediately after adding the hops and closing the lid.
Alternatively, you can hook up your CO2 and purge the keg with 3-4 PSI WHILE you’re adding your dry hops. This will fill the headspace with CO2 and prevent oxygen from getting in.
The last option is to transfer your beer to another fully purged keg reserved for dry hopping. This would entail adding your hops to a sealed and purged vessel and racking your beer to the new keg when it comes time to dry hop. My only hesitation with this method is having to potentially modify two kegs to avoid sucking up hops. In this instance, you would most likely need a method to contain the hops or use additional floating dip tubes.
Dry Hopping Under Pressure
My latest practice with NEIPAs is to dry hop under pressure in an effort to lock in as much of the volatile aromas as possible. Once I reach terminal gravity, I swap out the blowoff tube for the spunding valve and set it to 10 PSI. If you do this in conjunction with a dry hop addition you will also be able to naturally capture the pressure build-up as CO2 production will be retriggered by the yeast interacting with the hops. If you don’t reach 10 PSI in 3-5 hours, you can top it off with CO2 and let it sit.
My latest dry hop experiment was an 8-ounce charge in a 4-gallon batch. I did not have any issue with clogging during transfers. You can hop in a stainless hop tube or mesh bag but you’re not going to get the best dry hopping effect. If you cold crash adequately, you shouldn’t have any issues. Scott Janish writes about keg hopping in-depth on his blog and has some other creative options.
A simple picnic tap is the best way to easily pull samples during fermentation for testing specific gravity or tasting. This is a great option as it doesn’t risk oxidation. Do not open the keg lid unless you absolutely have to. It’s not worth exposing your beer to oxygen. Be sure to fully sanitize anything that comes in contact with your keg or beer.
These also come in handy for doing an initial runoff of cloudy/trub beer. This is helpful for ensuring your beer is clear before making any major transfers. You will inevitably suck up a little trub when you first rack.
Cold crashing has never been easier as you can simply throw your keg in your fridge/keezer and crash your beer before transferring to the serving vessel. The best part about keg fermenting is you don’t have to manage oxygen suck-back since you have a fully sealed pressure rated vessel. At this stage, you don’t need any airlocks, blowoffs, spunding valves etc.
Always cold crash for 24-48 hours at 33-35°F to settle anything left in suspension. You don’t want large hop particles to clog your lines.
Transferring from fermenting keg to serving keg is really easy using a jumper line. This is a piece of tubing with a liquid quick disconnect on each end. This allows you to transfer beer OUT from the liquid tube in your fermenting keg into the liquid OUT of your serving keg. If you build your own jumper (which I suggest you do), I would highly recommend you buy liquid disconnects with removable flare barbs for easier dissembling and cleaning. You will need standard 3/16 ID beer lines for this.
Steps to Transfer:
- Fully purge your serving keg using the Star San/water flush method. Clean and sanitize the serving keg and fill it to the top with Star San water solution.
- Connect your CO2 tank to the serving keg and push all the water out through a liquid line. This will leave you with a fully oxygen-purged keg. This is also a great opportunity to flush your kegerator’s beer lines with sanitized water.
- Add 3–5 PSI to both the fermenting and serving keg (from what I’ve read equalizing keg pressure will help with foam when transferring carbonated beer)
- Connect your CO2 tank to the fermenting keg with the regulator set to the same 3-5 PSI you initially filled each keg with.
- Attach the jumper line to each beer “OUT” post on the kegs.
- Vent the pressure from the serving keg to start the flow. CO2 will flow out of the serving keg as it slowly fills with beer. I use my spunding valve set to 0-1 PSI on the gas post of the serving keg. You can also just attach a naked gas post to vent or put the PRV in the open position. If beer flow is slow or not moving, increase pressure a few PSI on the fermenting keg.
- Disconnect the gas/jumper line once the flow stops and you’re ready to carbonate/serve.
PSA: If you beer has been carbonating or is carbonated at this point, You’ll want to be sure not to transfer too quickly. Stick with a slow 3-5 PSI transfer to prevent foaming.
All-in One Fermenting and Dispensing
A lot of homebrewers are fermenting and serving out of the same keg. This is an incredibly appealing option as it eliminates transferring and oxygen exposure to the highest degree. If I went through beer faster, I would absolutely go this route. The only drawback is leaving your beer exposed to the trub for several weeks. If I was brewing a batch I knew I would finish in a matter of days or weeks, then this would be the ONLY way to go. I’d be hesitant to do this for a month or two…but I’m sure you could squeeze by.
With all the benefits of fermenting in a corny keg, at the end of the day, a keg is a keg and does not have some of the advanced features of a true conical such as trub dumping or integrated temp control systems. Assuming you’re using a 5-gallon keg, you’ll also have to scale your batches back to about 4.5 gallons to accommodate the keg size and necessary headspace needed for fermentation. This is not a big deal for me as I lean towards smaller batches anyway.
Take good consideration when cutting your dip tube or go with a floating dip tube to reduce the risk of clogging your system with hops. A cold crash is going to really help prevent this.
Overall, I love keg fermenting and I think it’s a really efficient way to create a fully closed cold side process of your homebrewing on a reasonable budget. If you’re brewing delicate styles like NEIPAs, this process will be well worth your time and investment.