Serious Eats: Drinks
How Draft Systems Work: Getting Beer From Keg to Glass
Anyone who has ever been to a college keg party has seen a draft beer system in action. One chilled keg + one party pump = one red Solo cup filled with beer. It's a simple set-up, but that kind of beer service is a bit abusive to your beverage. So your favorite bars and restaurants don't keep perma-drunk frat boys in the keg coolers to give the party tap a few pumps every 20 minutes. The draft systems used to get beer to you from the keg at these places are more complicated than you might think.
In any draft system, you've got six main components: cooler, keg, coupler, gas, tubing, and faucet—it can get more complex, but that's the basic setup. These six pieces all work together in a delicate harmony to ensure that the beer served to you is properly chilled and carbonated, with an appropriate foamy head. Here's how it all breaks down.
Back to that keg party. See how the keg is stuffed into a bucket and packed with ice? Beer needs to be kept cold not just so that it tastes good, but also to prevent spoilage and warm, foamy pours. But there aren't any keg buckets in the backs of your favorite bars and restaurants either. Most restaurants have walk-in coolers that can keep a whole slew of kegs cold at once. This is where your beer's journey begins.
Your beer resides in kegs: this you already knew. Kegs come in a wide variety of sizes, shapes, and materials, but beer made in the US is usually packaged in 5 gallon, 7.75 gallon, or 15.5 gallon kegs made of stainless steel. From the outside, you can see a valve on top. This is the hole where gas flows in and beer flows out. Inside the keg, there's a long tube of metal extending from the valve to the vessel's bottom. To get beer out of the keg, gas pressure is applied to the top surface of the liquid, which presses the beer from the bottom of the keg up through that metal tube and out of the valve.
To use the valve, you need another piece of equipment. This is called the coupler—the party pump used at that keg party is a gussied up version of one. Proper couplers connect to two tubes: one brings gas to the keg and one lets the beer flow out on its way to your glass. The coupler plugs into the top of the keg and has a little handle that you pull down to open the valve and start the flow of gas into the airspace of the keg.
So, I've mentioned a few times that we need gas to push the beer out of the kegs. Most bars and restaurants use canisters of pure bottled carbon dioxide and nitrogen for this purpose. A regulator (that thing with the gauge) allows the operator to control the amount of pressure leaving the tanks.
Those keg party taps aren't as effective because the gas they use to pressurize kegs comes from your hardworking pumping action. So instead of using pure bottled gas, you're pumping the air around you into the keg to build pressure. Unfortunately, the air around you is packed with oxygen and wild yeast and bacteria that will quickly spoil your beverage. And one more thing: the imprecise, varying pressure applied by that pumping action will allow the bubbles in your beer to be released. This is why kegs only last one wild night when they are poured from a party pump—by morning the beer is oxidized, flatter than it should be, and down the road to spoilage.
Woohoo! The beer is on its way to you! On the trip from keg to faucet, beer travels through vinyl or polyethylene tubing measuring about a quarter inch in diameter. In systems where the beer has a long distance to travel from keg to tap, this tubing may be chilled to ensure the beer stays cold on its journey to your face.
The last piece of equipment standing between you and your beer is the faucet. Here, you run into another valve, which is controlled by pulling the tap handle. The tap needs be opened completely (by pulling the handle all the way forward), or you'll create turbulence that will cause foaming in the glass.
Getting it Right
It doesn't sound all that complicated...yet. This is where things get tricky.
The system detailed above is held in a delicate balance, hinging on the amount of pressure that's applied to the keg as it relates to the resistance imposed by tubing, gravity, and other hardware like faucets and couplers. The temperature and amount of carbonation in the beer matter too. So it's actually super easy to screw up. A system with too much or too little applied carbon dioxide pressure will produce foamy pours and kegs that are overcarbonated or super-flat—all things that cost the bar or restaurant money and leave the customer with a less-than-perfect beer.
There are a lot of factors that dictate what needs adjustment and how much applied pressure should be used. Here are just a few examples:
- If the refrigerator holding the kegs is in the basement, you need more pressure to counteract the force of gravity and the resistance coming from trying to squeeze beer through 15 or more feet of tubing.
- If the cooler is above the bar, you apply less pressure because gravity is on your side.
- The warmer the temperature in the cooler, the more pressure you need to apply—because the carbon dioxide in beer is less soluble at warmer temperatures, more pressure is needed to keep the bubbles from escaping.
- If a beer is very highly carbonated, more pressure needs to be applied as well.
- Pouring beer at higher elevations in the mountains? More pressure.
All of this wouldn't be too much of an issue if you could just crank up the bottled carbon dioxide pressure as high as you'd like. If the necessary pressure is pouring beer too fast, you can just lengthen the tubing between keg and faucet to add resistance, right? Unfortunately, that's not how it works.
If too much carbon dioxide pressure is applied, it will be absorbed by the beer as excess carbonation (and if not enough carbon dioxide pressure is applied, the beer will go flat). To avoid overcarbonation in systems that require higher pressures, bars use a blend of carbon dioxide and nitrogen. Nitrogen is less soluble in beer than carbon dioxide, so it will push the beer without being absorbed as carbonation.
If the system is dialed in properly, beer will pour from the taps at about two ounces per second. When proper carbonation has been maintained, it's easy to form an appropriate head of 1-2 fingers atop your glass and little beer is wasted to foaming.
One More Thing...
The job isn't done quite yet. Draft systems require a whole lot of maintenance to make sure the beer is tasting great. Ideally, bars and restaurants should be cleaning their draft systems every two weeks and replacing components regularly. If they fail to do so, you may end up with funky off-flavors in your beer. Lines left uncleaned are susceptible to build-up of crud which can harbor yeast and bacteria that can make your beer taste musty, buttery, or sour. Yuck.
About the author: Mike Reis is a Certified Cicerone working as Minister of Education for California-based distributor Lime Ventures. Previously, he co-managed the beer program at San Francisco's Abbot's Cellar and The Monk's Kettle. Follow him on Twitter @beerspeaks or find him behind a pint near you.