How to Tap a Keg

A keg is a fancy version of your basic barrel. In the factory, a sheet of stainless steel is rolled into a cylinder and welded together. Then, at the cylinder's midpoint, a set of ribs is pressed into it for added rigidity. Next, top and bottom steel plates are stamped out and likewise welded into place.

The steel in question, a particular alloy of chromium, nickel, manganese and several other elements, is used because it welds cleanly leaving a smooth joint -- this is very important for a food-grade container because you don't want any rough surfaces where bacteria can set up shop. [source: nickelinstitute.org]

The airtightness of a keg is incredibly important -- it's what keeps the beer from becoming flat and flavorless. Still, the beer has to come out somewhere, and that's why there's a spear. The spear is a long metal tube that reaches almost to the bottom of the keg. Despite its warlike name, the spear is basically a big metal straw through which beer can travel up from the bottom of the keg and out through the tap at the top [source: nickelinstitute.org].

Just what are taps, and why do English and American brews refuse to come out of the same ones? We'll get to that in a bit. Right now, let's find out why all kegs are not created equal.

Few things kick off a party more effectively than tapping a fresh keg. It's a dramatic too. But don't take my word for it, just watch how fast the line forms following the welcoming hiss of a freshly tapped keg. But there's a lot of pressure on the the one doing the tapping, both literally and figuratively. Fortunately, it really isn't all that complicated.

Before you get the beer flowing, make sure you do everything that you can to keep the keg cold. If the beer isn't cold enough, you'll likely have a lot of foam on your hands. So let the keg rest in one spot for at least a few hours before tapping it to prevent warm beer or excessively foamy beer [source: homebrewingadvice.com].

Once the beer is cold enough, and has acclimated to it's environment, it's time to tap. Do so by following the steps below:

When a pub orders up its draught beer supply, usually what's delivered is a truckload of standard kegs known as "half barrels." The half-barrel is also the garden-variety keg commonly nestled in a big bucket of ice at a frat party. It's called a half-barrel because the "barrel" is a legal unit of measure used for import and export statistics.

In the U.S., a barrel, by definition, is 31 gallons (117 liters) of fermented beverage. So, as you might have guessed, a half-barrel holds half that amount -- somewhere between 14 and 16 gallons (53-60 liters), or by a more agreeable measure, 124 pints (117 metric pints). This is slightly larger than the 50 liter (13 gallon) standard European keg. Fill either kind of keg with beer, and you've got an object weighing about 160 pounds (73 kilograms). By contrast, an empty half keg is a lonesome but hardly insubstantial object that weighs approximately 30 pounds (14 kilograms) [source: Wayland Works].

If you're at a smaller party where people aren't jumping naked into the pool and spraying shaving cream all over somebody's car, then you might encounter a more civilized vessel -- the quarter keg, aka the "pony keg." It's called a pony because, like ponies, the quarter keg is small -- but it can still hold 90 pounds (41 kilograms) of beer. [sources: Wayland Works, Word Detective].

For the sedate dinner party, there's the refined and aerodynamic "torpedo keg" (or the "sixth barrel") which holds a little more than 5 gallons (19 liters). And for an evening with friends, there's the very restrained "mini-keg," affectionately known as "bubba," which provides a pleasant 10 and a half pints (10 metric pints) with which to while away the football game. Last but hardly least is the Beer Ball, a plastic number cradling 5 gallons (19 liters) in its svelte interior [source: Micromatic].

There are other, more arcane vessels lurking in the margins of beer culture. The Cornelius, or "corny" keg, is the basic soda canister that servers are always changing at fast-food joints. Capable of holding 5 gallons (19 liters) of liquid, the corny keg was long ago identified by home-brewers as the ideal vessel for their purposes. [source: Brew Ware].

All kegs, no matter their size or style, must deal with the same vital issue: how to get the beer out. On the next page we'll talk about the complex challenges this seemingly simple problem presents.

Steel kegs are built to withstand pressure, both from the outside and the inside. With some beer containers (casks, mini-kegs, torpedoes and beer balls) the spout is often at the bottom of the vessel, and gravity forces the beer out of the tap when it's opened. But when it comes to the average keg, the tap is at the top. So what makes that beer defy gravity and flow through the tap and beer line at will? In a word, pressure. But creating that pressure is a tricky business [source: Wayland Works].

When people talk about pressure in beer kegs, they mean the force exerted down onto the surface of the beer, forcing it up through the spear. What exerts that pressure may differ. The average party keg uses, appropriately enough, a "party pump." Party pumps are simple and cheap. All you do is pump them a few times, and that builds up enough pressure to force a pint of beer up and out the tap. But what you're pumping into the keg is air, and one of the things that makes up air is oxygen.

Our lungs love oxygen, but beer really doesn't. Oxygen tends to destabilize the complex flavors and aromas in a beer while at the same time absorbing the carbonation. This means that in the space of a day or two, the contents of the keg will be both tasteless and flat. Not a problem for a well-attended party, but also not an option for a pub that might take weeks or months to go through a single keg [source: Brew Ware].

That's why some clever beer lovers came up with the idea of a tap system, which maintains the pressure inside a keg by feeding pressurized CO2 or nitrogen into it at a constant rate. Of course, this means that you have to have a tank of the stuff handy.

Carbon dioxide alone works pretty well, and for some beers, CO2 combined with nitrogen is even better. It's all about maintaining the natural carbonation in a beer -- and carbonation is … CO­2. So when pressurized CO2 dissolves into the beer, it's just adding more of what's already there.

But there can be too much of a good thing. Excess carbonation can make turn beer into a big glass of fizz. That's why nitrogen can be so handy: Like CO2 it's more or less tasteless and odorless, but unlike CO2 it doesn't dissolve easily into beer. A good mix of the two gases can maintain the natural carbonation in the beer without upsetting the balance of its flavors and aromas [source: Wayland Works].