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 rise out of the tap? 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 maintaining the pressure inside a keg by feeding pressurized CO2 or nitrogen into the keg 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].
Ok, but how do you keep the contents of the keg (the beer and the gas) under pressure and still allow the beer to come out without losing that pressure? This is where taps and couplers come in.