Manufacturers typically make beer kegs out of stainless steel, although aluminum is also occasionally used. Commercial kegs used in bars and restaurants have a metal tube inside the keg that runs from the bottom of the vessel to the valve through which beer is poured.
The coupler on a keg connects to the valve, attaching two tubes to the keg. The tubing attached to the coupler typically is made out of vinyl or polyethylene. One of these tubes controls the flow of gas into the keg, while the other controls the flow of beer out of the keg. Bottled carbon dioxide and/or nitrogen connect to the keg with the tubing, pressuring the keg for easy dispensing.
Historically, brewers transported beer in casks made of wood. These early kegs were made of oak strips held together with steel hoops. Once beer brewing became a flourishing, widespread industrial process, brewers began using stronger metallic materials for the vessels. In the mid-19th century, the tanks were made of copper. In the latter part of the 1950s, stainless steel replaced copper production vessels. During the next decade, lighter weight aluminum materials gained preference with strength comparable to stainless steel with approximately 30 percent less material weight.