Guinness is peculiar. It tastes creamy and has a fine-textured head you just don’t find in most other beers.
You can chalk that up to the presence of nitrogen.
Most beers just contain carbon dioxide. If you cut open a can of Guinness pub draught, you will discover a plastic orb with a pinhole opening — that’s a widget. This device allows Guinness in the can to have a fine, creamy head similar to Guinness on tap.
I don’t know when or why the good people at Guinness (now owned by Diageo, a British alcohol conglomerate) decided that nitrogen would be a good thing to add to their beers. Rumour has it that the introduction of nitrogen was devised in order to approximate the mouth-feel of a traditional cask ale — a low carbonation “living” ale dispensed simply by pumping air to push the beer from the barrel to a glass.
Nitrobrews or nitrogenised beer are a 20th century Guinness invention. Not only does the stout in the keg contain nitrogen, but it is dispensed from the keg with a mixture of roughly 25 percent nitrogen and 75 percent carbon dioxide. From 1969-1984, Guinness was devising ways to create a nitrogenised version of their beer in a can, to approximate what one might find in an Irish pub.
The American Patent and Trademark Office gives the Guinness widget patent number 4,832,968, filed in 1986 with the very sexy title “Beverage package and a method of packaging a beverage containing gas in solution.”
During the beer-canning process, the widget is put in the bottom of the can. The can is filled with stout (containing dissolved carbon dioxide and nitrogen gas), but the key to the process is that during filling a shot of liquid nitrogen is added to the can and then the can is sealed. Nitrogen has a low boiling point (-196ºC), so it essentially boils inside the can after it is added, becoming a gas that expands and forces stout and the dissolved gases into the widget.
The pressure in beer cans, of course, is greater than the air pressure outside the can. This is why beer gives that satisfying “pfft” when you open it; the gas is escaping. When you open a can of Guinness, the stout and dissolved gas inside the widget tries to get out of the plastic ball to equalize with the pressure outside the can.
The opening in the widget is so small that the beer literally rips itself apart trying to get out. Plus, this surging exodus agitates the rest of the beer outside the widget. As a result, millions of tiny nitrogen (and carbon dioxide) bubbles froth up forming the foamy head, which approximates the so-called “perfect pint.”