What is inside the Guinness Pub Draught Can? Nitrogen Widget
The purpose of the widget is to release the CO2 from some of the beer in the can to create the head. The widget is a plastic, nitrogen-filled sphere with a tiny hole in it. The sphere is added to the can before the can is sealed. It floats in the beer, with the hole just slightly below the surface of the beer.
Just before the can is sealed a small shot of liquid nitrogen is added to the beer. This liquid nitrogen evaporates during the rest of the canning process and pressurizes the can. As the pressure increases in the can, beer is slowly forced into the sphere through the hole, compressing the nitrogen inside the sphere.
When you open the can, the pressure inside immediately drops, the compressed gas inside the sphere quickly forces the beer out through the tiny hole into the can. As the beer rushes through the tiny hole, this agitation causes the CO2 that is dissolved in the beer to form tiny bubbles that rise to the surface of the beer. These bubbles help form the head.