Commercial brewers who use this fascinating technology require several pieces of specialized equipment. The first is a widget, or a plastic bladder-type doo-dad with several very small holes and/or check valves. Most widgets these days look like little footballs and move freely in the package, as opposed to the first model found jammed in the bottom of Guinness cans. Bottled widget beers require extra-heavy glass since the headspace has a higher pressure than carbonated beers. To start the process, an empty widget is inserted into the can. Next, they fill the bottle or can with beer and a small drop of liquid nitrogen is added immediately before it is sealed. Since liquid nitrogen has a very low boiling point, it boils in the container and creates enough pressure to partially fill the widget with beer. When the beer is opened and the headspace pressure is released, the gas in the widget rapidly expands and forces the beer in the widget through the holes or check valves and then the magic happens. Nitrogen bubbles lead to more nitrogen bubbles and, after several seconds, the beer has a head you can cut with a knife.
