What is that plastic ball doing in my can of beer?
To understand what that plastic ball or widget is doing floating at the top of your can of beer, we must start off with a short lesson in what makes beer fizz!
Most beers are carbonated with carbon dioxide (CO2). Some of the CO2 is dissolved in the beer and when the can is closed, the pressure inside the can is higher than outside of the can . When you open the can, there is a sudden drop of pressure causing some of the CO2 to bubble out of the beer with the agitation of the pouring. This results in a beer with a thick layer of head on top.
Stout beers like Guinness, which are canned with widgets inside, are pressurized with a mixture of CO2 and nitrogen. Nitrogen is not absorbed into the beer at the same extent as the CO2. While a can of stout beer may have the same pressure as a can of lager, due to its lower CO2 content, it is less fizzy.
Without the widget, a stout like Guinness, would not have a very thick layer of head because most of the CO2 would stay dissolved. The purpose of the widget is to release the dissolved CO2 from the beer, to create more head, giving the beer a creamier longer lasting head, that stout is known for.
The widget is a plastic, nitrogen filled sphere with a tiny hole in it. Just before being sealed, a small shot of liquid nitrogen is added to the beer. During the rest of the canning process, the liquid nitrogen evaporates and pressurizes the can. As the pressure increases in the can, the beer is forced into the widget through the small hole, compressing the nitrogen inside it.
When the can is opened, the pressure drops and the compressed gas in the widget forces the beer out of the plastic ball and into the can. This agitation causes the dissolved CO2 to form tiny bubbles that rise to the top of the beer and form the head!
ENJOY YOUR FROTHY BEER!