What is the Function of the Widget in a Beer Can?

The Widget in Guinness. Observe the hollow space near the center of the sphere, where a tiny opening is situated.

To address this question, we must first explore what gives beer its fizziness and how foam develops.

The majority of beers are carbonated using carbon dioxide (CO₂). While the beer is sealed in the can, part of the CO

₂ dissolves into the liquid, while the rest remains at the top. The dissolved CO₂ is responsible for the beer's fizziness. When the can is opened, the rapid pressure drop and the motion of pouring cause some of the CO₂ to escape from the liquid, creating foam on the beer.

Stouts like Guinness have a thicker, more enduring foam compared to canned lagers. Additionally, Guinness is less carbonated than typical lager beers. It is canned with a blend of carbon dioxide and nitrogen. Nitrogen doesn't dissolve into the beer as effectively as carbon dioxide, so even though a Guinness can may have the same pressure as a lager can, it contains less CO₂ (and thus less fizz) because nitrogen contributes to the overall pressure.

Since beers like Guinness have a lower amount of dissolved CO₂, pouring it from a can without a widget would result in a thinner foam, as most of the CO₂ would remain in the liquid.

The widget's role is to release CO₂ from a portion of the beer inside the can, which helps create the foam. This widget is a small plastic sphere filled with nitrogen, featuring a tiny opening. It is placed in the can before sealing and floats in the beer, with the hole positioned just below the liquid's surface.

Before sealing the can, a small amount of liquid nitrogen is injected into the beer. This nitrogen evaporates during the canning process, increasing the internal pressure. As the pressure builds, beer gradually enters the sphere through the hole, compressing the nitrogen inside.

When the can is opened, the pressure drops instantly, and the compressed gas inside the sphere forces the beer out through the small hole. This rapid release agitates the beer, causing the dissolved CO₂ to form tiny bubbles that rise to the surface, contributing to the foam.