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The Physics of Beer

In honor of Oktoberfest and my last day as an APS intern, here's a post dedicated to one of the greatest inventions of all time: Beer.


The secret to a good beer? Pressure!

When it comes to beer, it's all about carbonation. Every vessel containing beer, from a glass to a keg, to cans and bottles, is created to keep beer fizzy and foamy down to the last drop.

How is this accomplished?

Let's start with the keg. A keg is just a stainless steel, airtight barrel. The standard keg - the one you would likely see at a backyard party - holds about 60 liters, or 126 pints, of beer.

In the center of the keg is a big metal straw that almost reaches the bottom. To get the beer out of a keg, a tap - a one way valve - is added to the top of the straw. Beer is forced up through the straw and out of the tap by either manually pumping air into the keg (backyard party) or by attaching a carbon dioxide canister that pumps in CO2 (local bar).
[Keg image by quantum.]

This works because the beer inside the keg is under pressure. Nerd explanation warning! Liquids are incompressible so when more pressure is applied above the beer, it is forced down. Because the air outside the keg has a lower pressure, some beer flows up and out of the keg, equalizing the pressure of the whole keg system.

The carbon dioxide added to the keg in a bar also helps to keep the beer fizzy. Beer makes CO2 as it ferments. This is what makes it bubbly. Adding more CO2, therefore, is okay. Pumping outside air into a keg, though, causes the beer to go flatter faster, rendering a beer tasteless in about two days. Because bars go through kegs much more slowly than a bunch of revelers at a party, the CO2 keeps the beer fresher longer.

What about beer that doesn't come from a keg? Though beer makes its own CO2 bubbles and keeps itself fizzy, some say it tastes best when it comes out of the tap. To re-create the same foamy goodness that you get at the bar, a little beer company called Guinness came up with a cool solution.

They created a spherical widget that gives beer from a can head when it is poured into a glass. The widget, which is pressurized when the can is filled, contains both nitrogen and CO2. The ball loses pressure, releasing nitrogen and CO2 bubbles as soon as the can is opened. The bubbles add extra foam to the beer so that it has a nice head once it's poured into a glass. More recently, Guinness created a rocket-shaped widget to add fizz to bottles. That widget releases both nitrogen and oxygen each time you tip the bottle so there's no need to pour the beer into a glass.

So, for those aspiring young physicists out there (or perhaps not so young--there is a minimum drinking age in the U.S. after all) here's an experiment for you to try next time you're out at the bar. Order the following four beers: 1) Guinness on tap; 2) Guinness in a can or bottle with a widget; 3) Other beer on tap; 4) Other beer in a can or bottle.

Compare the head from the four beers. (You can do this with an iPhone ruler app.) Does the Guinness can/bottle have as much head as the Guinness on tap? How does the "other beer" stack up? Does the Guinness widget do its job? Report back.



  1. Very nice article. Most people don't understand that air is the enemy and CO2 is better for beer.


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