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The Great Beer Foam Experiment
NPR's Joe Palca Gets to Bottom of Beer Foam/Altitude Puzzle

Listen Listen to Joe Palca's beer foam experiment, conducted at sea level in NPR's Washington, D.C. headquarters.

Joe Palca

Joe Palca conducts his "experiment."
Photo: David Banks, NPR

Dec. 7, 2001 -- What do you do in your spare time when you're high up in the Andes Mountains? If you're NPR science correspondent Joe Palca, you do a little experiment -- with beer.

Palca wanted to see what would happen when he poured a beer into a glass at more than two miles above sea level, and he got his chance earlier this year when he was on assignment in La Paz, Bolivia (elev. 11,975 feet). In theory, beer or any other carbonated beverage poured at that altitude would have a tendency to produce more foam when poured, because there is less atmospheric pressure keeping the carbon dioxide gas in the liquid. In the rarefied air of La Paz, the atmospheric pressure is one-third to one-half as much as at sea level.

The first part of Palca's experiment involved a cold Pacena beer -- a Bolivian favorite.

beer poured in la paz
Beer foam from a warm Pacena

Top, a reenactment of Palca's beer foam experiment in La Paz, Bolivia, at two miles above sea level. Bottom, Palca measures foam from a "warm pour" at sea level. The experiment roughly shows about an inch of difference in the amount of foam that forms at the different altitudes.
Photos: Edward Olney, David Banks, NPR

In his radio report, Palca gave NPR listeners a play-by-play: "So I'll open the Pacena, pour it into the glass, and see how much liquid I get, and how much foam I get.

"Open the beer -- that's a nice sound -- and now pour the beer. I'm pouring the beer straight into the glass. And, gee that is a lot of foam.... I've got about one inch of liquid and two and three-quarter inches of foam. Now that's a lot of foam.

"But to prove that altitude is the key variable, I've got to do the same experiment with the same beer, chilled to the same temperature, in the same glass, at sea level -- and see what happens," he concluded.

Months after the La Paz visit, Palca, back in Washington, made good on the second half of his experiment. And he also added another variable: warm vs. cold beer.

"If I pour a warm beer into this same style glass, I should also get a foamier head -- that's because carbon dioxide doesn't dissolve as well in warm liquids as it does in cold," Palca said.

The results? It appears Palca's first hypothesis is correct. The foam from the Pacena poured at sea level rose about two inches in the glass -- slightly less than the beer poured in La Paz months earlier, and just about the level Palca anticipated for the higher atmospheric pressure in Washington, D.C. And the warm beer was indeed much foamier than the cold one.

And it was a good excuse to open a couple of beers, well before cocktail hour.

More from NPR's Joe Palca:

listen Ever wonder how many calories are in your Thanksgiving dinner? Ever wonder why you bother to wonder? Ever wonder how you'd even find out? Joe Palca answers at least one of those questions.

listen Joe Palca took a trip to the U.S. Department of Agriculture Food and Nutrition Laboratory to learn where all those calories you ate on Thanksgiving actually go.

Other Resources

• Learn more about beer 'fizzics' from Stanford chemistry professor Richard Zare.

Bolivia Web.com has information about the city of La Paz.

• The Web site for Cervecería Boliviana Nacional, makers of Pacena beer.