A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Why do the bubbles in Guinness Stout float down?

May 26, 2000

Dear Cecil:

A subject perhaps worth extensive research: Why is it that poured Guinness Stout bubbles appear to float downward in the glass?

Cecil replies:

Spending a lot of time staring at beer glasses, Manuel? Not that you're the only one. They say Crick and Watson, who discovered the structure of DNA, found daily inspiration at the pubs, and I've done some of my best work there myself. Although admittedly my breakthrough, instead of the double helix, was discovering Stroh's spelled backwards was "shorts."

I'm not a big drinker of Guinness, which always seemed to me like something you'd pour on pancakes. But dadgummit, this is the Straight Dope. I bought a bottle of Guinness Stout at the supermarket and got a genuine tapered pub-style beer glass given to me by my brother-in-law's English wife. (I recognize that Guinness is made in Ireland, not England, but it was the best I could do.) I poured the stout into the glass and observed the bubbles. Noticed they floated up. Thought: Manuel, you gotta lay off the sauce. But to be sure, I went online and searched for "Guinness Stout," "bubbles," and "down." Hmm. Turns out I shouldn't have bought a bottle of Guinness, but a can.

From my online reading I learned that the secret of Guinness's creamy mouthfeel, as the taste experts put it, is a mix of nitrogen and carbon dioxide rather than pure CO2 as the bubblizing ingredient. Nitrogen bubbles are much smaller than CO2 bubbles, a mere 50 microns in diameter, and produce a smoother head. But nitrogen doesn't produce bubbles as spontaneously as CO2. At the corner tap they deal with this by using a special nozzle that aerates the stout with nitrogen as it's poured. In packaged goods that's not possible, so for a long time those drinking Guinness at home were stuck with pure carbon dioxide. But science marches on. Using brain cells that might have solved the third-world debt crisis, Guinness engineers invented a plastic device, known in brewing circles as a "widget," that's placed in each can (not bottle) of Guinness Draught. (See patent info at www.ivo.se/guinness/patent.html.) When you pop the top and pour, the pressure in the can drops to ambient, stout squirts out of the widget, and nitrogen is liberated from solution and aerates the exiting beverage. Result: nitrogen microbubbles galore, same as if the pouring were done by your local barkeep.

I hustled back to the supermarket, grabbed a four-pack of the tall black cans, and gave them a shake. Something inside rattled — a good sign. Took 'em home, stuck 'em in the fridge for the prescribed three hours, then popped and poured. This was accompanied by a satisfying whoosh and — I tell you, it's a fascinating thing to watch — cascades of tiny bubbles, sliding down the inside of the glass.

Now for the part where I actually answer the question. Tiny or not, nitrogen bubbles ought to have more buoyancy than the surrounding liquid. Why don't they? I consulted Clive Fletcher, professor of computational fluid dynamics at the University of New South Wales, Sydney, Australia. Fletcher, a disciple of the Crick and Watson school of research, was wondering one day why the bubbles went down. A chemist friend, one of these if-all-you've-got-is-a-hammer-everything-looks-like-a-nail kind of guys, tried to explain the phenomenon in chemical terms. Bah, thought Fletcher. Using computational fluid-dynamics software from Fluent Inc. that modeled the activity of the bubbles in the glass, Fletcher found that what occurs is similar to convection — the bubbles rise in the center of the glass, where you can't see them because of the stout's opacity, then heel over and skitter down the sides, their buoyancy overcome by the viscous drag of the roiling brew. (See illustrations at www.fluent.com/news/pressre l/guinness/tsld001.htm — click on the thumbnails to get larger versions.)

To further extend the frontiers of knowledge, Professor Fletcher says he's thinking about trying to "persuade Moet & Chandon to fund some cross-industry research, with plenty of field trials." Sounds like a worthy goal to me. Naturally, professor, you'll want a second test site north of the equator, so as to control for the Coriolis effect. Let me know if I can be of help.

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