Foaming in Stout Beers

Stout beer, chips and salsa plus a foam-enhancing plastic "widget".

Three physicists walk into a bar. They've just read a new paper from the University of Limerick Foaming in Stout Beers and want to test its hypotheses first hand. "By Henry's Law, this Guinness should not foam," says one. By Fick's Law, the making of the froth will be too slow for a good head," says the second. "And by Laplace's Law," exclaims the third, "the bubbles should not form at all!" Yet our barmaid has drawn us three excellent pints with fine creamy heads. "It's a bloody miracle," they all shout and order another round.

Foaming in Stout Beers by William Lee and Michael Devereux introduces the paradox of stout beers. Unlike regular beers, champagne and soft drinks which are pressurized with carbon dioxide, stout beers are pressurized with nitrogen gas which contributes to the unique taste and creamy head of Guinness and other stout beers. But nitrogen is 50 times less soluble in water than carbon dioxide which hinders the foaming that so enhances the beer-drinking experience. To force draught Guinness to foam, the tap is fitted with a special nozzle. Stout in cans and bottles is foamed by a gas-injecting plastic "widget".

The lads at Limerick have authored a paper which tells you more than anyone should be allowed to know about the secret life of stout. Here is their webpage and here is the abstract of their mathematical analysis of how bubbles form in stout and other delightful drinks:

We review the differences between bubble formation in champagne and other carbonated drinks, and stout beers which contain a mixture of dissolved nitrogen and carbon dioxide. The presence of dissolved nitrogen in stout beers gives them a number of properties of interest to connoisseurs and physicists. These remarkable properties come at a price: stout beers do not foam spontaneously and special technology, such as the widgets used in cans, is needed to promote foaming. Nevertheless the same mechanism, nucleation by gas pockets trapped in cellulose fibres, responsible for foaming in carbonated drinks is active in stout beers, but at an impractically slow rate. This gentle rate of bubble nucleation makes stout beers an excellent model system for the scientific investigation of the nucleation of gas bubbles. The equipment needed is very modest, putting such experiments within reach of undergraduate laboratories. Finally we consider the suggestion that a widget could be constructed by coating the inside of a beer can with cellulose fibres.

Another round of the dark stuff to be honorin' William Lee, Michael Devereux and Scott McKechnie.

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