Apart from its smoky taste the other distinctive feature of stout is the thick creamy head that results from the stout being mixed with nitrogen as it is poured. According to the Guinness website
, the head should be like 'velvet' and is an integral part of the experience of drinking stout.
Most beers use carbon-dioxide to pressurise the beer but nitrogen is used as the main gas for most stout resulting in smaller bubbles being formed and giving the drink a creamier head of foam. The trouble is, adding nitrogen also changes the physics of bubble formation.
Who better to investigate the way stout bubbles form than a team of researchers from the University of Limerick,
A graduate student, supervised by applied mathematician William Lee, has discovered that in normal beer, bubble formation quickly starts at 'nucleation sites' around cellulose fibres in the beer and glass but that the large bubbles quickly fizzle out.
The researchers adapted a mathematical formula first developed for champagne and were surprised to find that bubble nucleation also happens when nitrogen is used, but much more slowly. Wired Science
Lee and his team recorded stouts under a microscope to watch bubbles form inside cellulose fibers. They discovered the bubbling rate was up to 20 times slower than in carbonated brews, which is probably why no one had noticed it before.
Currently widgets, a small plastic ball with a hole in it, are used in canned stout to create the creamy effect. When a can is opened, the widget releases pressurized nitrogen into the beer, which then triggers more dissolved nitrogen in the beer to bubble out.
The University of Limerick
researchers believe that a postage-stamp sized piece of cellulose attached to the inside of the can would have the same effect, which may make canned stout both cheaper and more environmentally-friendly. “If you line a can with enough of them, you can get a creamy head in less than 30 seconds,” Lee said.
"The global sales of stouts are so huge that even tiny savings could add up to quite significant profits," Lee said in an Australian Broadcasting Corporation report
It also seems the research is a matter of national pride for the Irish researchers with Lee saying to Wired Science:
“What happens around these fibers is really complex, so it’s a ripe area for research. This is also a matter of national pride. Stout beers are as culturally important to Ireland as champagne is to France.”
Lee has posted a paper titled Bubble Nucleation in Stout Beers
on the online physics archive arXiv.org
. Below is a YouTube post of the bubble nucleation of stout beer.