Beer Bubbles and Science
Usually, discussions related to the science of beer is to do with the biology of hangovers, or the chemistry of cocktails; but today I am going to explore the physics happening within the glass.
The Institute of Physics (IoP) recently looked very closely at a pint of stout and asked the question, “Are the bubbles rising or falling?“. After several rounds of drinks, they found that the bubbles are actually falling. When you look at your stout, it appears that some are rising and some are falling, but the bubbles that are falling have first risen from the bottom of the glass in order to fall. This acts somewhat similar to a convection current: The bubbles in the centre of the glass (where they are difficult to see) are projected towards the head of the beer, at which point they slide to the side of the glass, where they sink back from whence they came. This is a feature common in many fizzy drinks, but it is much more noticeable in stout because of its colour.
Another point of discussion, which probably isn’t frequent around the bar but should be, is the colour of the head of your beer. The IoP went ahead and asked the question, “Why is it that the body of a pint of beer is amber, but its head is white?”; while New Scientist went one step further and pondered why a pint of Guinness is a treacly black colour with a thick and creamy white head. The answer to both of these questions sits in the scattering of light that happens within the bubbles.
Inside your pint glass, the liquid absorbs blue light, but allows red and green light to pass right on through, and so it looks brown. Guinness absorbs a broader range of light wavelengths, and so it lets barely any light through- giving it that familiar black colour. Inside the bubbles on any pint; light enters and scatters through a large array of bubbles and leaves as a mixture of light, which is why it remains white.
In the Easter of 2012, I had the opportunity to receive a lecture from Dr. Cyril Isenberg, whose research focuses on the science behind bubbles. While his main work is on soap films and soap bubbles, he has looked under the microscope at beer heads before; and found that bubble surfaces meet at 120°, and whose lines meet at 109.5° (These numbers will definitely appeal to any Chemists reading). These bubbles occur in cavity wall insulation, in soufflés, and in soap bubbles.
The IoP are putting particular emphasis on communicating knowledge like this, in order to promote the fact that science is everywhere in daily life- even in the pub. Andy Moffat from Redemption Brewing Company, who are producing 5 short films about Beer Physics with the IoP said:
“People are interested in how science relates to their everyday life, rather than seeing equations and complex theories. You can take something quite complex and relate it back to someone and they get a real interest from it.”