Today, is St Patrick’s day. This cultural and religious celebration honours the death date of the patron saint of Ireland. It’s purpose, traditionally, is to commemorate saint Patrick and the arrival of Christianity in Ireland, although more recently it is a day of celebrating Irish culture in general. People dress in green, Christians attend church services and the restrictions on food and drink imposed by Lent are lifted. Perhaps because of this, drinking alcohol has become what the holiday is notorious for, with celebrations spreading across the world.

If you do choose to celebrate the day with a tipple, what stereotypically Irish drink would you be most likely to choose? Of course the mind springs instantly to a cool pint of Guinness, the Irish dry stout first brewed in Dublin in 1725. Guinness is a legendary drink, not only for being the best selling drink in Ireland but also for its gravity defying bubbles! After your local bartender pours a pint of the dark stuff (and if it’s only your first pint!) You might notice that the foamy white bubbles settle downwards rather than rising to the top.

In a beer or a fizzy drink, carbon dioxide is added to the drink at a high pressure and when you release this pressure by opening them, it begins to escape, creating bubbles and making the drink fizzy. The bubbles are less dense than the liquid and so flow upwards to release the gas at the surface. But in Guinness, the bubbles defy these physical laws and flow downwards. This sinking bubble mystery puzzled everyone: from the people in your local pub, to physicists in the lab. A scientific paper published in 2012 finally provided the answer.

The solution appears to be a combination of the shape of the glass and the type of bubbles in Guinness. The bubbles in Guinness, unlike in bitter, contain nitrogen as well as carbon dioxide. The diffusion of nitrogen is very different to the diffusion of carbon dioxide. The dynamics of the fluid in the glass are influenced by the diffusion of these gases. The other ingredients in Guinness also coat these nitrogen bubbles and influence their velocity in the liquid.

The largest surface area is the flat middle of the pint glass, as opposed to the curved sides. Therefore, comparatively, more bubbles will rise from the middle. Here, the Guinness bubbles in fact do flow upwards and in doing so they drag the liquid with them. This upward rush of liquid from the centre will then move out and flow down the walls of the pint glass dragging the bubbles down with it. It is similar to the way a water fountain works. This creates the illusion from the outside that the bubbles do, in fact, flow downwards. This knowledge has informed industrial processes and the food industry.

But why does the Guinness contain nitrogen? It has a low solubility in liquid and works to displace carbon dioxide, giving a unique, creamy and frothy head. Draught Guinness is dispensed from kegs pressurised with nitrogen to achieve this but it does not translate well into canned beers.

To overcome this, the scientists at Guinness developed a “widget”. This contains nitrogen and when the pressure in the can is lowered by opening it, the widget releases additional nitrogen. This successfully mimics the delicious head on a draught Guinness. In 2006, Guinness also introduced another option, a “surger” to sit under a pint glass and send out ultrasonic waves to drive bubbles out of solution and add to the creamy froth.

The head is bright white and more aesthetically pleasing than foams that appear on bitters. This is, again, thought to be down to the nitrogen in the bubbles. The bubbles are smaller than those of just carbon dioxide and when the light hits them, it scatters equally and the foam appears bright white to our eyes. Everyone can remember back in school being asked to sort various objects into the categories of solid, liquid and gas. However, Guinness foam complicates this classification as it satiates the requirements of all three!

Therefore, the packing of the particles in the foam of beers was also a big question. 2D foam always joins together in a honeycomb structure, however, 3D foam is much more complex. Researchers in trinity college, Ireland, discovered using Guinness that the 3D structure of foam is comprised of two slightly different types of 14-sided particles binding together in a lattice. This discovery is honoured in a statue in the university and inspired the design of the aquatic centre in the 2008 Beijing Olympics.

So where will you be raising your pint of Guinness this St Patrick’s day? Sheffield has some great options including a St Patrick’s day specials at the Students’ Union, the Guinness Tent of Fargate, the Uni vs Hallam Paddy’s Day bar crawl on Carver street and ‘Nice Like Potato’ at the Harley. Or if you want to go further afield; then the Guinness factory in Dublin is always a great trip where you could learn even more about the famous Irish stout.

Before you hit the town don’t forget to check out; for the government guidelines on drinking.

Happy St Patrick’s Day!

#StPatricksDay #Guinness #Chemistry #EllenMoye

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