St. Patrick’s Day is here, which means large quantities of the world’s most popular stout are about to be consumed with wild abandon.
But despite its simple beauty, there’s actually some very complicated physics behind Ireland’s most beloved beer: Dublin Guinness.
That’s why scientists have shared their secrets to getting the perfect pint of this ebony nectar, famous for its robust malt sweetness.
This iconic drink is packed with nitrogen gas and has long been considered unique because its bubbles flow downward.
Whether you’re pouring it yourself at home or watching a bartender do it for you, here’s how to get the perfect pint of Guinness, according to science.
According to experts, the perfect pint of Guinness should be served at 45 degrees and served at 42.8°F (6°C), with a head no more than 0.7 inch thick.
Firstly, a perfect Guinness should come in the classic contoured glass, which is wider at the top and slopes inward closer to the bottom.
William Lee, professor of industrial mathematics at the University of Huddersfield, said this tilted shape is ideal for Guinness bubbles to flow downwards, helping them settle faster.
The shape directs the bubbles downward near the wall of the receptacle, although there are bubbles rising in the center that we cannot see.
“Near a tilted wall, the bubbles rise leaving the bubble-free beer behind, then the bubble-free beer slides down the wall to the bottom of the glass,” he told MailOnline.
When pouring, the clean, cold glass should be held at a 45-degree inclined angle, the optimal point to let the beer flow smoothly into the glass.
Guinness itself officially recommends the classic contoured glass, not only because of the shape, but because the harp logo etched on the front helps with a perfect pour.
Experts told MailOnline that when pouring the spout from the tap should be as close to the harp of the glass as possible but without the two touching.
“As the beer begins to reach the golden harp in the glass, notice that your bartender begins to slowly straighten the glass, again without dipping the spout,” a Guinness spokesperson said.
The perfect Guinness should come in the classic contoured Guinness glass (pictured), which is wider near the top.
Why do Guinness bubbles go down?
Experts at Osaka University said the bubbles of many newly opened carbonated drinks move upward, but Guinness has bubbles downward.
As fans will know, the bubbles in a glass of Guinness descend in a spectacular “waterfall” while those in the beer rise from the bottom.
This is actually due to the contoured shape of the Guinness glass, which typically curves downward from the top.
This changes the density of the liquid inside the glass as the bubbles move away from the wall, forming a dense region near the edge.
This region sinks under its own weight because it floats less than the surrounding fluid, dragging the bubbles with it.
In essence, bubbles obey the laws of physics in that they “try” to rise through the liquid, they are simply dragged down by the circulation of the fluid.
Bubbles from a pint of Guinness fall forming a “waterfall”
Guinness also recommends the famous “two-part pour,” which It involves pouring two-thirds of the pint and letting the bubbles settle, before pouring the remaining third.
This ensures a creamy white foam with a perfect thickness: 12 to 18 mm (0.4 to 0.7 in).
Ideally, you want to stop the first part of the pour at the top of the harp because any higher and it will be too full and will give the pint too thick a head.
But stopping lower than the harp, which is at the widest part of the glass, will result in a head that is too thin after the final pour.
Guinness, which recommends a serving temperature of 6°C (42.8°F), had been adding nitrogen gas to its barrels for more than half a century, resulting in smaller, more stable bubbles that don’t dissolve as easily.
This results in a light white foam with a mousse texture while softening the burnt flavor of the roasted barley for the perfect balance.
Professor Philip Moriarty, an Irish physicist at the University of Nottingham, says this probably gives Guinness whiskey (poured from a barrel through taps) an advantage.
This is despite the fact that the can contains the famous ‘widget’: the small white ball filled with nitrogen that releases the gas the moment it is opened.
“I think the debate between eraser and bottle or can is definitely leaning heavily towards the eraser side,” Professor Moriarty told MailOnline.
‘It’s the nitrogen in Guinness that sets it apart from other brands and the flow of nitrogen bubbles will be very different for the drink, particularly when compared to the rather more “explosive” effect due to the nitrogen element they put in the cans.
“Again, whether that flow makes a difference in terms of overall flavor may well be a question of psychology rather than the physics of fluid flow.”
Pub-goers will be used to having to wait ages for their beloved Guinness while bar staff perform the “famous” two-part pour. Pictured are glasses of Guinness before the second part of the two-part pour.
Many Guinness fans would probably agree that a glass of Guinness served in a can or bottle is never as good as drinking it straight from the pub tap.
Recognizing this, Guinness last year launched an “ultrasonic” device called “Nitrosurge” that works only with a special line of Guinness cans.
The £25 electronic gadget sits on top of the can and pours out a perfectly straight stream of liquid, as if coming from a stream of air.
It means that technically the perfect Guinness could come from a can, as long as the Nitrosurge device is connected.