How to pour the perfect pint of Guinness this St Patrick's Day, according to science

St Patrick's Day is here which means vast quantities of the world's most popular stout are about to consumed with wild abandon. 

But despite its simple beauty, there's actually some very complicated physics going on behind the most beloved of Irish beers – Dublin's Guinness. 

That's why scientists have shared their secrets for the perfect pint of this ebony nectar, famed for its robust malty sweetness. 

The iconic brew is packed with nitrogen gas and is long considered unique because its bubbles flow downwards. 

Whether you're pouring it yourself at home or watching a bartender doing it for you, here's how to get the perfect pint of Guinness, according to science. 

The perfect pint of Guinness should be poured at 45-degress and served at 42.8°F (6°C), with a head no thicker than 0.7 of an inch, according to experts  

READ MORE £25 device lets you pour a perfect Guinness at home

 The 'ground-breaking' £25 electronic contraption lets fans experience the 'ritual' that is the two-part Guinness pour at home

First of all, perfect Guinness should come in the classic contoured glass – which is wider at the top and slopes inwards nearer the bottom.

William Lee, professor of industrial mathematics at the University of Huddersfield, said this sloping shape is ideal for Guinness bubbles to flow downwards, helping them settle faster. 

The shape directs bubbles downwards near the wall of the receptacle – even though there are bubbles going upwards in the centre that we can't see. 

'Close to a sloping wall, the bubbles rise leaving 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 tilted, 45-degree angle – the optimum point for letting the beer flow into the glass smoothly. 

Guinness itself officially recommends the classic contoured glass, not only due to the shape, but because the harp logo etched on the front helps with the perfect pour. 

Experts told MailOnline that when pouring the nozzle on the tap should be as close to the harp on the glass but without the two actually touching. 

'As the beer starts to reach the golden harp on the glass, watch to ensure your bartender slowly starts to straighten the glass, again without dipping the nozzle,' a Guinness spokesperson said. 

The perfect Guinness should come in the classic contoured Guinness glass (pictured) which is wider nearer the top

Why do Guinness bubbles go down? 

Osaka University experts said the bubbles of many just-opened carbonated beverages move upwards - but Guinness has downward bubbles 

As fans will know, bubbles in a glass of Guinness go downwards in a dramatic 'cascade' while those in lager rise from the bottom. 

This is actually due to the shape of the contoured Guinness glass, which typically curves downwards from the top. 

This changes the liquid's density within the glass as bubbles are shifted away from the wall, forming a dense region near the edge.

This region sinks under its own weight because it is less buoyant than the surrounding fluid, pulling bubbles down with it.

In essence, the bubbles are obeying the laws of physics in that they are 'trying' to rise through the liquid, they are just being pulled down by fluid circulation.

Bubbles in a pint of Guinness flow down in a 'cascade' 

Guinness also recommends the famous 'two-part pour', which involves pouring two-thirds of the pint and letting the bubbles settle, before pouring the remaining third.

This ensures a creamy white head of perfect thickness – 0.4 to 0.7 inch (12 to 18mm). 

Stopping the first part of the pour at the top of the harp is ideal because if it's any higher, it will be too full giving the pint too thick a head.

But stopping lower than the harp – which is at the widest part of the glass – will result in too thin of a head after the final pour. 

Guinness – which recommends a serving temperature of 42.8°F (6°C) – had added nitrogen gas to its kegs for more than half a century, resulting in smaller, more stable bubbles that don't dissolve as easily.

This results in a light, mousse-textured white head while also tempering the burnt flavour of the roasted barley for the perfect balance.

Professor Philip Moriarty, an Irish physicist at the University of Nottingham, says this likely gives Guinness draught (poured from a keg through taps) an edge. 

This is even though the can contains the famous 'widget' – the small, white nitrogen filled ball that releases the gas the second it's opened. 

'I think the draught versus bottle or can debate is definitely heavily weighted towards the draught side,' Professor Moriarty told MailOnline. 

'It’s the nitrogen in Guinness that distinguishes it from other brands and the flow of nitrogen bubbles will be very different for draught, particularly when compared to the rather more “explosive” effect due to the nitrogen widget they put in cans. 

'Again, whether that flow makes a difference in terms of the overall taste 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 as bar staff perform the 'famed' two-part pour. Pictured, glasses of Guinness prior to the second part of the two-part pour

Many Guinness fans would probably agree that a glass of Guinness poured from a can or bottle is never quite as good as getting it straight from the taps in the pub. 

Recognizing this, Guinness released an 'ultrasonic' device called 'Nitrosurge' last year that works only with a special line of Guinness cans. 

The £25 electronic contraption attaches to the top of the can and pours a perfectly-straight jet of liquid – as if it's coming from draught. 

It means the perfect Guinness could technically come from a can – as long as the Nitrosurge device is attached.