Madeline Darnell, of Athens GA (USA) but visiting New Zealand, asks :-

Why does the "head" on a glass of Guiness beer seem to be finer and longer-lasting than other beers?

Ralph Bungard, a biochemist who is brewer and owner of Three Boys Brewery, responded.

I love this question as it was one that, as a student and young home brewer, I asked myself. My solution at the time was to add lots of gelatine just as I bottled my brew – bad idea! Unless you like beer-jelly that is!

Although Guinness traditionally had a “strong head” of foam, it probably never reached the long-lasting examples we see in the modern brew. The trick the modern Guinness brewers use is in fact a process not naturally associated with brewing.

In most traditionally brewed beers the “fizz” or carbonation comes from dissolved carbon dioxide that is a natural by-product of fermentation. Guinness, instead, artificially substitutes most of this carbon dioxide with nitrogen gas.

Why that substitution with nitrogen gas then causes the fantastic head retention on Guinness is explained by Dalton’s law of partial pressures. In short, Dalton’s law says that the total pressure exerted by a mixture of gases is made up of the pressure of each individual gas in the mix and those pressures are simply related to each gases concentration. In other words, carbon dioxide at low concentration in a gas mix will also have a lower pressure than if the carbon dioxide was at a higher concentration.

Foam bubbles in beer make for an interesting demonstration of this law. In naturally carbonated beer, the foam bubbles contain almost pure carbon dioxide that is escaping from the beer. Outside of that bubble is the atmosphere, which contains only a fraction of one percent of carbon dioxide. Dalton’s law says that the high concentration of carbon dioxide in the foam bubble creates a greater pressure inside the bubble compared with the low concentration and pressure of carbon dioxide in the atmosphere. The result is that the foam bubbles expand because of that high internal pressure and eventually pop. Just like over-inflated balloons.

Unlike the low concentrations of carbon dioxide, nitrogen gas makes up nearly 80% of the atmosphere. The clever people at Guinness realised that by filling the bubbles of their beer with nitrogen gas, the make-up and therefore pressure of the gas inside the foam was much closer to that of the surrounding atmosphere. The result being that the foam bubbles filled with nitrogen gas did not expand and burst at the same rate as bubbles fill with carbon dioxide.

Why don’t all brewers use the Guiness trick? For a few good reasons. For a start you need some special equipment to force nitrogen into beer, nitrogen makes the beer feel a little “flat” and you lose that lovely “sparkly” feeling that comes from carbon dioxide. Although clever, it’s not really a natural process associated with traditional beer making.