Sometimes beers are rather capricious. They foam when they’re not supposed to, and when you want to have a nice head, it collapses all too quickly.
Basically, beer only begins to foam when the beverage comes into contact with air, i.e. when it’s being poured.
This triggers a natural chemical reaction: the carbon dioxide contained in the beer no longer has any counterpressure from the bottle or the keg, is released and rises to the top in the form of small bubbles. The warmer the beer is, the faster this happens. The gaseous carbon dioxide here brings small protein molecules with it, which are responsible for the colour of the foam. The protein molecules adhere to the carbon dioxide, and form small skins around the bubbles. Thanks to these skins, the bubbles then stick to each other at the surface. And there it is: the white foamy head.
But unfortunately the head doesn’t last for ever. The culprit’s name is evaporation, which causes the fragile skins around the carbon dioxide to become progressively thinner, until finally the carbon dioxide bubbles burst and the foam dwindles into nothingness.
One beer that’s famous for its head is wheat beer. Here, the main purpose of the head is to keep the carbon dioxide inside the beer, and thus prevent it from acquiring a stale taste. But of course, appearances also play a major role, since it’s really the head that makes the beer look fresh and quaffable.
To put a nice-looking head on the beer, the main thing is to get the technique right. The basic principle involved is this: the more vigorously the beer comes up against the side of the glass, the more it will foam. So you should hold the glass at an angle when starting to pour, and only move it upright towards the very end.
If despite the best of techniques you don’t manage to get a nice-looking head, the problem probably lies with the glass. If, for example, it has any fat on it, the protein molecules will be prevented from forming a skin around the bubbles, and the foam simply cannot get started.
This may not be news to many of you, but the head is in fact an indicator for the quality of the beer. The primary factors involved here are the time the head remains on the beer, plus the height and consistency of the foam.
So a nice foamy head should not be underestimated! And if despite a clean glass and the technique described above you still can’t manage it, my advice is: practice makes perfect