One of the most important aspects of my job is training new bartenders, and one of the most important aspects of that training is showing them how to pour a pint of beer and get it right. Oh sure, anyone can pull on a tap handle and fill a glass with the amber nectar, but if you don't pay attention to detail you're likely to lose one very big part of the experience - presentation. A pint of beer with a head on it just looks good! It looks right. Not only that, it's an important part of enjoying the beer's flavour. But where does it come from?
Like any carbonated drink, beer will foam up when it's poured because of the carbon dioxide that's dissolved in the liquid. Beer is naturally carbonated during fermentation by yeast as it eats up the sugars and burps out CO2. For centuries this was the only way to 'condition' beer (condition = how much fizz it has), and in the case of traditional cask- and bottle-conditioned beer and most homebrew, it still is. Kegged beer, and most bottled beer, has been force-carbonated: extra carbon dioxide is persuaded, under pressure, to dissolve into the beer. This is the biggest reason why British beer is often thought of as being flat. It isn't, if it's been looked after properly, but more on that later.
If soda pop and beer are both fizzy because of dissolved carbon dioxide, why doesn't soda have a head like beer does? The simple answer is that it would if it were made of the same stuff. The process of bubble formation is the same in both - there's more CO2 in the liquid than there is in the atmosphere and it's trying to achieve equilibrium, which is why it'll eventually go flat if you leave it out long enough as the amount of gas in the liquid reaches the same volume (about 0.03%) of CO2 as air. The liquid might originally have had up to four volumes of CO2 dissolved in it (four gallons of gas dissolved in one gallon of liquid), and once the container is opened the gas suddenly gets the opportunity to make like John Dillinger and escape its confines. Once it goes below about one-to-one, it's just not fizzy enough any more.
Beer, like soda, might be mostly water, but it also contains proteins which come from the grains (barley malt, wheat etc) it's brewed with, and these help to stabilise the bubbles that form as the carbon dioxide in the beer comes out of solution and forms those streamers of rising bubbles that you see in a pint. The more proteins and polypeptides in a beer, the better the head retention, which is why wheat beers keep their heads longer - there's more protein in wheat than there is in barley. Given enough time though, any head will shrink and disappear. It's made of bubbles. What do bubbles do best? They go pop! And when they do, they release a delicious whiff of the beer they came from, which is why you can taste so much more beer when you drink it from a glass instead of a bottle. Savouring the aroma of a beer is an integral part of the experience; don't deny yourself the pleasure by swigging it straight from a longneck!
There are certain compounds in a beer - volatiles - which evaporate quickly, and in doing so take some of the beer's flavour and character with them. A good head of foam will prevent those from disappearing into the ether.
A lot of fuss is made by some beer geeks about using the correct glass for your beer. Yes, the shape of some glasses can enhance the beer by concentrating the aroma and enveloping your nose in it as you tip the glass towards you to take a mouthful, like the tulip glass for instance. And to be sure, I like to use one of those, or a goblet, chalice or snifter, and I sure don't have the same prejudice against the straight pint glass as some do. But as far as I'm concerned the best glass for your beer is a clean one, and I'm not talking about dirt.
Detergent is a killer of protein-based bubbles. If your glass hasn't been properly rinsed and dried you won't get a decent head (or much head retention) on your beer. It's the same with eggs - if you haven't got all the detergent off your whisk or your bowl, that meringue or Baked Alaska isn't going to be as impressive as it might have been if you'd rinsed properly the last time you washed the dishes. I'll often review a beer when I'm at a bar or pub, but when describing the appearance I'll always make the disclaimer that I don't know for sure how thoroughly rinsed the glass has been, particularly if it comes with little or no head. If I know that bar chills its glasses I'll also make a point of asking for one that's at room temperature if the bar's not too busy. The one drawback of a clinically clean glass is that you'll probably lose some of that gorgeous-looking latticework of foam (aka lace) that clings to the inside of the glass as you drain it because the foam needs something to cling to.
How much head there should be is often a bone of contention where draught beer is concerned. The bigger the head on a pint, the less beer you're getting because, obviously, the head is mostly gas, and some publicans and bar owners will try to get away with serving pints that have more head than there should be. My personal view is that anything more than a finger of head is too much, but I do want to see some foam on my beer, for reasons both aesthetic and nasal. Some countries limit the size of the head with laws that enforce either the size of the glass (exactly one pint), the size of the head (exactly one pint to a line but with more glass above the line), or with numbers (95% of the pint must be liquid).
Temperature plays an important role in the head of a pint and is the main reason you get your keg beer cold, despite what detractors may say about macro beers needing to be cold so that you can't taste how awful they are. Most kegged beer is kept and poured at 38°F (3.3°C) because that's the optimum temperature for pouring it. If it's a little colder than that the carbon dioxide will be more reluctant to come out of solution and make a head; if it's four or five degrees warmer the bartender's going to have a hard time pouring your pint without sending several ounces of beer down the drain as it foams up, and that means the CO2 isn't where it should be - in your beer, which means your beer will be flatter than it ought to be.
And that brings us back, rather neatly, to cask-conditioned beer. Often the butt of jokes about it being warm and flat, cask beer is neither of those things. Cask-conditioned beer is best suited to being served at around 50° - 55°F (roughly 12°C) which, conveniently enough, is the temperature of the average pub cellar. Since it hasn't been force-carbonated but instead relies solely upon the action of the yeast for its CO2, it's much less fizzy than kegged beer, and cask aficionados will point out the smooth, velvety carbonation of a pint of real ale, as opposed to the comparatively harsh carbonation of keg. This as an effect on the head: it's more difficult to make one on a hand-pulled pint, but there is a remedy. A sparkler is nothing more than a nozzle attached to the spout which agitates the beer as it's forced through several tiny holes and encourages the formation of a head. Some say that it strips the beer of its body, others make the same claim about flavour, and any good bartender should remove the sparkler before pouring your pint if you request it.
There is one kind of beer over which the bartender has almost no control of the size of the head: beers pushed with nitrogen instead of carbon dioxide, the most well known example being Guinness.
Nitro beers are a very different beast to all others because of the nature of the gas. Nitrogen is less soluble in beer than carbon dioxide so a much higher pressure is used to dispense it (around 32lbs psi for nitro as opposed to 12 - 14lbs for CO2). As the beer is poured it's forced through five small holes in a metal plate in the spout (a bit like the sparkler on a cask beer spout) which is how it gets its tiny bubbles that give rise to that hypnotic cascading effect, and give it its creamy head and mouthfeel. Nitro beers keep their head much longer than carbonated beers because there's far more nitrogen in the atmosphere (78%) than CO2 (0.03%), so the nitrogen in the beer isn't in quite such a hurry to equalise itself with the nitrogen in the air.
Now, I'm not overly keen on nitro beers, with two exceptions: stouts and porters. There's something about the dark/high roasted malt flavours that either work well with nitrogen or are powerful enough not to be altered by it, and nitrogen, although flavourless and odourless, does alter the taste of beer somehow. I have a theory (and it may well be complete tosh) that it's because of what it lacks rather than what it adds. Carbon dioxide, when dissolved in water, makes a very small amount of carbonic acid (the same stuff that dissolves limestone so that it can make stalagmites and stalactites) which slightly sours the water. In beer it's almost undetectable because of all the other flavours present, but the fact is nitrogen doesn't do the same thing. What it does do though, in my opinion, is ruin a pale ale or a bitter. It's not for me.
Don't diss the head - it's there for several good reasons and it's an intrinsic part of appreciating a beer. Just insist there's not too much of it.