When I read reviews and ratings of beers on this site the format is often the same - how it looks (clarity/colour) with a comment on the colour, fineness and retention of the head - e.g. fine white head with good retention/light brown head that dissipates quickly etc.
Looking predominantly at draught here - keg and cask rather than bottles.
Now obviously we can infer things about the taste and nature of the beer from its colour and clarity - such as the type of malts, the filtration process or the presence of finings (assuming that the beer is in good condition) but what can we infer about a beer from looking solely at the nature and longevity of its head?
First of all: I don’t have evidence for everything I state below. Most things I’ve learned as a chemist, from stories or from other literature about beer.
There are some factors and compounds in beer that positively or negatively effect the longevity of the head.
Colour or the head. Foam is just a gas (carbondioxide or nitrogen) captured in a liquid (the beer). That means that color of the head is just caused by the colour of the beer.
The gas. Carbondioxide tends to produce bigger foam bubbles than nitrogen. In some countries, like the UK, nitrogen is used to produce a softer, smaller and more dense head than the traditional carbondioxide head. Almost all bottled and canned beers are produced with carbondioxide, since it’s naturally produced by the yeast cells. Also the amount of foam will say something about the carbonation level of the beer. A big head on your beer is often caused by a highly carbonated beer (or by an unskilled bartender of course).
Viscous beers tend to have longlasting heads. This is because the liquid film of the bubble dehydrates and bursts quicker if the beer has a lower viscosity.
Certain proteins in malts, unmalted grains and compounds in hops act as foam stabilizers. Medium sized proteins in the beer cause foam formation, while small sized proteins/amino acids effect the longevity of the head in a negative way. This all depends on the mashing programme and other variables during the brewing process. Some say that Caramalt increases is a good type of malt to increase retention, but I dont have any evidence for this claim.
Just from the perspective of a chemist I would say that longevity of the head of a beer could say something about the abv. Ethanol decreases the surface tension of water. The foam bubbles in the head of a beer are less stable if the beer has a high abv. Not only ethanol but other alcohols can cause this effect.
The process of barrel aging and barrel fermenting can introduce certain compounds into the beer aswell. Resins and methanol from the wooden barrel, alcohols (like butanol), esters and lactones, originating from for example wine, whisky, cognac etc., have the same negative effect. Most times when I pour a barrel aged beer I don’t see the formation of a nice foam head on the beer, just because it has been barrel aged. You can check this yourself by taking 2 versions of the same beer (non-barrel aged versus barrel aged) and see for yourself if the barrel aging makes any difference in head formation and longevity. An even easier test to see the effect is to pour coke in 2 glasses: 1 without and 1 with a small portion of liquor, like whisky or rum.
Besides that, you’ll probably know that the longevity of the head also says something about the cleanliness of the glassware the beer is served in. Fatty acids on the glass surface cause the foam head to disintegrate.