What causes a stout's creamy head?

So I asked the same question to myself about 10 years ago and did a bunch of research.

Similar to shaving cream, or whipped cream from the pressurized can, the answer is related to Nitrogen. Nitro beer: It is a classification born from Guinness projects to simulate a freshly squeezed "cask conditioned" ale to all the people without access to cask ale.

The beers you describe are known as Nitro beers, and you see it more and more these days, like Samuel Adams Nitro White Ale, etc. Do not confuse Nitro beer with just plain old normal good head retention. Also don't get confused by the beers that are served "Nitro" AND also served normal (Smithwicks, for example). You can usually tell when its nitro or not, because of the tap. Nitro taps usually have long (often black) attachments on the tips.

Nitro tap tip:

Non nitro:

Nitro beer is pressurized with a special Nitrogen gas mix known as "G-Mix" or "Beer Gas" which is 25% CO2 and 75% Nitrogen.

Planet Earth's atmosphere composition dictates that CO2 will bubble-out of liquid easily, but Nitrogen does not. (already lots and lots of nitrogen in the atmosphere, who needs more?). Nitrogen creates stable bubbles, and CO2 does not (in comparison). Nitrogen has to be coaxed to come out of the liquid by 'bumping' some nitrogen molecules against each other.

So, to get that nitrogen "out of solution", Special taps with "sparklers" in them are used to help release the Nitrogen when serving, creating that creamy Nitrogen shaving-cream style head.

Or when its served from a can, a "widget" is used to create pressure difference when you crack the can, leading to the head being created. (Lasers are used to make tiny hole(s) the widget!). I used to think that these widgets had something special inside them, but they just cause a sudden pressure variance when opening the can.

Or you might use a Surger with canned nitro beer, which uses ultrasonic vibrations on the liquid, coaxing Nitrogen out, leading to that shaving cream head.

Once upon a time, the widgets were not plastic containers inside the can, they were syringes!

More notes: Some beer manufacturers that carbonate only using CO2, like Pilsner Urquell, also strongly suggest the use of Beer Gas when serving beer in cold places. They actually go so far as to change the gas mix recommendation depending on how cold the storage temperature is (50/50). The hotter the storage of the area, the more likely they will recommend pure CO2 for a tastier, colder beer.

When CO2 bubbles-out of a liquid it is Exothermic (it makes the liquid colder, and maybe so much that it feels like it burns your mouth and stomach).

So in colder places, Beer Gas is advised to be used to serve otherwise normal beer. Cheap bars are totally unaware of this advice while certain bars are likely required to adhere to this advice. (But I think there is a corporate witch hunt going on for violators). So you may see different style head in a "fancy place" in a cold city. Prague in the winter time?

Finally, to get seriously off topic, there is a new Fad in Coffee Houses known as Nitro Coffee (and also Nitro Tea, for that matter). Cold-brewed coffee (or tea) that is charged with "Beer Gas" and served like a Nitro Beer. I won't even post a pic, because it looks exactly like guinness. The foamy head has a very nice taste, like crema on espresso (but more stable!). Try one sometime.

All the nitrogen does is allow the beer to be pushed at a higher pressure than CO2 would, since nitrogen is much less soluble than CO2. It's this higher pressure that contributes to the head and "creamy" mouthfeel, since the high pressure pour strips out much of the carbonation. Many years ago, before Guinness used nitro, the 6 packs came with a syringe. You'd suck up some beer and shoot it back into the glass in order to create the same effect. It's this high pressure pour, much moreso than any ingredients, that creates the classic head and mouthfeel of Guinness.

In the pubs the creamy nead is achieved through the CO2/Nitrigen gas mix as mentioned already. It is also achieved by using a stout tap. A stout tap is similar in all respects to a regular tap, however the one significant difference is that inserted into the tap is a small disk that diffuses the beer through a number of small holes around the perimeter of the disk, this allows the beer to cascade through the disk, thus helping to make the creamy head.

Adding some wheat to the recipe can give some good body and head retention. There are a bunch of other methods as well. Check out this BrewWiki article on Head Retention. The main methods are:

• The use of body and head enhancing malts such as crystal, wheat, or carafoam
• The altering of the mash schedule to enhance head retaining proteins
• The use of heading agents - additives that enhance head retention
• Addition of high alpha hops - which will increase bitterness, but also increas isohumulones that enhance head retention
• Limiting the use of household soaps on drinking glasses and homebrew equipment
• The use of a nitrogen and CO2 mix for carbonation and serving
• The shape of the glass used to serve the beer

I seem to recall something about having to use a Nitrogen cannister instead of CO2 when you keg it. I have no idea how to do it if you bottle condition.