Ever wondered why the head on a nitro beer, like Guinness, sticks around longer than on a regularly carbonated beer?
Today’s dispense digest is all about nitro beer.
During fermentation, yeast consumes sugar and produces alcohol and carbon dioxide – the booze and bubbles in our beer, respectively.
So most beers traditionally get their sparkle from carbon dioxide gas. Either naturally, like with cask- or bottle-conditioned beers, or the CO2 can be added by a process called force carbonation.
But not all beers are carbonated equally.
Beer carbonation is measured in volumes of CO2: an absolute measure that’s relative to the volume of the container holding the beer.
Let’s say an average lager contains around 2.5 volumes of CO2. For a standard 30L keg of beer, this means that 2.5 kegs’ worth of uncompressed CO2 gas has been compressed and dissolved into that single keg of beer.
But some beer styles are significantly more highly carbonated. Take the Belgian Golden Strong Ale, for example. This style typically has 3.5 to 4.0 volumes of CO2, giving it that characteristic head of foam that fills nearly half the glass!
On the other hand, there are also some very lightly carbonated beer styles, like cask beers, or real ale, which typically contain 0.8 to 1.5 volumes of CO2.
Historically, when casks were made of wood, they couldn’t hold very high pressures, so a gentle carbonation is characteristic for this style. (Even after metal casks became the norm.)
As you can imagine, different approaches are needed to accommodate these different carbonation levels so each beer pours at its best.
My next post will delve into the details on cask beer conditioning, but for now, all you need to know is that most cask beers are dispensed using a “pull” system – the beer is drawn from the cask using a hand pump, also known as a hand pull or beer engine.
When kegged beer was introduced in the 1960s, draft dispense moved to a “push” system – gas pressure was applied to a keg and that pressure then pushed the beer through the lines and to the tap for dispense.
Looking to find a way to replicate the creaminess and low carbonation of a cask conditioned ale on a “push” system, breweries turned to nitrogen gas.
(Did you know we had cask ale to thank for nitro beers? It was news to me!)
Enter: the nitro beer
Here’s how they work: nitrogen gas is only one-sixtieth (1/60th!) as soluble as carbon dioxide, so in order to get it to dissolve into a beer it must be forced into the beer under high pressure. (This step happens at the brewery).
Then, to keep that pressure in the beer at the time of dispense, bars will use a blended gas mixture containing 75% nitrogen and 25% CO2 gas (instead of CO2 only).
It’s important to note that nitrogenized beers don’t only contain nitrogen gas – they still contain carbon dioxide, but at much lower volumes than normal. Typically closer to 1.1 to 1.4 volumes… which is conveniently in the range of traditional cask ale. (You see what they did there?)
Because of the high pressure needed to force the nitrogen into the beer, special equipment is needed to help break the gas back out of the beer during dispense.
All nitro beers are all served through a special nitro faucet.
Inside the faucet there is a restriction plate which forces the beer through several tiny holes of varying diameter. In order for the beer to continue flowing through this restricted diameter at the same rate, its flow needs to speed up, which causes a drop in pressure.
This pressure drop then causes the nitrogen gas to break out of solution and the small nitrogen bubbles begin to rise to the top of the beer. The pressure drop also causes the CO2 gas that’s in the beer to break out, too, but at a different rate.
Carbon dioxide breaks out faster than the nitrogen does, causing the beautiful cascading effect seen when pouring a nitro beer.
Once those small nitrogen bubbles rise to the top, forming the beer’s head, they tend to stay there. Why?
Molecules want to be in equilibrium and will readily move from an area of high concentration to an area of lower concentration to even things out.
But our atmosphere is 78% nitrogen gas (which is a very high concentration, compared to the less than 0.1% of our atmosphere made up by CO2), so the nitrogen in the head is in no real hurry to leave!
So, as the gas has nowhere to go, the head on a nitro beer will stick around for a longer period of time, compared to the head on a regularly carbonated beer.
There you have it – nitro beer in a nutshell.
Check back next time to learn what happens to a cask ale as it conditions.
Note: there are a number of factors that determine how much head a regularly carbonated beer forms and how long it lasts, but it’s an incredibly complex topic for another time!
Sources: Brewer’s Association Draft Beer Quality Manual, Tasting Beer by Randy Mosher, Cellarmanship (5th ed) by Patrick O’Neil
Image sources: Guinness Instagram, London Pride Instagram, Duvel Instagram
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