Anyone who has ever had a Guinness Stout on tap knows the cascading off-white head surging and swirling above the nearly black liquid. The head is full of extremely fine bubbles, creamy on the tongue and an integral part of the signature of this world-famous beer. This is now emulated in cans and bottles of Guinness Draught, as well as by several other dry stouts, including Beamish and Murphys. Additional beers — such as Boddington’s Pub Ale, Caffrey’s Irish Ale and even Pyramid DPA in the US — have extended the concept to include other ale styles carbonated and served with nitrogen blends rather than solely with carbon dioxide.
Homebrewers need not feel left out, we can serve such distinctive beers, too. It’s not that difficult to duplicate the “nitro pour” of your favorite pub draught. All it requires is some additional equipment beyond a standard keg set up and somewhat different carbonation and serving methods. In this article, I’ll explain what it takes to do the job right and pour your beer proudly.
First, it’s important to know what is occurring in these beers. For thousands of years, brewers have relied on the carbon dioxide (CO2) produced during fermentation, and naturally dissolved in the beer, to provide bubbles and a tingling sensation when served and consumed. As the beer warms, the CO2 comes out of solution as it is less soluble at higher temperatures. This is part of the flavor profile of nearly all beer styles; it’s less so for some British ales, but almost no beer is entirely flat. Apart from cask ales, most tap beers are pressurized with additional carbon dioxide in order to force the beer through the lines and prevent staling due to contact with oxygen in the air.
Nitrogen gas (N2) comprises around 78 percent of the Earth’s atmosphere. Nitrogen is much less soluble in water or beer than carbon dioxide — around 80 times less at beer serving temperatures. And, it does not react with beer. In beers served with nitrogen — or, much more typically, a nitrogen-carbon dioxide blend — the nitrogen is forced along with the beer through tiny holes in the tap that create millions of nearly microscopic bubbles and a creamy, long-lasting cascading head.
American Pilsners have around 5,000 mg/L of CO2 dissolved in them. In contrast, beers served with nitrogen typically contain around 2,400 mg/L of CO2 and only about 20 mg/L of N2.
The components of a mixed gas dispensing system are similar to that for regularly carbonated beer, with a few important differences. Obviously one is the gas itself. The usual blend is 75% nitrogen and 25% carbon dioxide (occasionally the percentages vary) and is sold by many gas distributors. If your supplier does not have it, look in the yellow pages under “carbonic gas” and ask for “mixed gas,” “beer gas” or “Guinness gas.”
Mixed gas cylinders are slightly different from those that contain carbon dioxide. The threads on mixed gas cylinders are left-handed so that a mixed gas regulator cannot accidentally be attached to a CO2 cylinder. Additionally, the gauges typically have higher maximum readings because both the storage and dispensing pressure of nitrogen is higher. If your homebrew supplier does not have mixed gas equipment, these items are available from many gas suppliers.
A major difference is the faucet for dispensing nitro beers, which is easy to distinguish from a typical tap because it is both taller and thinner. The key element is a small disc called a “restrictor plate” that impedes the flow of beer and gas, and forces them through tiny holes (usually five). Don’t confuse a stout faucet with a creamer faucet; they are not the same thing.
The dispensing pressure for beers on mixed gas is considerably higher than for other beers, both because of the restrictor plate and the nature of nitrogen itself. Guinness recommends, and most bars push nitro beers with, about 30 PSI of gas pressure.
While nitrogen beers have a dense, creamy head, these beers are not highly carbonated. In fact, excessive carbonation may be objectionable because it adds a sharpness (from dissolved carbon dioxide that reacts with water to form carbonic acid) that is not part of the flavor profile of these beers. It also results in excessive foaming when being dispensed. Guinness Draught, for example, is carbonated only to about 1.1–1.2 volumes of CO2. Most of this already occurs as the gas is dissolved in the beer during fermentation; little CO2 is added later for force carbonation.
It’s worth considering which beers may benefit from dispensing with mixed gas. Not all styles are appropriate for this method. For one thing, it tends to increase the perception of a beer’s body or mouthfeel. This is ideal for low-gravity styles such as dry stout, but is much less desirable for beers that already have a high final gravity and considerable body. Another result of mixed gas is to decrease the perceived bittering and hop aroma. Highly hopped and bitter beers will appear less so when served this way. Lagers tend to have an odd creamy quality that seems out of place.
Beer served on nitrogen is not the same as cask ale, which shares a low carbonation level but is typically dispensed with a hand pump that mechanically uses the vacuum pressure of air to draw the beer from the cask. This mixing of air with the beer causes flavor changes over time that traditional “real ale” partisans consider the hallmark of a properly-cellared beer. Some pubs attempt to imitate cask ale by serving their regular beers through a stout faucet with mixed gas, but this is a poor substitute for real ale and produces quite different results.
Consider nitro beer as practical only if you keg your beer. While some commercial breweries have developed systems for canning and bottling such beers, this is beyond the technology of homebrewers. The “widget” in the bottom of these cans and bottles is injected with a droplet of liquid nitrogen and added during filling. When the can or bottle is opened, the change in air pressure forces the nitrogen out of a tiny hole in the widget and diffuses it throughout the beer, resulting in a cascading head when it is poured, much like the tap version. Bottling without the widget does not result in the creamy head of a “nitro pour.”
To prepare a beer for mixed gas, brew and ferment an appropriate style in the normal way, but carbonate it very lightly. For example, if a dry stout is fermented at 68 °F (20 °C), it will already have nearly 0.8 volumes of dissolved CO2. Increasing this to the recommended 1.2 volumes would require only a little force carbonation if the beer is chilled to the recommended serving temperature of 43 °F (6 °C). Force carbonating should be done with pure CO2; trying to accomplish this with mixed gas requires much more time. If you opt for priming the keg, use less than 1 oz. (28 g) of corn sugar for 5 US gallons (19 L).
Although some brewpubs simply begin pushing carbonated beer with the nitrogen mix, others claim that you need to equilibrate the beer with the mixed gas. (Guinness kegs are shipped with nitrogen already dissolved in them.) This requires diffusing the gas into the beer. This can be accomplished by letting cold beer sit under high pressure, although — given the low solubility of nitrogen — this can take a week or more. A better way is to inject mixed gas into the beer through a stone. Turn the regulator to a pressure higher than the pressure in the headspace of your keg and bubble gas through the beer (releasing the headspace pressure occasionally). Ashton Lewis, of Springfield Brewing says, “We gas for 30 minutes, rest for 30 minutes and gas for 30 minutes.”
Once the beer is ready to go, connect the gas fitting of the keg to the mixed gas regulator and cylinder, and the beer fitting to the dispensing line. Open the mixed gas cylinder fully, adjust the pressure at the regulator to a reading of 30 PSI (205 kPa) at 43 °F (6 °C) and test for a smooth pour, a cascading head and no excessive foaming of the beer. Adjust as necessary.
Guinness very carefully instructs pubs and servers in the proper dispensing technique, but it really boils down to these six steps:
1. Use a clean, dry unchilled glass.
2. Hold the glass near the faucet at a 45-degree angle.
3. Pull the handle forward to the fully open position.
4. Fill the glass approximately three-quarters full.
5. Allow the head and the beer to settle for one to two minutes.
6. Fill the glass to the top as necessary so that the cascading head just climbs above the rim.
That’s all it takes for “a perfect pint” and nitro nirvana. As the Irish say, “Slainte!”
Bill Pierce is Brew Your Own’s Advanced Homebrewing columnist.