Nitrogen and Stout Faucets

What is the standard response when non-beer nerds hear someone order a Guinness Draught? “Meal in a glass!” So evocative are the creamy head and dark body that they are convinced it has a nutritional content somewhere between beer and a chocolate milkshake. The magic that turns a 4.0% ABV 125-calorie session ale into a decadent dessert is a special faucet and a whole lot of pressure supplied by nitrogen. Nitrogenation and stout faucets were originally intended to replace finicky real ale served through a sparkler — where air provides the nitrogen (along with beer-staling oxygen) and a human-powered hand pump provides the pressure. Nitro pours are a poor imitation of real ale, but that doesn’t mean they aren’t enjoyable on their own merits!

Irish dry stout, bitter, and dark mild are the most traditional styles served on nitro, but amazing things happen when you take a flavor-packed imperial stout, old ale, or pumpkin ale, and give it the same treatment! While that may sound fun, as I’ll explain, you can’t simply fill your kegerator’s carbon dioxide (CO₂) tank with nitrogen (N₂) to achieve the same results.

Pros

In addition to that eat-it-with-a-spoon head and creamy body, the other advantage of a stout faucet is how pretty it pours. A standard draft should settle quickly and without drama. The high serving pressure (20-30 PSI, 138-207 kPa) and restrictor plate cause the nitrogenated beer to shoot out as whitewater, churning liquid and tiny bubbles. As it settles, the liquid cascades through the rising bubbles creating a mesmerizing appearance. I post a lot of beer photos on social media, and nothing gets more likes/retweets/upvotes than mid-settling nitro pours.

Cons

Serving a beer on nitrogen (more precisely beer gas, Guinness gas, or G-mix all describe a roughly 75/25 blend of nitrogen and carbon dioxide) does have drawbacks. Even if you already own a kegerator, the initial setup will cost about $300: Roughly $100 each for the stout faucet, nitrogen tank, and regulator. There is also the continued expense; my gas supplier charges more to exchange 5-pound (2.3-kg) tanks of beer gas than 20-pound (9.1-kg) tanks of CO₂. To make matters worse, each beer gas fill contains half as much total gas as the same size tank of CO₂ (despite higher tank pressure, nitrogen doesn’t liquefy as easily as CO₂), and because you’re also serving at twice the pressure you’ll empty it about four times quicker!

Even if money is no object and the idea of having an ultra-creamy head on all of your homebrews is appealing, there are still two potential cons to consider. Aroma is carried by CO₂ coming out of solution. With almost all of the carbonation knocked out during the turbulent pour, a nitrogen-poured beer won’t have much of a nose in the glass. That’s the reason I’ve never loved IPAs on nitro, their aroma is usually my favorite part! Beers can also taste flabby, lacking sharpness because the carbonic acid tingle is missing.

Equipment

Before purchasing a regulator, ask your gas supplier if they swap beer gas tanks with CGA-580 nitrogen-style (female) or CGA-320 CO₂-style (male) fittings. Don’t worry if you buy the wrong type or want to change suppliers, CGA-580 to CGA-320 adaptors are available.

Next you’ll need a stout faucet with a restrictor plate (a tiny disk with five pinholes the beer is forced through). This is where the real magic happens. In fact, many bars push all of their drafts with beer gas to avoid over-carbonating when using high pressure to overcome the resistance of long liquid lines. At home we aren’t pushing the beer far and want lots of pressure at the stout faucet, so ignore line balancing charts and use a relatively short liquid line — I use 4 feet (1.2 m) of 3⁄16-inch (0.5 cm) line.

You’ll also want a good drip tray. Stout faucets are messy because there is a space between the valve and the restrictor plate where a teaspoon or two (5-10 mL) of beer collects and then drips for 10 minutes after each pour. Unscrew the bottom of the faucet and rinse it periodically.

The Science

The reason beer is traditionally carbonated rather than nitrogenated is that yeast produce carbon dioxide during fermentation. Even beer right out of the fermenter has a small amount (~0.8 volumes) of CO₂ in solution because it is soluble at fermentation temperature and atmospheric pressure. Nitrogen is only 1% as soluble as carbon dioxide (at 1 atmosphere of pressure N₂ has a solubility of .024 g per kg of 50 °F (10 °C) water compared to 2.5 g of CO₂)^¹.

The result of this low solubility is that very little nitrogen dissolves, even at 25 PSI (173 kPa). Rather than just holding N₂ in solution, most of what nitrogen provides is the force to shove the beer through the restrictor plate. If you didn’t mind the effort and wasted gas, you could achieve a similar effect with CO₂ alone: Carbonate at 6 PSI (41 kPa) and pour at 25 PSI (173 kPa). To prevent it from over-carbonating you would need to reduce the pressure to 6 PSI (41 kPa) and vent the headspace between pours. Beer gas allows beer to sit at high pressure indefinitely without over-carbonating/nitrogenating.

Non-reactive gasses behave according to Dalton’s Law — 25 PSI (173 kPa) of 75/25 beer gas is equivalent to applying:

25 × 0.75 = 18.75 PSI of N₂ and 25 × 0.25 = 6.25 PSI of CO₂ or 173 × 0.75 = 130 kPa of N₂ and 173 × 0.25 = 43 kPa of CO₂

This example uses gauge rather than absolute pressure to illustrate the point, but actual calculations must account for the effect of the atmospheric pressure on the gauge. Each gas behaves according to its own partial pressure, so if you have a beer with CO₂ dissolved in it, no matter how much nitrogen pressure is in the headspace, CO₂ will come out of solution until the CO₂ pressure in the beer and the headspace equalize. This is why we use beer gas. Beer carbonated with CO₂ and served with pure N₂ would gradually go flat as it sat in a partially consumed keg.

Best Practices

You can take a favorite recipe and pour it with your new tank and tap, but there are two recipe adjustments to consider. The aroma will be muted by the low carbonation, so increase the specialty malt, late-boil hops, or spices to compensate. The beer won’t have the acidity provided by carbonic acid (dissolved CO₂), so aim for a slightly lower final pH. This is what Guinness achieves with the lactic secret sauce they blend into their clean base. Dark grains are acidic, so for dark beers the solution can be to reduce chalk, baking soda, or slaked lime water treatments. If your water is high in carbonate, add 1-3% acidulated malt once the saccharification rest is complete. Lowering the pH of the wort or fermented beer with a small amount of food-grade phosphoric or lactic acid is another solution.

There are a variety of options for preparing fermented beer for service: Prime in the keg with sugar, force carbonate with 5-7 PSI (34-48 kPa) of carbon dioxide, or force carbonate with 25 PSI (173 kPa) of beer gas. I pre-carbonate (pure CO₂ or priming sugar) aiming for 1.8-1.9 volumes of CO₂. Commercial brewers who specialize in nitro beers often force gas through a diffusion stone into the brite tank to overcome the low solubility of N₂. Others vent standard carbonated kegs and push with beer gas. With the beer chilled to 40-45 °F (4-7 °C) attach the beer gas and adjust to 25 PSI (173 kPa). You’ll get a good pour right away, but the head will become denser as nitrogen slowly dissolves.

When it’s time to pour, remember this is all about presentation. Use a clean, dry glass for the best effect when pouring. Rather than the traditional pint glass, I prefer Scottish thistles (see picture on pg. 109) because they funnel the creamy head into your mouth with each sip! Tilt it to 45 degrees and pull the faucet forward until it is parallel to the floor, slowly bring the glass upright until the foam reaches the rim. Stare transfixed by the swirling and tumbling bubbles for a minute and then push the faucet back slightly to top-off. You may need to adjust the pressure towards 20 PSI (138 kPa) if you are pouring a glass of foam, or 30 PSI (207 kPa) if the beer isn’t coming out with enough head.

In the now-famous refrain of Donna Meagle and Tom Haverford, sometimes you’ve got to “treat yo self!” No homebrewer needs a nitro setup, but it adds special something to the right beer. If you need one more reason to spend the money, you can pour cold-brewed coffee on nitro!

References

¹ http://www.engineeringtoolbox.com/gases-solubility-water-d_1148.html

Written by Michael Tonsmeire