Pouring draught stout on CO2 versus mixed gas
Draught stout lives or dies on the gas behind it. A nitro stout needs a nitrogen rich blend and high pressure through a restrictor faucet to build its creamy head. Put it on straight CO2 and the carbonation drifts within days. Here is how the two systems work and how to set them up.
Nitro stout is a low carbonation beer under high pressure
A typical draught ale or lager carries about 2.3 to 2.7 volumes of dissolved carbon dioxide. A nitro stout carries much less, roughly 1.1 to 1.4 volumes, plus a small amount of dissolved nitrogen. Nitrogen dissolves poorly in beer, which is exactly why it forms such fine, persistent bubbles when it is forced out.
To get that effect, the beer is pushed hard, often around 2 to 2.8 bar (30 to 40 psi), through a stout faucet with a restrictor plate: a small disc with several tiny holes. The beer is forced through the holes, nitrogen breaks out as millions of tiny bubbles and you get the surge and cascade, then a dense white head.
Why pure CO2 ruins a nitro stout
Gas pressure in the keg headspace sets the carbonation of the beer over time. At 3 °C, around 0.8 bar (12 psi) of pure CO2 holds a beer at about 2.5 volumes. Push a nitro stout at 2.5 bar of pure CO2 and the beer slowly absorbs carbon dioxide until it reaches somewhere near 3.5 volumes or more. The stout becomes fizzy, sharp and pours as a glass of foam through the restrictor.
Turn the CO2 down to the low pressure that matches 1.2 volumes and you lose the drive needed to push beer through a stout faucet. You get a slow, flat pour with no cascade. Either way the beer is wrong.
The damage is slow, which is what makes it hard to spot. A keg hooked up to the wrong gas on Monday may pour fine on Monday night. By Thursday the carbonation has crept up and the head has turned loose and bubbly. Staff blame the faucet or the beer, rarely the cylinder.
Mixed gas lets you push hard without over carbonating
The answer is a blend. Nitro stout is usually dispensed on beer gas of about 70 to 75% nitrogen and 25 to 30% CO2. The total pressure is high enough to drive the stout faucet. The CO2 part, about a quarter to a third of the total, is close to what that low carbonation beer needs to stay stable. The nitrogen does the pushing without dissolving much.
Bars with long runs sometimes use a different blend, around 60% CO2 and 40% nitrogen, for normal ales and lagers. That is a separate tool. Do not put a nitro stout on a 60/40 blend and expect it to hold.
The stout faucet is part of the system
A standard faucet will not give a nitro stout its head, whatever the gas. The restrictor plate inside a stout faucet is what knocks the nitrogen out of solution. Keep it clean. A partly blocked plate gives a slow, uneven pour with large bubbles. Many bars remove and soak the nozzle and plate every day. Our guide to draught line cleaning covers the full routine.
The pour itself is two parts: fill about three quarters at a 45 degree angle, let it settle for a minute or so, then top up straight. The guide to the two part nitro pour walks through it.
Standard stout on CO2 is a different beer
Not every stout is a nitro stout. Many craft stouts are carbonated like any ale and served on CO2 through a standard faucet. They pour with a tan head of larger bubbles, a livelier body and a more roasty, sharper finish. That is a legitimate choice and many brewers prefer it for a stronger or sweeter stout.
The mistake is moving a beer between systems without changing its carbonation. A stout carbonated to 2.4 volumes for CO2 dispense will pour as foam through a stout faucet. A stout carbonated for nitro will taste thin and lifeless on a standard faucet at low pressure.
Setting up a nitro line in a hot climate
India adds two complications. First, beer gas cylinders can be harder to source than CO2 in smaller cities, so some bars try to make do with CO2. Do not. If blended gas is not available, serve the stout as a CO2 beer through a standard faucet instead.
Second, temperature. Higher beer temperature means lower gas solubility, so a stout line that warms to 8 or 10 °C will pour foamy and lose its cascade. Keep the keg at 3 to 4 °C and make sure the line is cooled right up to the tap. Check the gas blend on the cylinder label every time it is changed. A cylinder of the wrong blend is a common, invisible fault.
Diagnose before you change anything
If a nitro stout is pouring badly, check in this order: cylinder label and blend, regulator pressure, keg temperature, faucet plate and nozzle, then the beer's carbonation with the brewery. Change one thing at a time and pour a test glass after each. Taste matters here too. A stout that has drifted onto too much CO2 tastes prickly and sour at the edges long before the pour looks wrong.
Your first step: read the label on the cylinder behind your stout tap today and write down the blend and the regulator setting. If it says pure CO2 and the tap has a restrictor plate, you have found your problem.
Common questions
What gas is used for nitro stout?
Usually a beer gas blend of about 70 to 75% nitrogen and 25 to 30% carbon dioxide, at around 2 to 2.8 bar through a stout faucet.
Can I serve nitro stout on CO2?
Not for long. At the pressure a stout faucet needs, pure CO2 over carbonates the beer within days. At low pressure it pours flat.
Is 60/40 mixed gas the same as nitro gas?
No. A 60% CO2 and 40% nitrogen blend is for ales and lagers on long draws. Nitro stout needs a nitrogen rich blend of about 70 to 75% nitrogen.
