How to Pressure Ferment a Lager

Why it works

Pressure is an ester suppressant

Lager yeast fermented warm produces esters and fusel alcohols that do not belong in a clean lager. That is the entire reason the traditional lager schedule is slow and cold: you ferment at 50 to 55 F to keep the yeast from making the compounds you do not want, and you accept that a cold fermentation takes a long time.

Fermenting under pressure changes that equation. Elevated CO2 pressure in the headspace suppresses the production of the ester compounds that warm fermentation would otherwise generate, so the yeast can work at a temperature it finds comfortable without putting its stamp on the beer. The result is a lager that ferments in a week rather than a month and still tastes like a lager.

This is not a free lunch. Pressure also slows yeast activity somewhat and it makes the fermentation less forgiving of a weak pitch, because you are asking stressed yeast to work in a less hospitable environment. It rewards a healthy pitch rate and a vessel you can actually hold pressure in. The mechanism in general terms is covered in pressure fermentation explained.

Step 1

Pitch cool, with the vent open

Pitch at 54 to 58 F and leave the spunding valve fully open for the first 12 to 24 hours. Two reasons. First, yeast needs the small amount of dissolved oxygen in the wort for its growth phase, and sealing the vessel immediately does nothing to help that. Second, the lag phase produces very little CO2, so there is nothing to spund yet and a sealed vessel just gives you a false reading.

Pitch rate matters more here than in an ale. Lager yeast at lager temperatures under pressure is working against three headwinds at once. Pitch generously -- a double pitch or a healthy starter -- and the schedule below behaves. Pitch thin and you will be watching a stuck fermentation wondering whether pressure caused it.

Step 2

Close the spunding valve to 10-15 psi

Once you see active fermentation, set the spunding valve to 10 to 15 psi and let the fermentation pressurize itself to that setpoint. Lower than 10 psi and the ester suppression is weak. Higher than about 20 psi and you start measurably slowing the yeast for diminishing returns.

Ten to fifteen psi is also useful because it is close to the carbonation pressure a lager wants at serving temperature, which means the beer arrives at the end of fermentation already substantially carbonated. That is a real time saving -- no separate force carbonation step, no three days on the gas. The mechanics of that are in how to force carbonate.

Step 3

The diacetyl rest is not optional

Lager yeast produces diacetyl as a fermentation intermediate and reabsorbs it afterwards, but only if it is warm enough and still active. Skip the rest and the beer tastes of butter or butterscotch, which is the single most recognizable lager fault there is.

When gravity is within a couple of points of terminal, ramp the temperature to 65 F over several hours and hold it there for two days. Keep the pressure on. Do not crash first -- cold yeast flocculates and stops working, and once it has dropped out it will not come back to clean up the diacetyl.

If you want to be certain, a forced diacetyl test is worth doing: warm a sealed sample to around 140 F for twenty minutes, cool it, and smell it against an untreated sample. If the warmed one smells buttery, the rest is not finished.

Step 4

Crash, then transfer closed

1. Ramp down to 32 F over a day or two. A programmable controller does this properly; a dual-stage controller does it in one step, which is fine for a lager that has already finished.
2. Keep pressure on through the crash. Cooling beer contracts and will try to pull a vacuum. A vessel on a spunding valve with CO2 available fills that space with gas instead of air, which is the whole point of doing this in a sealed vessel.
3. Give it two to three days cold. Yeast and chill haze drop out, and a lager that has sat cold for three days pours noticeably clearer than one transferred the same day.
4. Transfer under pressure into a purged keg. Push with CO2 from the fermenter into a keg that has been filled with sanitizer and emptied with gas. The beer never meets air, which for a pale lager is the difference between three weeks of freshness and three months.

Where this came from

The basis for this schedule

The schedule reflects widely documented homebrew pressure-fermentation practice and the published pressure ratings of the vessels named above. Ester suppression under pressure is well established in brewing literature; the specific psi and temperature figures here are the consensus homebrew working range rather than results from our own instrumented trials. See how we pick.