Pressure Fermentation: The Complete Guide (Benefits, Downsides, Equipment, PSI Chart)
What fermenting beer under pressure does to yeast, esters and hop aroma, the gear you need, a step-by-step method, and a pressure and carbonation chart you can pin to the fermenter.
In this post
Pressure fermentation means fermenting beer in a sealed vessel and letting CO2 build to a set pressure instead of venting it through an airlock. Breweries have done it for a century. Homebrewers started when pressure-rated fermenters became affordable, and it is now the single biggest reason people move from a bucket to a stainless unitank.
This guide covers what pressure actually does to a fermentation, where it helps and where it hurts, the equipment, a method you can follow on your next batch, and the pressure and carbonation numbers in one chart. If you already ferment under pressure and want the style-by-style detail, read pressure fermentation for every ale style next.
What pressure does to yeast
Yeast under a few psi of CO2 behave differently in three ways that matter to flavour.
- Fewer esters. Dissolved CO2 inside the cell suppresses the enzymes that make fruity esters. A lager strain fermented at 65°F under 12 psi tastes almost as clean as one fermented at 50°F with no pressure. That is the headline benefit: clean lagers in ten days, at ale temperatures.
- Fewer fusel alcohols. The hot, solvent-like alcohols that show up in warm or high-gravity fermentations are reduced. Big beers finish smoother.
- Slower growth. Pressure applied too early, during the first day or two when yeast are still multiplying, stresses them and can stall the ferment. This is why the timing matters more than the number.
The benefits
- Fast, clean lagers. Ferment at 62 to 68°F under 10 to 15 psi and a lager is done in the time an ale takes. No six-week wait, no dedicated lager fridge.
- Hop aroma stays in the beer. An open fermentation blows off CO2 by the litre, and it carries volatile hop oils with it. Under pressure the CO2 stays dissolved and so do the oils. Hazy IPA brewers notice this immediately.
- Natural carbonation, for free. Close the spunding valve when a few gravity points remain and the yeast carbonate the beer in the tank. No CO2 bill, no two-week wait on a keg.
- No oxygen, ever. The beer is under CO2 from day one. Dry hopping through a port, cold crashing, and transferring to a keg all happen without air touching it. Oxidation, the thing that turns a pale ale to cardboard in a fortnight, does not get a chance.
- Closed transfers. Push finished beer to a keg with the tank's own pressure. No siphon, no splashing.
The downsides
- It kills character in ester-driven beers. A saison, hefeweizen, Belgian tripel or English bitter gets its personality from esters and phenols. Pressure flattens them. Ferment those open, or apply pressure only in the last third.
- Early pressure can stall a ferment. Pitch, let the first 24 to 48 hours run at 0 to 2 psi, then close the valve. Yeast need that growth phase.
- It needs a pressure-rated vessel. Carboys, buckets and most plastic conicals are not. See equipment below.
- Bottling is harder. Beer that is already carbonated in the tank needs a counter-pressure filler to bottle. Kegging is the natural partner.
- You lose the airlock as a progress meter. You will watch a gauge and take gravity samples instead. Most people find that an upgrade.
Equipment
- A pressure-rated fermenter. Look for the working pressure, not a marketing number. Our Unitank is rated to 30 psi working and ships with a 15 psi relief valve, which covers every fermentation and carbonation pressure in this guide. Kegmenters (a keg with a wide lid) work to 15 psi but have no cone or jacket.
- A spunding valve. This is the whole trick. It is an adjustable relief valve with a gauge: set it to 8 psi and the tank sits at 8 psi, venting anything above. Ours is all stainless, 0 to 15 psi, with a built-in safety relief.
- A gas post. A ball-lock post on the lid to purge with CO2 and to push transfers. On a Unitank it is a 1.5" tri-clamp gas-in post.
- A way to take gravity samples without opening the tank. A sample valve on the side of the vessel.
- Temperature control. Pressure lets you ferment warmer, but it still needs to be a controlled warm. A jacketed tank with a controller and solenoid, or a fridge for a kegmenter.
- Optional: a carbonation stone. If you would rather force-carbonate in the tank than spund, a stone does it in a day.
How to ferment under pressure, step by step
- Chill, transfer, pitch as usual. Oxygenate the wort. Yeast still need oxygen at pitch, pressure or not. Pitch a healthy, full-size starter; stressed yeast and pressure do not mix.
- Seal the tank, valve wide open. Fit the spunding valve, set it to its lowest setting, 0 to 2 psi. For the first 24 to 48 hours you want the yeast to grow and the sulphur and green-apple compounds to blow off.
- Close down once fermentation is rolling. When kräusen is up and the gauge is climbing on its own, set the valve. Ales and hop-forward beers: 5 to 10 psi. Lagers fermented warm: 10 to 15 psi. Big beers: 5 to 8 psi.
- Hold temperature. 62 to 68°F for most ales and for warm-fermented lagers. Pressure is not a substitute for control, it is what lets you control at a warmer, easier setpoint.
- Dry hop under pressure, if the beer wants it. Through a port with a valve, or a HopDrop, while the beer is still a few points above terminal so the yeast scrub any oxygen you let in.
- Spund for carbonation. When the beer is 3 to 5 gravity points above your expected final gravity, raise the valve to the carbonation pressure for your temperature (chart below). The last of the fermentation carbonates the beer.
- Diacetyl rest, then crash. Two or three days at fermentation temperature after terminal gravity, then drop to 34°F over a day with the valve still set, so the tank never goes negative and draws air. More on the diacetyl rest, and on cold crashing.
- Dump yeast, transfer. Open the cone valve for a second to drop the yeast, then push the beer to a purged keg through the racking arm using the tank's own pressure.
Pressure chart
Fermentation pressure by stage. These are working ranges, not rules; your yeast and your beer decide.
| Stage | Clean ales, IPAs | Lagers fermented warm (62 to 68°F) | Belgian, English, wheat |
|---|---|---|---|
| First 24 to 48 h | 0 to 2 psi | 0 to 2 psi | 0 psi |
| Active fermentation | 5 to 10 psi | 10 to 15 psi | 0 psi |
| Last third, dry hop | 8 to 12 psi | 12 to 15 psi | 1 to 3 psi |
| Cold crash | Keep the valve at its current setting so the tank stays positive as it cools | ||
Carbonation chart: psi by temperature
Carbonation is temperature and pressure together. The same 12 psi gives a flat beer at 65°F and a lively one at 38°F. Set the spunding valve, or your CO2 regulator, to the pressure for the temperature the beer is actually at.
| Beer temperature | 2.0 vol (English ales, stouts) | 2.5 vol (most ales, lagers, IPAs) | 3.0 vol (Belgian, wheat) |
|---|---|---|---|
| 34°F | 4 psi | 9 psi | 14 psi |
| 38°F | 6 psi | 11 psi | 16 psi |
| 45°F | 9 psi | 15 psi | 21 psi |
| 50°F | 11 psi | 18 psi | 24 psi |
| 55°F | 14 psi | 20 psi | 27 psi |
| 60°F | 16 psi | 23 psi | 30 psi |
| 65°F | 18 psi | 26 psi | 33 psi |
Two practical notes. Spunding at fermentation temperature (65°F) to 2.5 volumes needs about 26 psi, which is above a 15 psi relief valve. So most brewers spund to 12 to 15 psi warm, which gets the beer most of the way there, then crash and top up on the gas at 34°F, where 9 to 10 psi finishes the job. And never exceed your vessel's working pressure; on a Unitank that is 30 psi, with the relief valve set at 15.
Which styles benefit
- Lagers, pilsners, kölsch, cream ale: the best case. Warm, fast, clean.
- IPAs, pale ales, hazies: hop aroma retention and zero oxygen. Use moderate pressure mid-ferment, not from day one.
- Stouts, porters, ambers: tolerant. Moderate pressure smooths them.
- Imperial stouts, barleywines: fewer fusels, so they taste less hot young.
- Saison, hefeweizen, Belgian ales, English bitter: mostly no. Open fermentation until nearly done, then seal to carbonate and protect from oxygen.
The full style-by-style pressure curves are in pressure fermentation for every ale style.
Common questions
Does pressure fermentation speed things up?
Not directly. What it does is let you ferment warmer without off-flavours, and warmer is faster. A lager at 65°F under pressure finishes in 7 to 10 days instead of 3 to 4 weeks at 50°F.
Can I pressure ferment in a bucket or carboy?
No. Neither is rated for any pressure, and a glass carboy under pressure is dangerous. Use a vessel with a stated working pressure and a relief valve.
What psi should I use?
Start at 8 to 10 psi for ales and 12 to 15 psi for warm lagers, applied after the first day or two. Adjust from there by taste.
Do I still need a diacetyl rest?
Yes, and it is easier: the beer is already warm. Hold it 2 to 3 days after terminal gravity before crashing.
Will pressure hurt my yeast if I want to harvest it?
Repeated generations under high pressure select for less flocculent, sometimes weaker yeast. Harvest from lower-pressure batches, or buy fresh every few generations.
Where to start
If you have a Unitank, the only part you may be missing is the spunding valve. If you are choosing a fermenter, here is the honest case for and against a unitank. And if the beer you brew most is a lager, pressure fermentation is the reason to make the jump: lagering, start to finish.