Under Pressure: Pressure Fermenting

Pressure fermenting means fermenting in a sealed vessel that holds some of the yeast’s own CO2 inside, instead of letting it all bubble out through an airlock. It sounds like the plot of a disaster film. It is actually one of the most useful tricks in modern home brewing.

• DifficultyEasy, if you can already keg
• Hands-on timeAbout the same as a normal brew
• Ready in2–3 weeks
• You’ll needA pressure-rated fermenter and a spunding valve

Why Ferment Under Pressure? #

Normal fermenting is a bucket with a lid and an airlock, venting gas into the laundry like a slow, contented sigh. Pressure fermenting swaps the airlock for a spunding valve, a small adjustable valve that lets gas out only once the pressure inside passes a number you choose. Everything below that number stays in the beer.

This has three happy side effects:

• Clean lagers without a fridge running at 10°C. Pressure calms the yeast’s fruity by-products, so lager yeast at room temperature behaves much more like lager yeast in a cold cellar. In an Australian summer, this is close to a miracle.
• Beer that comes out already partly carbonated. The yeast does the gassing for you, for free.
• No oxygen. The beer can go from fermenter to glass, or fermenter to keg, without ever meeting air. Hoppy beers in particular stay fresher for longer.

It is not harder than normal fermenting. It is just normal fermenting with a lid that has opinions.

What You Ferment In #

Any vessel with a gas post, a beer post and a pressure rating can pressure ferment. The difference is size, and how much the vessel was designed for the job.

Vessel	Batch size	Why choose it
Apollo Snub Nose Pressure Fermenter 30L	A full 23L brew	A dedicated pressure fermenter. Room for a full kit batch plus the foam, and built for fermenting first, serving second
Oxebar 20L Amber PET Keg & Pressure Fermenter	About 16L	Cheap, light, and does double duty: ferment in it, then serve from it. Comes with a floating dip tube already fitted
Keg King Junior Fermenter/Keg 20L	About 16L	The same ferment-and-serve idea in a compact fermenter-keg

Why only about 16L in a 20L vessel? Because fermenting beer grows a thick head of foam called krausen, and it needs somewhere to go. If it reaches the top, it gets pushed into the spunding valve and gas post, which is a mess at best and a blocked valve at worst. Leave around 20% of the vessel empty.

The other essential: a spunding valve #

Good to know: what is a spunding valve? It is an adjustable pressure release with a gauge on it. It clicks onto the gas post, and a knob sets the pressure you want to keep: below that number the gas stays in the beer, above it the valve lets the extra out. The name comes from the German Spund, the bung brewers hammered into wooden casks to trap the fizz. To set one, start with the knob loose, let the yeast build some pressure, then tighten a little at a time until the gauge holds at your number.

A spunding valve is not a pressure relief valve. The ring-pull valve on the lid is a safety valve. It opens by itself at a fixed, much higher pressure, and it is not accurate enough to hold a fermenting pressure. You need both: the spunding valve does the daily work, and the relief valve is the backup if anything goes wrong. The colours and ratings are explained in Setting Up Your Regulator.

Valve	Notes
Integrated BlowTie Spunding Valve Kit	Clicks straight onto the gas post. Our usual pick
BlowTie Spunding Valve Kit	Same valve, set up to connect with your own disconnect and line
Keg King Spunding Valve	A good alternative that does the same job

All of them do one thing: you turn the knob to set a pressure, and the valve vents anything above it. A gauge shows what is happening inside, which is far more satisfying to watch than an airlock.

How to Do It #

1. Sanitise everything: vessel, lid, posts, dip tube and spunding valve.
2. Fill with your wort or kit, leaving that 20% headspace.
3. Pitch the yeast and seal the lid.
4. Fit the spunding valve to the gas post, but leave it wide open for the first 24 hours. Pressure slows the yeast down while it is getting started, so give it a day to get going first.
5. After the first day, set it to 10–12 PSI. Tighten the knob a little at a time until the gauge holds there. When it reaches your setting, the valve will start to vent with a faint hiss. This is normal, and frankly rather pleasing. Never let it get to 35 PSI or more.
6. Sample without opening anything. Click a Bronco Picnic Tap onto the beer post and pour a little into your hydrometer jar. The first splash will be cloudy with yeast; tip it out and take your reading from the next pour.
7. When the gravity reads the same on 2 days in a row, fermentation is finished.
8. Cold crash under pressure. Move the vessel to the fridge. As the beer chills, it absorbs gas and the pressure on the gauge drops. Connect your CO2 at about 10–12 PSI while it chills, to keep the pressure up and finish the carbonation.

Yeast	Temperature	Spunding setting	Notes
Lager (Saflager W-34/70, Saflager S-23)	18–20°C	Open for the first 24 hours, then 10–12 PSI	The headline trick: a clean lager without lager temperatures (why warm lager yeast misbehaves)
Ale (Safale S-04, US-05, kit yeasts)	Its usual range, on the packet	Open for the first 24 hours, then 10–12 PSI	British-style ale yeasts cope well with pressure

Yeast carbonates the beer at fermenting temperature, and warm beer holds less gas than cold beer. So beer that finishes at 12 PSI on the bench will be under-carbonated once it is cold. That is what the CO2 top-up in the last step is for. Think of the yeast as having done most of the work, with the gas cylinder signing off on it.

Not for every beer. Belgian beers and saisons get their flavour from the yeast, and pressure holds that flavour back, so ferment them the usual way. The same goes for a hefeweizen if you want the banana and clove.

Dry Hopping Under Pressure #

The one thing you can’t do is take the lid off a fermenter that is under pressure. Luckily the hops don’t need you to. There are three easy ways:

When	How	Good for
On brew day	Add the hops with the yeast, before you seal it	The simplest. Lots of fruity hop character
In the first 24 hours	While the spunding valve is still open, open the lid, add the hops and seal it again before you set the pressure	A bit more fresh hop aroma, still no fuss
Later on	Let the pressure off slowly over about an hour with the relief valve, open it, add the hops quickly and seal it. The yeast builds the pressure back up	A big late aroma. Easiest in a wide-mouthed fermenter like the Apollo Snub Nose

Pellets in a fine hop sock are easier to keep out of the beer post and the spunding valve. Pressure fermenting is quick, so there is rarely a long wait before the hops go in.

So What Happens to the Sediment? #

It stays in the bottom of the vessel, and that is perfectly fine. Yeast and trub settle into a layer on the floor of the keg, exactly as they do in a normal fermenter. The only question is how you get the beer out without the sediment coming with it.

The catch is the dip tube. In a standard keg, the beer is drawn up through a tube that reaches almost to the very bottom, which is precisely where the sediment is sitting. It is a bit like putting the straw at the bottom of the milkshake where all the lumps are.

You have three ways around it:

Option	What happens	Good for	Watch out for
Serve straight from the fermenter	The first glass or two pull up sediment, then the beer runs clear	The simplest approach. No extra keg, no extra gear	Do not move the keg once it has settled, or you stir it all up again. Not one for the BBQ
Use a floating dip tube	A 100cm Silicone Dip Tube Kit with a weighted filter and stainless float draws beer from just below the surface, well above the sediment	Ferment and serve from the same vessel with clear beer from the very first glass	The Oxebar 20L already has one in the box. For other vessels, fit it before you fill, and sanitise it along with everything else
Transfer to a clean keg	Move the finished beer, under pressure, into a second keg, and leave the sediment behind	Clear beer you can move, transport or store, and frees the fermenter for the next batch	Needs a second keg and a little extra time. See the next section

Cold crashing first helps whichever option you choose. Chilled yeast clumps together, sinks, and packs down firmly, so there is less loose sediment for anything to pick up. If cold crashing is new to you, it is explained in our clear beer guide.

Do I Need a Different Keg? #

No, you do not have to. An Oxebar 20L can ferment the beer and then serve it, and the floating dip tube it comes with means clear beer from the first glass. Leave it in the fridge, connect the gas, and pour.

A second keg earns its place when you want to:

• take the keg somewhere, since moving a fermenter stirs up the sediment
• get clear beer from the first glass without a floating dip tube
• start the next brew while you are still drinking this one

The closed transfer #

A closed transfer moves beer from one keg to another without it ever touching air. It works on a simple principle: beer flows from high pressure to low pressure, like gossip.

CO2 → Fermenter gas post → Fermenter beer post → Jumper line → Serving keg beer post (spunding valve on the serving keg gas post)

Gas pushes into the fermenter’s gas post, beer leaves through its beer post, travels along a jumper line, and enters the serving keg through its beer post, filling from the bottom without splashing.

1. Prepare the serving keg. Clean it, fill it right to the top with no-rinse sanitiser, seal it, then push all the sanitiser out through the beer post with CO2. The keg is now sanitised and full of CO2 instead of air.
2. Make a jumper line: a short length of EVA Barrier line with a liquid (black) disconnect on each end, such as two Duotight 8mm Disconnects – Liquid.
3. Pressurise the fermenter to about 10–15 PSI with your gas.
4. Fit a spunding valve to the serving keg’s gas post and set it a few PSI lower than the fermenter. It lets the CO2 out as beer comes in.
5. Connect the jumper to both beer posts and watch the beer move across. If it stops, lower the spunding valve or raise the fermenter pressure a little.
6. Stop when the line spits gas or turns cloudy. That is the bottom of the beer and the top of the sediment. Disconnect, and the serving keg is ready to chill and finish carbonating.

For beer that is properly crystal clear, the 10 Inch Beer Filtering System – Keg to Keg fits in the jumper line and filters the beer on its way across.

New to kegs? Our main kegging guide covers gas, pressure and serving, and Kegging on a Budget covers the Oxebar range.

Don’t Panic: Safety and the Short Version #

Pressure is perfectly safe when it has somewhere to go. The spunding valve is that somewhere, so treat it like the most important part of the setup, because it is.

• Always leave that 20% headspace. Foam in the spunding valve can block it, and a blocked valve stops doing its job.
• Check the gauge each day for the first few days. If the pressure keeps climbing past your setting, release it with the pressure relief valve and clean the spunding valve.
• Stick to the pressures in this guide. A fermenter is not the place to see what happens at 40 PSI.
• Keep PET vessels out of the sun and away from heat, and retire any that are dented, cloudy or deeply scratched.
• Never open the lid under pressure. Vent it first with the pressure relief valve.

The short version #

• Vessel filled to about 80%, sanitised, lid sealed
• Spunding valve on the gas post, open for the first day, then 10–12 PSI
• Gauge checked daily, samples taken through the beer post
• Gravity steady, then cold crashed with CO2 connected
• Served from the fermenter, through a floating dip tube, or closed-transferred to a clean keg

If your gauge is doing something you did not expect, bring the whole thing into Nerang or Oxenford, or send us a photo and a message. A spunding valve with a story to tell is one of our favourite things to look at.

Happy brewing, and may your gauge always read exactly what you set it to.