Pressure Fermentation and Its Impacts On Yeast
How Do Brewers Ferment Under Pressure?
To ferment under pressure, you need a tank that can handle pressure safely. Most brewers use 1 bar or less (about 15 PSI). Always check your tank’s pressure rating. If you're not sure, ask the maker.
Most pressure-ready tanks use a spunding valve. This tool controls how much pressure builds up during fermentation. It connects to the tank’s gas-out line and holds a set pressure. Homebrewers can buy small versions too.
A spunding valve:
Why Use Pressure When Fermenting?
There are 3 big reasons to ferment under pressure:
1. Keep CO2 in the Beer
CO2 prices are up, and some places face shortages. Pressure helps trap CO2 in the beer. This gives you “natural carbonation” and cuts how much CO2 you need to add later.
2. Change the Flavor
CO2 slows down yeast’s flavor-making. It blocks the yeast from making esters (fruity flavors). This gives a cleaner taste. It also means you can ferment warmer than normal without strong fruity notes. Some brewers use lager yeast at 20ºC or more and still get clean results.
3. Protect from Oxygen
More CO2 in the beer helps guard against oxygen damage. CO2 also boosts levels of SO2—a natural antioxidant. That helps keep the beer fresh.
Do You Need to Change Anything When Fermenting Under Pressure?
Yes. More pressure means more stress on the yeast. To help the yeast:
- Add enough nutrients (both micro and macro)
- Oxygenate the wort well
- Use a slightly higher pitch rate (more yeast cells)
What Counts as “Fermenting Under Pressure”?
Most brewers use about 1 bar (14.5 PSI). You can go lower or higher depending on your goal. You may need to adjust the pressure to hit a target carbonation level (like 2–3 volumes of CO2). Just make sure your tank and pressure relief valve can handle it.
Are there certain yeast strains where this is particularly advantageous/disadvantageous?
In general, strains that are more stress-resistant overall with also have an easier time with CO2 pressure. At a given temperature (say, 20ºC) with all other variables controlled, a yeast fermented under pressure will ferment a bit slower than an unpressurized ferment. However, pressure offers the possibility to raise the temperature and get identical results. So for strains like those used for kveik pseudo-lagers, you can ferment warmer and get more neutral flavour profiles which may offer some benefit.
When fermenting under pressure, should any other parameters be changed? When cooking under pressure, the food can reach higher temp and cook faster. So, could I ferment under pressure at a higher temp and still achieve the flavours I want in shorter period of time?
Yes, although we find you need to increase the temperature by about 5ºC or more before the benefits of higher temperature outweigh the costs of high dissolved CO2 toxicity on yeast.
Should I relieve pressure for diacetyl rest?
Diacetyl uptake isn't impacted directly by pressure to our knowledge, although slightly releasing pressure near the end of fermentation could help with rousing the yeast at the bottom of the tank, which would add more yeast cells into suspension which could help with diacetyl reuptake.
Does fermenting under pressure affect yeast viability when re-pitching? Should brewers re-pitch for fewer generations (because of more rapid mutations)?
Since CO2 is mildly toxic to yeast, brewers may find that the viability of cropped yeast is a bit lower when fermenting under pressure. We do not recommend leaving yeast in the fermentor beyond final gravity and diacetyl-clear when holding the beer at pressure. Another challenge with cropping yeast from pressure ferments is that the yeast will come out of the tank wit dissolved CO2, and yeast slurries can hold onto CO2 really well. This means you may get a really foamy yeast crop.
What else can a spunding valve be used for?
For small brewery fermentors, applying some back pressure to the tank with a spunding valve during cast out can work wonders for keeping wort dissolved oxygen in solution, which can help with yeast health and fermentation performance.