The 3 proven benefits of pressure fermentation (Time, Esters, Oxidation)

There’s is some clear benefits from fermenting under pressure compared to standard fermentation in a bucket. Its a technology leap in fermentation and it will cost you some bucks to get everything needed when you want to take that leap. From my own experience, I will never go back to buckets after starting with pressure fermentation and here is why.

The benefit of pressure fermentation is that the yeast will produce fewer esters and fusel alcohols in your beer. This gives the ability to ferment at higher temperatures resulting in a quicker fermentation combined with the natural carbonate inside the vessel. When racking under pressure there is a reduced risk of oxidation when racking to a keg.

Ester production and pressure fermentation

Studies show that pressure has an effect on both ester and fusel alcohols production in yeast [R]. In the test, the wort was split and fermented with lager yeast under different temperature and pressure. The researchers found that temperature was independent of pressure in increasing fusel alcohols. The amount of dissolved CO2 reduced yeast growth and subsequently decreased ester production. Dissolved CO2 is depending on the temperature and pressure. Higher temperature will result in less dissolved CO2 on the other hand higher pressure will result in a higher amount of dissolved CO2.

pressure fermentation and ester production

Another study performed in 1992 has shown the same results [R]. Basically, pressure and CO2 content restricts growth, reduces both esters and fusel alcohols.

To get a good understaning of volume of CO2 at different pressure an temperature use a CO2 Carbonation Chart like the one down below from Keg King. It also comes handy when you keg your beer and want to set the right CO2 level.

Pressure fermentation CO2 chart, KegKing
KEGKING Set Forget O2 Carbonation Chart

Oxidation and the benefit of pressure fermentation

Before pitching the yeast a good oxygenated worth has great benefits for your yeast at the start of fermentation. After this initial step, oxygen and its reactivity will become the enemy and should be avoided. Oxygen will react with the flavour compounds coming from the hops and change the character. An oxidized beer might show stale flavours such as cardboard, sherry, and what some would describe as rotten fruit.

Pressure fermentation and oxidation

Pressure fermentation helps to minizine the greatest oxidation risk, moving the finished beer from the fermentor to the serving keg. Because air consist of 21% oxygen the goal is to minimize the potential exposure to air. There is mainly two practises used by homebrewers.

The simplest way is to connect and add CO2 into the keg and try to drive the air out. Set the pressure at a low value and then venting off the excess pressure from the overpressure valve. The main drawback of this approach is that when adding the CO2 it will be mixing and with the air inside the keg. The approach will not completely remove all oxygen present and only reduce the percentage of oxygen from the initial 21%. This is not recommended for a hoppy beer like NEIPA or a beer that should be store for a long period of time.

The better approach is to fill the keg with a no-rinse sanitiser like StarSan and push the liquid out with CO2. Since the keg is filled with liquid there is no air present to dilute the CO2. The incoming CO2 will fully displace the contents ensuring a good purge. Set the pressure at a low value and dispense the sanitiser through the tap. If you have multiple kegs you can daisy-chain them together and push the liquid from one keg to another to save on sanitiser.

Do commercial breweries use pressure fermentation?

The benefits of low ester and fusel alcohol production makes it possible for commercial brewers to ferment lager under pressure at higher than normal temperature. In a commercial setting speed from fermentation to glass is important. It frees up fermentation chambers, lowering the capital investment, and therefore a reason why many microbreweries are starting to use this approach.  By fermenting under pressure at high temperature, you can make very clean lagers or IPA in a fraction of the time. Homebrews have made lagers in less than a week, I have yet to try that but have done an IPA that was drinkable in only 5 days.

Reinheitsgebot, Germany’s beer purity law, forces the German brewers to use pressure fermentation to naturally carbonated the beer in large pressurized tanks. The law prohibits forced carbonation of beer with external CO2.

Equipment needed for pressure fermentation

Choosing a vessel

When pressure fermenting you need a vessel that can handle the pressure built up during the fermentation. The normal fermentation bucket will pop during pressurised fermentation, the lid doesn’t hold the pressure, and glass carboys can in the worst case shatter.

Today there are special pressure fermentors in PET plastic that are built for pressure fermentation. Another option that you might already have laying around is to ferment in a Cornelius keg. Most Cornelius kegs can take around 2.4 bar (35PSI) of pressure. When using a plastic or keg be sure to not overpressure the vessel and check the manufacturer’s pressure rating as it can vary.

  • KegKing Fermentasaurus snub nose
  • KegLand Fermzilla
  • KegKing Jr
  • Cornelius Keg

I use both the Fermentasaurus snub nose, Fermzilla and a KegKing Jr. The KegKing is rated to 2.4 bar (35PSI) compared to Fermzilla.

Choosing a spunding valve.

spunding valve for pressure fermentation

The next important piece of equipment is the adjustable pressure relief valve, spunding valve. This is the regulator used to set the desired fermentation pressure in your vessel. When the pressure goes above the setpoint it starting to release any excess carbon dioxide being produced. Today there are many commercial options available so I would not recommend you to DIY your own if it’s not something you really would like to do.

Commercial spunding valves
DIY spunding valve
  1. Ball or Pin Lock Gas Connector (depending on the vessel)
  2. Pressure gauge in the right range (0-3 bar) (0-43,5 PSI)
  3. A pressure relief valve in the right range

Personally, I have only used the Kegland BlowTie and it works for my brewing. The SpunDit 2.0 is 3 times the cost of the BlowTie. For more information about how does a spunding valve work see this post.

What is the right pressure during fermentation

The maximum pressure you can use will come from the pressure vessel you will be using. Some are rated to 2.4 bar (35PSI) or above and others less. The other thing to consider is the ideal pressure that promotes yeast growth. Different kinds of yeast respond to different levels of pressure, the usual recommendation is to start at around 0.7 bar (10 PSI) and work your way upwards until you get the result you want without getting any side effects.

There is no risk of killing the yeast, a study shows that yeast dies at around 500 bar (7,252 PSI) [R], no fermenter can take that. What can happen when you start going above 2.5 bar (37 PSI) is that yeast will stop growing [R]. At these pressures, the yeasts ability to divide and reproduce will get hindered resulting in reduced metabolism of sugars and stuck fermentation.

When fermenting at around 1-2 bar there is no problem with yeast growth, and there will be positive benefits of a low ester and fusel alcohols production.

How do you dry hop during pressure fermentation?

When you dry hop your beer during pressure fermentation there are some more things that you need to think about compared to normal dry hopping. In traditional fermentation, there will be some carbon dioxide that’s dissolved in the beer, this amount is massively increased when pressure fermenting.

Just release the pressure and open the lid on a pressure fermenter that might trigger uncontrolled bubbles that with the help of the krausen will result in a foam eruption. Adding dry hops to your beer provide nucleation points where carbon dioxide can be released thanks to nucleation. Just like placing mentos in a Coca-Cola bottle. I good idea is to use whole-leaf hops that are less prone to catastrophic discharges of carbon dioxide compared to pellets.

My recommendation is to take it slowly and release the pressure in the vessel unit you can set it close to the atmospheric pressure. Let it stabilise for a couple of hours before adding your hops. When adding be prepared to close the lid if the beer starts to foam up.

 Dry Hop Device for FermZilla

Another way if you use a pressure fermenter is to use the Dry Hop Device for FermZilla. This is giving you a way to drop the hop pellets in the fermenter without the need to depressurize the system before. The hop is added to a regular PET bottle and drops into the fermenter by opening the ball valve. You can read more about it in this post, Pressurized dry-hopping is now easy! Dry Hop Device for FermZilla.

The Downsides of Fermenting Under Pressure

As described above fermenting under pressure has some downsides if you don’t take care. To high pressure will reduce the yeast cell division and affect the fermentation.

Take it carefully when dry hopping to reduce the risk of a beer volcano in your kitchen.

There is an increased cost if you buy a dedicated fermentor or a need to get more kegs if you decide to repurpose an old Cornelius keg for your homebrew.

If you want to have your beer in a bottle you then will need to invest in a counter-pressure bottle filler.

In my mind pressure fermentation is the way forward and the benefits outweigh these downsides. To get the beer faster, cleaner and reduce the risk of oxidation. Pressure fermentation makes it possible to even at home do a really hoppy beer style like NEIPA.

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