Under Pressure: Capturing CO2 During Fermentation

There are some who say that a beer whose entire fermenting and aging life is maintained at positive pressure — around 15 psi —tastes much better. Dr. Chris White of White Labs believes that not only pro brewers, but homebrewers can benefit from pressure during fermentation. 

“It is an especially nice practice for fermenting lagers because you can ferment at higher temps and still get lager characteristics. The added pressure retards yeast growth, which lowers esters and fusel alcohols. Since the yeast don’t grow as well, the viability of the crop will be less,” says Dr. White. 

Many homebrewers get into pressurized fermenting in hopes of retaining more of the delicate hop aromas usually pushed out of the airlock by escaping CO2 gas. Others are interested in the possibility of reduced ester production and faster fermentation at higher temperatures (think lagers here). Whether you are trying to enhance your beer’s flavor or speed up your brewing process, putting the squeeze on those yeasties can most likely add something positive to your brewing experience. 


In a normal unpressurized fermentation environment yeast is affected by the concentration of CO2 present at any given time. The volumes of CO2 present can have pronounced effects on yeast’s cell properties and fermentation byproducts. This may be why it is so hard to reproduce the same beer recipe in different shaped fermenters even if you follow your recipe perfectly.

In commercial brewing environments they often have tall fermentation tanks of different sizes and shapes. In these taller fermentation tanks, the hydrostatic pressure is greater and pressure can near 20 psi. When pressure starts to exceed 30 psi the yeast can have issues and fermentation can be negatively affected. This usually isn’t the case for homebrewers and our small-scale fermenters. Hydrostatic pressure is low in small fermenters, so the brewer can add low levels of CO2 to achieve the target pressure. 

Many studies have been done on the effects on yeast under pressure, but one study published by the Journal of the Institute of Brewing discusses in depth how fermenting under pressure leads to reduced fusel oils and ester concentrations while the diacetyl and acetaldehyde levels increase.1 The study reveals how adding low levels of CO2 can make beer production faster, yet the viability of the yeast crop suffers. 

For the average homebrewer that buys a new vial or packet of yeast for each batch, the degradation of the yeast’s cell structure and propagation is not a big concern. If you harvest yeast from batch to batch, then it is something to consider when thinking about adding pressure to the

One thing that should be noted is that while pressurized fermentations retards growth of yeast cells with many strains, which provides positive characteristics, there are some yeast strains that do not seem to do well in pressurized fermentations and shut down. These strains that do not do well in this environment are generally ones that are traditionally used in open fermentations, such as some British and Belgian ale strains.


Fermenting under pressure allows the beer to carbonate (at least partially) naturally, as part of the fermentation process. The risk of contamination is certainly reduced from such a practice. Glass carboys are NOT pressure rated and should never be used to ferment under pressure, but for those of you dedicated to the glass, you can start maintaining pressure after you transfer your beer into a stainless steel keg for secondary fermentation. Keep in mind that if you are only fermenting under pressure in a secondary vessel, it will result in a beer that will not be as carbonated as a beer that is fermented entirely in a positive pressure environment, and the taste will be affected. 

An additional benefit with the natural carbonation formed in a closed fermenter is the ability to easily pull a carbonated sample from either a sample valve or tap. Typically, non-carbonated samples are pulled using a wine thief, which introduces opportunities for contamination. With a closed pressure fermenter you can quickly grab a safe sample with almost no chance of spoilage. You also can immediately counter-pressure fill directly from your pressurized fermenter into dispensing kegs or bottles. 

Scott Ciampa, Head Brewer of Idle Vine Brewery in Austin, Texas, likes to ferment under pressure simply as a way to save money on CO2 and speed up the carbonation process. He applies pressure toward the end of fermentation when the beer reaches a few degrees above the target terminal gravity. At that point he caps off the fermenter and allows the final stages of fermentation to naturally bring the tank up to around 7–10 psi. This allows the brewer to skip any carbonation time spent when it’s time to cold crash the beer. 


For those achieving primary fermentation in a ball- or pin-lock keg, you will benefit greatly from the use of a spunding valve. The spunding valve will maintain a certain amount of pressure in your keg, and open up to release excess CO2 at a pressure predetermined by you. (You can purchase spunding valves pre-assembled from several homebrew suppliers or build your own, plans for which can be found at One of the great benefits of the spunding valve is that it will keep the beer from foaming out during high kräusen, when your yeast is most actively fermenting the beer. 


Let us start from the moment that your yeast has been pitched. Let’s assume we have already properly aerated our wort with either pure oxygen by injecting, stirring, shaking or some other method. You will flood the head (non-liquid-occupied) part of your primary fermenter with a small amount of CO2 through the liquid tube or racking arm, thereby aerating the wort-yeast mixture with CO2 as a mixing device and purging the remaining air and other possible contaminants through the airlock or opened spunding valve (pressure release valve). Now the spunding valve should be set to open only when 15 PSI or so has been reached. You will need to pressurize the keg slightly with CO2 when starting out to ensure that there is, in fact, an airtight seal and that no air is in the fermenting vessel. 

Once your beer has fermented to the point that the desired specific gravity is achieved, it is now time to set the carbonation level to the desired volume of CO2. You will want to re-set your spunding valve accordingly, if the desired level of CO2 requires a different setting than you have already dialed-in. Leaving your beer to settle out for a week after specific gravity is reached will help with the finishing. 


A vigorous fermentation tends to blow out many of the hop oil compounds that give a hoppy beer its aroma. Adding a layer of top pressure in the fermenter reduces the kräusen and, in theory, the hop aroma loss. Some of the hop oils will still escape via a spunding valve even under pressure, but the idea is that many more of these aromatic oils will be captured inside.

I dry hop in a ball-lock keg fitted with a tabbed lid to hang a hop canister or bag inside. I sanitize everything, hang the hops inside the keg, and then purge the keg with CO2 for a few minutes to try and reduce the amount of oxygen present. I attach my spunding valve and set it to my desired pressure and transfer the beer under equal pressure from the fermenter to the awaiting kegs with hops. 


Moving carbonated beer from the fermenter to a secondary, brite, or serving keg needs to be countered with equal pressure in the receiving vessel. This is another area where your spunding valve can be handy. 

In order to do this, it is advisable to chill the beer to just above freezing, and leave it there for three days to a week. This is called cold crashing, and it causes most of the yeast remaining to fall out of suspension in the beer. This will clear the beer and prevent some of the yeast sediment from being transferred to the serving keg, as well as making it less likely that the CO2 inherent in the beer will come out of the beer as foam. 

The receiving vessel will have to be purged and filled with CO2 to an equal pressure as that which is maintained in the fermenter. If you can chill the receiving vessel as well, it will be helpful. The idea is that, as the already-carbonated beer is flowing from one place to the next, you do not want the temperature to change, and want to maintain a similar pressure. This will reduce foaming. 

The spunding valve will be mounted on the gas-out port of the target vessel to let pressure out as beer enters. You could also create an airlock with a hose going into sanitizer instead of a spunding valve.

You will have to use counter pressure to move the beer slowly from the fermenter to the receiving vessel. You can also use a filter here if you want the beer to be exceptionally clear. Once the vessels are attached, and the pressure is equalized in both, you should turn up the pressure slightly in the fermenter to begin the transfer of beer. You should be able to hear the beer start moving. If the sound stops, turn up the pressure just a little more. This should move all the beer from the fermenter to the target vessel. You should be able to see through the beer line when you have accomplished this. Now your beer is ready to age and serve whenever the time is right. 


Some homebrew-sized fermenters on the market now have pressurized fermenting capabilities. You want to make sure the vessel is rated for pressurized fermenting and not just pressurized transfers. Look for fermenters with pressure relief valves and transfer fittings to keep all moves closed off from outside air. While spunding valves usually will be able to hold pressure up to 15 psi, some of these systems go much higher. A few I have brewed with include the Fermentasaurus (with a pressure kit) that has a max psi of 35, the Blichmann Cornical has a max psi of 50, and the BrewKeg 25 from WilliamsWarn has a max psi of 58.


For me, FuP (fermenting under pressure) came about naturally. I didn’t spend weeks running exhaustive split experiments to prove that it can make a better beer or try to copy a big brewery’s fermenting method. I simply used a homebrew keg and a spunding valve to make sampling, transferring, and carbonating easier. Whatever your reason is for adding pressure to the fermenter, make sure you do it safely by following these tips. 

Most vessels should never exceed 30–35 psi. Read what the vessel you plan to pressurize is rated for, and do not exceed it. 

Always check that your pressure relief valve is clean and automatically opens under pressure. 

Perform a hydrostatic test on the fermenter every 24 months to identify any leaks or weaknesses. To perform the test, first put on your safety glasses and then fill the fermenter with water to remove the air contained within. Pressurize the system up to its designed pressure limit. Hold the pressure for around 2 minutes to visually inspect the fermenter for leaks. If any leaks are found do not use the fermenter until repairs are made or you have replaced the vessel. 

If any part of the fermenter looks damaged do not use it under pressure. 

DO NOT ferment under pressure in any vessel without a pressure relief valve (PRV). Otherwise you could blow up your fermenter and cause serious injury.

Don’t confuse fermenters that can ferment under pressure with ones that can only transfer under light pressure (1–2 psi). Exploding vessels are exceptionally dangerous and can cause severe injury or death.

Issue: December 2017