Under Pressure: What is Pressure Fermentation?

As beer ferments, yeast converts sugar to ethanol (alcohol) and carbon dioxide (CO2). Most brewers allow the CO2 to escape from the fermenting vessel. If you trap the gas inside the fermenter, this is called fermenting under pressure.

Pressure fermentation refers to beer that is fermented under a pressure that is higher than atmospheric pressure (0 PSI). Brewers ferment under pressure to reduce esters, speed up fermentations, and naturally carbonate. Despite these benefits, pressure fermentation can be expensive and not suitable for all yeast strains and beer styles.

how to pressure ferment

We’ll take a dive into why brewers choose to pressure ferment and tell you what you need to know to try out pressure fermentation at the homebrew level.

What is pressure fermentation?

Pressure fermentation is the process of fermenting beer inside a closed and pressurized vessel. Typical fermentations allow CO2 to escape the fermenter through an airlock or blow-off tube. In pressurized fermentation, the fermenter is sealed and the CO2 produced by the fermentation is trapped inside.

As fermentation progresses, CO2 continues to be produced and the internal pressure builds. Eventually, the beer absorbs all of the gas and becomes carbonated.

The level of pressurization depends on the rating of the fermenter, temperature, as well as the beer style, and the desired end result. Fermenting under different pressures can have specific effects on certain beers.

Ales Vs Lagers

Both ale and lager yeast perform well under pressure. Many brewers ferment lager under pressure at higher than normal temperature. This speeds up the fermentation process. Pressure also helps suppress unwanted esters caused by higher fermentation temperatures. By fermenting under pressure at high temperature, you can make very clean lagers in a fraction of the time.

Ales are more sensitive to pressure and start to experience yeast growth problems at elevated pressures. Keeping the pressure lower will allow most ale yeasts to ferment cleanly, quickly, and attenuate fully. For hoppy ales, like IPA, pressure fermentation is said to retain more hop flavor. Instead of being removed by CO2 production, all of the delicate hop oils remain in the beer.

German Tradition

German brewers use pressure fermentation to carbonate their beers. Because of the Reinheitsgebot, Germany’s beer purity law, brewers are prohibited from adding CO2 to their beer. Instead, most beers are naturally carbonated in large pressurized tanks.

Benefits of fermenting under pressure

Pressure fermentation has many benefits. Creative brewers continue to experiment and find new ways in which beers can improve under pressure.

Suppression of undesired flavors

Fermentation is a complex reaction which generates many byproducts. By pressurizing fermentation, the creation of many of these byproducts is suppressed. This is especially beneficial to limit ester production. Esters produce fruity aromas and flavors in beer. For many beer styles, especially lagers, these fruit-forward flavors are undesirable. On top of esters, diacetyl – that buttery-slick characteristic – is suppressed under pressure.

Ferment at higher temperatures, faster

Since fermenting under pressure suppresses off-flavors, the upper limits for temperature are higher. Fermenting at higher than normal temperatures typically produces many off-flavors. These include higher alcohol and solvent-like flavors. Under pressure, brewers can ferment hotter. This greatly speeds up fermentation without the risk of these typical off-flavors.

Save on CO2

If you keg your beer, you know there’s nothing worse than running out of CO2 when it comes time to carbonate. By fermenting under pressure, beer can be fully carbonated in the fermenter – completely naturally!

Better hop flavor

When brewing hoppy beer, the goal is to highlight all of those delicious volatile hop oils. During fermentation and CO2 production, hop oils are driven off as the gas escapes the fermenter. Capping the fermentation and fermenting under pressure keeps hop oils in the beer for maximum aroma and flavor.

Smaller krausen

Beer fermented under pressure does not produce a large krasen, the foam on top of fermenting beer. Because of this, less headspace is required in the fermenter. This is great if you want to brew a bit larger volume. Or, you can use a smaller fermenter and not worry about a messy or explosive fermentation.

Downsides of pressure fermentation

Stressed yeast

Yeast can become stressed under pressure, leading to unhealthy fermentations. Under-attenuation can be a result of lower yeast growth due to increased fermentation pressure.

Certain yeast flavor components are desired for some styles

Esters are a desirable flavor in many styles, like hefeweizen, witbier, and even IPA. Suppressing the production of esters can remove that fruity aspect from the finished beer.

Dry hopping considerations

Dry hopping into pressurized beer can cause a “dry hop volcano”. Adding hops to carbonated beer causes the CO2 to quickly come out of suspicion due to the nucleation points by the hop matter. This can cause a burst of beer – almost like adding mentos to cola.

Cost

Specially designed fermentation vessels are required to safely ferment under pressure. On top of the fermenter itself, transfer tubing, fittings, and external CO2 are all required for pressure fermentation.

How can I ferment my homebrew under pressure ?

Most importantly, you need to have a fermenter that is designed to be used under pressure. This is absolutely necessary. Do not attempt to pressure ferment unless you have a specifically designed and pressure rated fermenter.

At the moment, there are many commercial options available. As interest in pressure fermentation grows, companies will be releasing new pressure fermenters. If you’re on a budget, we highly recommend the FermZilla. Spike, Ss Brewtech, and Blichmann all have high-end stainless steel options that make great pressure fermenters, as well.

For a DIY option, you can safely ferment in typical 5 gallon corny kegs. Since they’re rated for up to 60 PSI, they make a great, affordable pressure fermenter.

No matter what fermenter you choose, you need a special pressure release valve called a spunding valve. Fermentation pressure is dialed in on a spunding valve. If pressure builds up higher than the set value, the spunding valve releases CO2 to prevent over-pressurizing.

Final Thoughts

Pressure fermentation is a powerful and practical tool for brewers. Pressure helps increase fermentation turn-around time and control off-flavors. It’s also a great option to naturally carbonate beer. Fermenting under pressure is a worthwhile consideration.

If you would like to brew crisp, clean lagers, but don’t have the time or required temperature control, consider pressure fermenting. For hoppy beers, try out fermenting under pressure to preserve all those dank and juicy volatile hop oils.

Frequently Asked Questions

What is the ideal fermentation pressure?

Fermentation pressure depends on beer style, yeast selection, and fermentation temperature. When using a new yeast, always start on the low side. 5 PSI is a great starting point, but pressure fermentation can produce great results all the way up to 15 PSI. Experimentation is key to finding the sweet spot for your particular fermentation.

Always make sure your fermentation vessel is rated to handle the pressures you wish to ferment at.

At what point should I put the beer under pressure?

Most lager yeasts can be fermented under pressure from the very beginning. Suppressing ester formation from the start will help keep lager beers clean. With ale yeasts, however, a pressurized fermentation can suppress desirable esters. Ale fermentations can start with an airlock and finish under pressure. This allows for ester production but also speeds up the fermentation at the end.

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