Updated: January 7, 2025

Benefits of Fermenting Under Pressure

Pressurized fermentations are becoming increasingly popular, and for a good reason. Some of the benefits that you gain from fermenting beer under pressure are: Lower ester production, being able to ferment at higher temperatures without producing off-flavors, and having your beer carbonated by the end of fermentation. Pressure fermenters are also called uni-tanks because you can use them for both fermentation and carbonating. You can serve beer directly from uni-tanks, counter pressure fill bottles or easily transfer beer into kegs using pressure.

Related: Benefits of Using Kegs for Fermentation

Fermenting lagers under pressure is a huge benefit of uni-tank. When yeast ferment under pressure, the production of fruity esters is greatly reduced. This can allow you to brew a lager at 70F an obtain traditional lager flavor. – via MoreBeer

Since off-gassing is reduced added hop flavor and aromas are also potential benefits of fermenting under pressure.

This article is by Homebrew Finds Contributor Brad Probert.  Brad is an engineer, expert homebrewer and experienced reviewer.  Grab a link to Brad’s website at the end of this review.

What is Fermenting Under Pressure?

I’ll start first by defining “normal fermentation” as fermenting with an airlock or blow-off tube on your fermentor. In the chemical process of your yeast converting wort sugars to alcohol, CO2 gets naturally produced. In a normal fermentation, this CO2 pushes out of the liquid wort/beer into the headspace above your liquid level. As more and more CO2 gets produced, it starts to get crowded up there, so pressure builds up and then pushes a glug of air/CO2 through your airlock. As fermentation progresses, more CO2 is created, creates pressure in the headspace, and then vents through your blow-off tube or airlock.

• 10 gallon or 15 gallon keg Ball Lock Kegs – Hands on Review
• 12 Gallon Ball Lock Kegs
• 10 Gallon Ball Lock Kegs
• Williams Warn BrewKeg
  • WilliamsWarn BrewKeg12.5™ | Stainless Conical | Unitank Fermenter | 12.5L | 3.3G FE165
  • WilliamsWarn BrewKeg50 | Pressurizable Conical Fermenter | Draining Sediment Bottle | 50 L | 13.2 Gallon FE161
  • WilliamsWarn BrewKeg25 | Pressurizable Conical Fermenter | Draining Sediment Bottle | 25 L | 6.6 Gallon FE160.V2
• FermZilla All Rounder Fermenter
• BrewBuilt X1 Uni+ Conical Fermenter
• Spike Brewing Flex Fermentor
• Spike Conical Fermenters
• Kegland Kegmenters – at Wiliam’s Brewing, at MoreBeer

Fermenting under pressure follows the chemical process, but instead of an airlock or blow-off tube, which has a very low pressure threshold before it relieves the pressure, you have a spunding valve attached. The spunding valve is typically set to open and release headspace pressure until a much higher level, and therefore keeps everything at a higher pressure in your fermentor (headspace and beer). This increased pressure on your yeast during fermentation changes how they behave. The two main important things that happens with yeast under pressure are: 1) It slows down fermentation rate, and 2) It suppresses production of esters and fusel alcohols.

Spunding Valves for Fermenting Under Pressure

There are multiple makers of spunding valves, with different looks and features/limitations. They can be grouped into two major types- 1) Spring & Poppet, and 2) Diaphragm. Both operate with the same basic principles. Pressure from your fermentor pushes up against the pressure regulating mechanism inside. In the Spring & Poppet, the “mechanism” is a small poppet like what you have inside the gas or liquid posts on your keg. The Diaphragm design uses a large flexible rubber/silicone disc as its “mechanism”. Resisting this pressure, on the other side of the mechanism is a spring. By turning an adjustment knob on the spunding, you can compress the spring more, which in turn pushes harder on the mechanism. Then it’s just a force balance between the pressure in your fermentor and the compression of the spring. Once the pressure overcomes the spring force, it creates a path for the compressed CO2 to escape and it bleeds off. This then regulates your pressure.

Keep Reading: Lots More About Fermenting Under Pressure