Under Pressure: Ferment like the pros with extra CO₂

The hobby of homebrewing is loaded with what most brewers love; fads, innovations, different processes and best of all, new pieces of equipment and gadgets. However, all those are just tools, and it is the brewer that makes the beer and the level of success depends, in part, on the selection and utilization of those tools.

One of the “hot” tools for the last two or three years is fermenting under pressure. The process entails fermenting the wort in a airtight container so that the CO₂ released by the yeast is contained and builds up pressure in the fermentation vessel. If you’ve ever bottled-conditioned beer and had bottles explode because of excess pressure, you’re probably raising an eyebrow at the last sentence. So what is it, really, how does it work and what are the advantages and disadvantages of using pressure fermentation?

Equipment and Process

• First a fermentation chamber that can be made airtight, will withstand pressure up to 15 psi and will allow you to control the temperature of the wort during fermentation. The most common ones are stainless steel conical fermenters. Fermzilla has some products that claim to fit that requirement but caveat emptor. Do not try to use glass or plastic carboys.
• Next you will need a spunding valve. A spunding valve is an adjustable pressure relief valve (PRV). A standard PRV has a pressure indicator and a poppit valve that is normally set to release at 15 psi. However, a spunding valve has a larger valve and is far less likely to become clogged than is a standard PRV - you don’t want it to get clogged. Also, the maximum pressure is adjustable.
• Place the wort in the fermentation chamber, oxygenate the wort, pitch the yeast and close the chamber. However, you may want to use a blow-off tube for a day or two to ensure that the yeasties have multiplied and are in a healthy and active state (see disadvantages).

Advantages of Pressure Fermentation

• The ability to ferment at higher temperatures. This means faster fermentation and the reduction of esters, phenols and off-flavors like diacetyl.
• Reduces the chance of oxidation since the fermentation occurs in a closed environment. There is a caveat to this advantage. If the beer is exposed to oxygen during the packaging (kegs, bottles, etc.) process then oxidation can still raise its ugly head.
• Natural carbonation of the beer. Yes, when fermentation is complete, the beer will be carbonated and the CO₂was free.
• Better hop flavor since the hop flavors and aromas don’t disappear through the air-lock or blow-off tube.

Disadvantages of Pressure Fermentation

• Pressure fermentation can stress the yeast cells and will slow their growth. This is why you may want to wait a day or two before closing the fermenter and allowing the pressure to build.
• Reduces or removes esters, phenols and off-flavors. While this is considered an advantage, if you’re brewing Belgian style ales, hefeweizens and other beer styles that acquire much of their flavor from the yeast, pressure fermentation will minimize or eliminate those flavors.
• Increased costs. Pressure fermentation requires equipment you may not already have. Some of it comes with a hefty price tag. See the first paragraph.
• Dry hopping is difficult and if you want to remain oxygen free, requires additional equipment.

I really like pressure fermentation. Currently I have a lager that has reached expected FG in 4 days. I have made lagers that took almost two weeks to finish fermentation when not using pressure. After numerous experimental pressure fermentations my approach is only for lagers. Ales usually ferment out in 2 to 4 days anyway so many of the advantages aren’t applicable. Also, since using pressure fermentation on a Belgian Ale that didn’t taste like a Belgian Ale I decided I didn’t need to make that mistake again.

As a postscript, there are a lot of articles on the interweb and videos on YouTube. Many are good, some are just wrong. As you can see from the photo below, the claim that it will keep aggressive fermentations in check is wrong.

— Steve Russell, President Emeritus