Pressurized Fermentation

                                                                                                              by Mike Retzlaff

A rather hot topic on various homebrew forums lately is fermenting under pressure.  The people touting this technique are quite rapt over the concept.  The technique goes against everything I’ve ever read or observed but rushing to judgment can be a very bad thing.  I’m slowly learning my lesson in arbitrarily dismissing ideas that aren’t my own or that I don’t fully understand.

It is claimed that fermenting under pressure does a number of things.

  • It retains some of the fermentation esters in the beer.
  • It suppresses the formation of fusels and other fermentation by-products.
  • It keeps atmospheric oxygen out and maintains a “sterile” environment for the beer.
  • It speeds up the fermentation by working at higher ferment temps.

What I’ve gleaned from credible sources is that when you pressurize the ferment, CO2 becomes saturated in the beer which stresses the yeast and can lead to several bad things.

  • The stress on the yeast causes a decline in optimal health which can promote eventual mutation.  If you want to “bank” your yeast for reuse, this can be a real problem.    
  • It can cause an increase in sulfur production. 

On the other end of the spectrum, proponents of open fermentation often point to the fact that traditional breweries use yeast for hundreds of generations without problems.  Even using the typical air lock, the increase in CO2 saturation will often weaken and/or mutate yeast after only 8  to 10 generations.

Some esters generated during fermentation are desirable and some are undesirable.  I have no idea if it is even possible to retain the good esters while excluding the bad esters from a fermenting beer.

An active fermentation keeps out atmospheric oxygen as there is a CO2 blanket on the surface of the beer, at least in the beginning, that shields it from the air unless you have serious drafts in your brewhouse or a fan blowing on your fermenter.  This “blanket” is not a sheet of plastic wrap but it does separate the beer from the atmosphere.

Pressurized fermentation proponents give a plethora of guidelines such as a range of 2 to 15 psi.  This can be regulated by using a spunding valve.  These valves are usually applied to bright tanks to aid in carbonation while the yeast drops out of solution.  The text books reveal that at 37 psi, yeast stops the budding process so the cited range is well below that threshold.


I asked about this and received the following answer from a professional brewer with far more education, experience, and knowledge than I. 

Fermentation under pressure has been studied for a long time.  The big brewers and a lot of big regionals use open fermenters that vent to the atmosphere or airlocks.  The Coors brewery uses large, box fermenters that vent to foam chambers, which are actually open fermenters and not airlocks.  The ABI breweries I have visited have vertical-conical tanks with venting to the atmosphere.  A lot of large craft regionals use VCT’s with airlocks that are usually foaming over during high krausen. 

Regarding applied pressure during fermentation, I’ve found that it seems to be strain dependent.  Years ago I tried to see if pressure could be used to inhibit fermentation and I found that with lager yeast, it took a lot of pressure to stop yeast.  I think I tested 10, 20 and 40 psi and the yeast kept fermenting, so I tested up to 60 or 70 psi and I think that is when fermentation slowed drastically.  I did not test to see the effects on yeast health or beer quality.  However, the fact that yeast kept fermenting meant that some amount of pressure is normal.  In fact, when you think about it, yeast can experience a lot of pressure in a large tank, from the CO2, but more so from the hydrostatic pressure of the liquid keeping CO2 in solution and keeping pressure on the yeast.  It’s kind of like a person swimming in a 6-ft deep swimming pool, and then trying to swim in a deep ocean where special precautions have to be made to protect the diver from the high pressure.  I’ve tested lager fermentations in large box fermenters and in large VCT’s.  There is a slight flavor difference, mainly in ester production, with a slightly higher ester profile seen in VCT’s.  I’ve always believed this was due to more scrubbing action in a shallower box fermenter vs. less scrubbing action in a VCT with high hydrostatic pressure.  We kind of see this played out in real life when a lot of traditional breweries still use box fermenters because they believe there is a traditional taste that results when using them.  In this case, I think traditional means a slightly cleaner taste because some esters have been scrubbed away, especially for lagers.  Also, when looking at fermentation tanks, a production box fermenter can have a liquid level of 9 to 12 feet deep.  However, a production VCT can have a liquid level around 40 feet deep.  Again, a higher degree of hydrostatic pressure.

Regarding yeast health, as long as yeast has adequate oxygen, (or a source of unsaturated fatty acids to build healthy membranes), zinc, minerals and fermentable sugars, it will usually ferment just fine.

Let me know if this answers your question. 

                     Keith Villa, Ph.D. (Univ. of Brussels) Ceria Brewing Co.  


After all of this, what is real and what is imagined?  What have I learned? 

  • Fermenting under pressure is not a magic bullet or universal cure-all for brewing. 
  • It is yeast strain dependent.  Some lager yeasts do well under pressure while others produce fusels and other higher alcohols along with the corresponding esters. German Weissebier yeasts are more prone to damage or mutation under pressure.
  • The pressure isn’t particularly critical.
  • Tall fermenters produce hydrostatic pressure similar to a closed system.
  • Pressurized fermentation provides less scrubbing action which yields more esters as in deeper VCTs.

Which styles of beer are actually suitable for fermenting under pressure?  Suggestions include lagers, blondes, pale ales, IPAs, stouts; anything that isn’t driven by yeast character.  It is also speculated that some sour or mixed ferments might be included where you need a totally anaerobic environment. 

There is ongoing research into this which leaves us with mostly speculation.  What seems to be the criteria is the biology of a yeast cell.   Besides having a cell membrane, brewer’s yeast has a thick cell wall that provides a lot of protection, unlike other cells that lack a cell wall and are adversely affected by pressure, pH, etc.  It could be that differences in the cell walls of different strains of brewer’s yeast are the reason that some yeasts ferment fine under pressure, while others do not.  The research continues and there is always more to learn.

Often, pressurized fermentation is used in conjunction with yeast strains which ferment very fast at higher temperatures such as the Norwegian sourced Kveik.

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