Pressure fermentation and impacts on yeast
How do brewers ferment under pressure?
Fermentation under pressure requires a fermentation vessel that is properly rated for safe pressurization. Typically, the amount of pressure used for fermentation is at or below 1 bar of pressure (roughly equivalent to 15 psi). It is critical for safety reasons to ensure the tank or fermenting vessel is properly rated for the desired pressure. If the brewer is not sure, they should consult the equipment manufacturer. Many pressure-compatible vessels can be fitted with a device to control pressure during fermentation, commonly called a spunding valve. A spunding valve typically affixes to a fermentation vessel's blow-off (gas out) arm and can be calibrated to a certain pressure. Homebrew-sized spunding valves are also available.
A spunding valve:
Why/when should brewers consider fermenting under pressure?
There are 3 main reasons to consider pressure fermentation and/or spunding.
1. Preserve CO2. This is useful to reduce costs on CO2 especially as costs have skyrocketed in recent years. Some regions even face CO2 shortages. Using a pressurized fermentation helps ensure CO2 stays in suspension which creates a "natural carbonation" and reduces the need for added compressed CO2.
2. Control flavour. Dissolved CO2 is mildly toxic to yeast and inhibits the genetic pathways that lead to a lot of the flavour development of yeast. In particular, dissolved CO2 suppresses production of esters by yeast. This means that a pressurized fermentation can have more neutral flavours than an unpressurized fermentation. This also opens up the possibility of fermenting at a higher temperature than the brewer would normally pick for a given yeast since the fruity esters are suppressed. Some brewers pressure-ferment with lager yeasts up to 20ºC or higher.
3. Control oxidation. Spunding or pressure fermenting helps preserve dissolved CO2 in the beer, which tends to mean the beer is less prone to accidental oxidation as it is stored and processed in the brewery. Dissolved CO2 can also suppress yeast growth and cause a slightly higher concentration of dissolved SO2 generated by the yeast. This is relevant to oxidation because SO2 is an anti-oxidant compound.
Are there any special considerations (eg. since there will be more dissolved CO2, should you pitch more yeast, over-oxygenate or add more nutrients)?
In short, yes. If you are adding to yeast stress through higher dissolved CO2, you should compensate for this by giving the yeast proper nutrition in terms of micro and macro nutrients (vitamins and FAN) as well as oxygen. Same goes for trying a slightly higher pitch rate.
How much pressure is "fermenting under pressure"? If packaged beer is typically 2 to 3 volumes of CO2 (depending on style), what pressure can be achieved during fermentation under pressure?
In our experience, most brewers go with 1 bar (14.5 PSI), although it's possible to pressure ferment at lower or higher pressures. In order to hit the desired level of dissolved CO2 it may be necessary to use a slightly higher pressure. Once again, ensure the vessel you are using is rated for the pressure and that the pressure does not exceed the safe range for the vessel or its pressure relief valve.
Are there certain yeast strains where this is particularly advantageous/disadvantageous?
In general, strains that are more stress-resistant overall with also have an easier time with CO2 pressure. At a given temperature (say, 20ºC) with all other variables controlled, a yeast fermented under pressure will ferment a bit slower than an unpressurized ferment. However, pressure offers the possibility to raise the temperature and get identical results. So for strains like those used for kveik pseudo-lagers, you can ferment warmer and get more neutral flavour profiles which may offer some benefit.
When fermenting under pressure, should any other parameters be changed? When cooking under pressure, the food can reach higher temp and cook faster. So, could I ferment under pressure at a higher temp and still achieve the flavours I want in shorter period of time?
Yes, although we find you need to increase the temperature by about 5ºC or more before the benefits of higher temperature outweigh the costs of high dissolved CO2 toxicity on yeast.
Should I relieve pressure for diacetyl rest?
Diacetyl uptake isn't impacted directly by pressure to our knowledge, although slightly releasing pressure near the end of fermentation could help with rousing the yeast at the bottom of the tank, which would add more yeast cells into suspension which could help with diacetyl reuptake.
Does fermenting under pressure affect yeast viability when re-pitching? Should brewers re-pitch for fewer generations (because of more rapid mutations)?
Since CO2 is mildly toxic to yeast, brewers may find that the viability of cropped yeast is a bit lower when fermenting under pressure. We do not recommend leaving yeast in the fermentor beyond final gravity and diacetyl-clear when holding the beer at pressure. Another challenge with cropping yeast from pressure ferments is that the yeast will come out of the tank wit dissolved CO2, and yeast slurries can hold onto CO2 really well. This means you may get a really foamy yeast crop.
What else can a spunding valve be used for?
For small brewery fermentors, applying some back pressure to the tank with a spunding valve during cast out can work wonders for keeping wort dissolved oxygen in solution, which can , in turn, work wonders for yeast health and fermentation performance.