Tuesday 26 July 2022

What Pressure Should I Pressure Ferment At?

If you're looking into pressure fermentation, one of the first questions you'll be asking is what pressure you should set on your spunding valve for pressure fermenting.

Any amount of pressure from 1 up to 18 psi will give you the benefits of pressure fermenting - in particular the ability to ferment with your wort at a warmer temperature, which results in a faster fermentation as well as a reduction in esters and off flavours being created by the yeast.

There are some other things to consider though before making your decision.

Firstly, certain types of beer yeast are sometimes chosen because of the desirable esters and phenols that are imparted into the beer by them - like Saisons, Belgian yeast strains, and other ale yeasts like Verdant IPA. Fermenting these yeast strains under pressure will likely suppress or totally prevent these flavours which may not be desirable in your particular beer.

A popular way around this is to not ferment under pressure for the first 3-4 days which is when these yeast specific flavours are generally created. For the first 3-4 days, the spunding valve can be left wide open, preventing the build up of pressure in the fermenter (or simply fit an airlock), then either remove the air lock and fit a spunding valve, or adjust the spunding valve to your desired pressure after the first 3-4 days of fermentation have passed.

If your beer style calls for a relatively clean/neutral yeast profile - such as lagers, or if using a clean fermenting yeast strain like US-05, then it's fine to pressure ferment from the very beginning of fermentation.

Another consideration is whether you will be dry hopping your beer. If you are dry hopping then you will more than likely need to depressurise your fermenter in order to open it to add the hops. This rapid depressurisation can lead to a sudden expansion of the krausen in the fermenter sometimes resulting in a "volcano" like effect with the krausen rapidly rising to the top of the fermenter - caused by the dissolved CO2 rapidly coming out of solution as the fermenters atmospheric pressure is released.

We manage this by setting a low pressure (~5psi) until the dry hops are added which minimises the impact of depressurising as outlined above. After the dry hops are added, if fermentation is still active the pressure can then be increased by adjusting the spunding valve accordingly and allowing the CO2 created by the remaining fermentation to build up. You can alternatively add pressure manually from a CO2 tank to increase the pressure and help with carbonating.

There's no conclusive evidence suggesting that higher levels of pressure give any additional benefits other than increasing carbonation levels of beer in the fermenter. If anything, higher amounts of pressure will put increased strain on yeast which is more likely to cause other problems affecting yeast health and viability. The generally accepted range is up to 18 psi, but as little as a couple of psi will work just as well. Sure you could go higher but likely won't reap any additional benefits from doing so.

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