Fermenting Under Pressure

[bubblyflows]

I first fermented under pressure by casting out as usual, oxygenating the wort, pitching yeast, capping the fermenter (I ferment in kegs), then pressurizing with CO2 and immediately questioned why I wouldn't have done so with O2. I haven't seen this discussed anywhere and most discussions and blogs (Brulosophy for example) suggest pressure is initially established with CO2 despite having the ability to use oxygen. The resulting increase in ppm of O2 is likely negligible no matter the headspace in your FV as most if not all of it will eventually be vented so I can understand it probably makes little to no difference, but it seems counterintuitive to use CO2 to do so at that stage so I've used oxygen ever since. Of course, if you don’t have the ability to oxygenate your wort it still makes sense to initially pressurize with CO2 to suppress yeast flavors during the initial phases of fermentation, but unless I’m missing something perhaps consider using oxygen.

 

[Vale71]

... but it seems counterintuitive to use CO2 to do so at that stage so I've used oxygen ever since.

Why? Whatever is in the headspace, be it either O2 or CO2 or N2 or plain air, is just going to be vented out and it won't make a bit of a difference so why should one be better than the other?

 

[jddevinn]

Why pressurize at all? It will naturally pressurize as soon as fermentation starts.

Most people have larger CO2 tanks than O2 tanks, so it is 'cheaper' to pressurize with CO2.

 

[bubblyflows]

Why pressurize at all? It will naturally pressurize as soon as fermentation starts.

Most people have larger CO2 tanks than O2 tanks, so it is 'cheaper' to pressurize with CO2.

It's beneficial to pressurize initially to suppress yeast derived flavours generated during the initial stages of fermentation, which why pressure ferments are done. Pressure will be established later anyway but not quite when fermentation starts but a little further into the growth phase.

Yes, CO2 is around a third the price of O2, but to fill 1 gallon of headspace to pressure you might save between $0.10 to $0.20, which I'm not too concerned about.

 

[bubblyflows]

Why? Whatever is in the headspace, be it either O2 or CO2 or N2 or plain air, is just going to be vented out and it won't make a bit of a difference so why should one be better than the other?

If you've just finished oxygenating your wort, and now apply CO2 to bring the FV to pressure, this will pull the oxygen out of your wort to establish equilibrium between the gases. You're undoing the work you've just done. Better to have O2 in that headspace. It doesn't vent out immediately and could take several hours or days before fermentation picks up.

 

[jddevinn]

I believe that most people that pressure ferment allow the pressure to build naturally

 

[bubblyflows]

I believe that most people that pressure ferment allow the pressure to build naturally

I absolutely agree. And doing so will still allow the beer to retain carbonation at the end of fermentation (this is one benefit of fermenting under pressure). However, if your goal in fermenting under pressure is to suppress yeast character, then you may consider initially pressurizing your FV for an overall reduction in yeast ester and fusel production generated in the initial stages of fermentation. In which case, you may also consider using oxygen to do so as proper oxygenation of wort also reduces the production of undesirable flavor and aroma compounds.

 

[Vale71]

If you've just finished oxygenating your wort, and now apply CO2 to bring the FV to pressure, this will pull the oxygen out of your wort to establish equilibrium between the gases. You're undoing the work you've just done. Better to have O2 in that headspace. It doesn't vent out immediately and could take several hours or days before fermentation picks up.

You're not undoing anything. Using CO2 to pressurize will not "pull" any other gas out of solution as diffusion of gases is independent for each gas. Even if it could take several hours for fermentation to become apparent (if it takes days then you're doing something wrong) the yeast will absorb all the O2 already in solution in a matter of minutes, an hour tops. Basically by pressurizing with O2 you've only wasted pure oxygen when you could have used cheaper CO2 or even compressed air if you have a source of clean compressed air available.

 

[Vale71]

In which case, you may also consider using oxygen to do so as proper oxygenation of wort also reduces the production of undesirable flavor and aroma compounds.

Actually the opposite is true. The more oxygen yeast have available in the reproduction stage (log phase) the more they will be stimulated to reproduce and that will increase the ester production. In warm lager fermentation the three variables that are used to obtain a clean lager profile are:

- pressure
- decreased oxygenation
- increased pitch rate

 

[Vale71]

I believe that most people that pressure ferment allow the pressure to build naturally

In kegs this often has the undesirable effect of not obtaining a proper seal (because of the lid design) so that you might end up not doing a pressure fermentation at all. Applying pressure from the start is usually sufficient to keep this from happening.
In fermenters that are pressure capable and don't need positive pressure to seal properly you can of course let pressure build up naturally.

 

[bubblyflows]

You're not undoing anything. Using CO2 to pressurize will not "pull" any other gas out of solution as diffusion of gases is independent for each gas. Even if it could take several hours for fermentation to become apparent (if it takes days then you're doing something wrong) the yeast will absorb all the O2 already in solution in a matter of minutes, an hour tops.

Pulling wasn't the best way to describe diffusion but the effect was implied. Nevertheless, assuming you oxygenated your wort, you will have diffusion of oxygen into the headspace which will at least in part be replaced by CO2. This will all occur during the lag phase. The O2 used during yeast metabolism won't start until the log phase. I agree that if it takes a day there are other issues. I've never really been concerned with using oxygen as my headspace is probably much less than a gallon but understand doing so is not realistic for anything other than a homebrew scale.

 

[bubblyflows]

- pressure
- decreased oxygenation
- increased pitch rate

I don't think this technique will result in over-oxygenation, and if being the case, the wort was probably already over-oxygenated to begin with. But perhaps this is also a good discussion as I haven't seen much commented on whether you need to oxygenate your wort during pressure ferments at all, or if doing so, only at very low doses. I'll try to read up on some literature to see what I can find! Thanks!