thaen (thaen) wrote in geekbeer,
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will fermentation continue while force carbonating?

Originally published at GeekBeer. Please leave any comments there.

This long overdue edition of Search Term Answers comes to us from Plainfield, Illinois (IP address 98.213.234.45). Michael Skinnyspeak (I’m just making up names here) wants to know, “will fermentation continue while force carbonating?”

Good question, Michael!

The answer is, as with most things homebrew-related, a bit more complicated that you might want. The short answer is, yes, fermentation will continue.

The main things that you’re doing when force carbonating are putting the beer under pressure and increasing the saturation of CO2 in the liquid. So the real question here is: do yeast perform the same in high-CO2 environments under pressure?

For a homebrewer, primary fermentation is the stage at which most alcohol and fermentation flavors are produced. This stage lasts between 2-7 days depending on initial pitching rate, yeast health, yeast strain, etc. The other major yeast activity is conditioning — the addition of sugar before bottling or kegging, wherein the yeast predominately produce carbon dioxide which serves to carbonate the beer.

The latter is easier, though I’m not sure how useful it would be. Force carbonating during a cask conditioning period will work just fine. Yeast can chew up the sugar you add during secondary fermentation just fine when under pressure. I can’t think of a good reason to do this, but hey, it’s your beer.

Primary fermentation under pressure is a bit more complicated, but the short version is: It will work just fine for the purposes of alcohol production. There are lager fermentation schedules that intentionally introduce pressure.

There are a few caveats to watch out for here:

  • Fermenting under pressure changes the character of the beer. It will reduce the production of fruity esters and higher alcohols. For some styles (notably lagers), this is desireable. For others (perhaps Hefeweizens), this might change the character of the beer.
  • The amount of gas produced during primary fermentation is more than enough to blow up a standard homebrew keg. You’ll want to have a way of measuring and releasing the pressure.
  • If you are using a keg for primary fermentation, you’re going to have yeast, trub, and krausen in your serving vessel. You really want that?

The pressurization issue raises the question — if there’s enough pressure to blow up the keg in the first place, why would you perform primary fermentation under artificial CO2 pressure? And if you’re force carbonating, why would you add priming sugars to cask condition the beer? To my eyes, it doesn’t seem like adding artificial pressure during any stage of fermentation is going to help you. If you want to ferment under pressure, the yeast will give you that pressure by themselves — you just have to control it. If you want to force carbonate, there’s no reason to artificially increase the fermentation rate by adding priming sugar.

You can effectively reduce the time from brewing to drinking by keeping the keg pressure at a steady 30-50PSI during primary fermentation, then transferring to a second keg and carbonating that as normal after primary fermentation finishes. There should be no need to add CO2 to the keg to keep it at 30-50PSI, but when you start force carbonating in what is essentially the “secondary” fermentation vessel, you’ll have a higher CO2 concentration that you would if the liquid were left under normal pressure, which will decrease the time it takes to carbonate the finished beer. I don’t think you’re going to save enough time this way to make up for the extra effort.

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