Composition of gasses in the headspace during active fermentation?

[DSorenson]

More out of curiosity than anything, but I was wondering if anyone has found out how the gas composition changes within the carboy, assuming perfect seal and that it started out with regular air composition?

I know CO2 is heavier than O2. My theory is that as fermentation is happening, the CO2 produced forces the less dense O2 out of the airlock.

I guess I'm kind of interested to know, because this means that fermentation does it's own CO2 purge, which should mean that taking the airlock on and off, or testing the SG is less intrusive than it would be otherwise.

 

[GSul]

Good question. Remember that the key to a healthy yeast fermentation is plenty of O2. But indeed a good thought in where does that O2 go...

 

[Beaker]

Not entirely correct, the yeast only needs O2 in the very beginning, so to say it's needed for fermentation is actually false.

That O2 gets used during the yeast cells lag phase, eventually being used to build healthier cell walls. it's no longer in the fermentor in the same way free O2 is.

The CO2 produced by the fermentation does displace the lighter residual air out of the carboy and essentially blankets the surface of the bitter wort.

 

[passedpawn]

More out of curiosity than anything, but I was wondering if anyone has found out how the gas composition changes within the carboy, assuming perfect seal and that it started out with regular air composition?

I know CO2 is heavier than O2. My theory is that as fermentation is happening, the CO2 produced forces the less dense O2 out of the airlock.

I guess I'm kind of interested to know, because this means that fermentation does it's own CO2 purge, which should mean that taking the airlock on and off, or testing the SG is less intrusive than it would be otherwise.

CO2 is heavier than O2, but they are gasses and they will mix. The idea of a "blanket of CO2" is not completely accurate. If you remove the airlock, O2 immediately and slowly begins to mix with the other gasses in the headspace. Given enough time, the headspace would be room air due to this mixing.

 

[Beernik]

The gases mix, but don't mix instantaneously (not a lot of wind movement in a primary). Plus you have a source of CO2 generation from the fermenting wort.

For these reasons, I would tend to think of the headspace in a primary during fermentation as being more of a plug flow reactor (PFR) than a Continuously Stirred Tank Reactor (CSTR). What ever off gasses from the top of the wort is going to march it's way up and out of the primary.

This changes when active fermentation ends. Then its more like a Batch Reactor and random Brownian Motion can mix gases (like if oxygen is introduced by opening the carboy). But Brownian Motion still isn't an instantaneous process.

 

[passedpawn]

Partial pressures of the gasses move the gas molecules in/out of the fermentor.

The gases mix, but don't mix instantaneously (not a lot of wind movement in a primary). Plus you have a source of CO2 generation from the fermenting wort.

For these reasons, I would tend to think of the headspace in a primary during fermentation as being more of a plug flow reactor (PFR) than a Continuously Stirred Tank Reactor (CSTR). What ever off gasses from the top of the wort is going to march it's way up and out of the primary.

This changes when active fermentation ends. Then its more like a Batch Reactor and random Brownian Motion can mix gases (like if oxygen is introduced by opening the carboy). But Brownian Motion still isn't an instantaneous process.

 

[feinbera]

For the practical purposes of homebrewing, you can treat the headspace as an impermeable blanket of CO2 -- while some minuscule amount of oxygen will diffuse through if you pop your carboy bung or bucket lid, as long as you're careful not to slosh things around and disturb the blanket, that amount will be so small as to have no detectable effect on your beer.

Oxygen also diffuses through the water in your airlock, and, if you really wanna geek out, straight through the walls of your fermentation vessel via quantum tunneling -- but, again, in amounts so tiny in the scale of a few-week fermentation that the effects are not detectable, so, we happily pretend these effects do not exist at all, without our beer suffering any adverse effects for the pretending.

 

[passedpawn]

For the practical purposes of homebrewing, you can treat the headspace as an impermeable blanket of CO2 -- while some minuscule amount of oxygen will diffuse through if you pop your carboy bung or bucket lid, as long as you're careful not to slosh things around and disturb the blanket, that amount will be so small as to have no detectable effect on your beer.

Oxygen also diffuses through the water in your airlock, and, if you really wanna geek out, straight through the walls of your fermentation vessel via quantum tunneling -- but, again, in amounts so tiny in the scale of a few-week fermentation that the effects are not detectable, so, we happily pretend these effects do not exist at all, without our beer suffering any adverse effects for the pretending.

That's perfect. I didn't mean to suggest that removing the airlock momentarily was a problem.

 

[DSorenson]

Wow guys... Thanks! It's reassuring homebrewers are so scientifically inclined.

 

[Beernik]

Partial pressures of the gasses move the gas molecules in/out of the fermentor.

Partial pressures do not move gasses into the fermentor unless you don't have an airlock on it. The airlock works as a one-way valve allowing gas to exit and not enter.

 

[beerluvva]

Speaking of headspace...I've read a number of threads that worry about the amount of headspace in the fermenter. Given what's been said above, what's the reason for this? Seems like if you do primary only, and basically leave your fermenter alone, the co2 pushes the o2 out, and so a lot of headspace should not be a big issue?

I typically do 3 gal in a 5 gal bucket, so wondering if I'd taste a difference in my beer if I matched the vessel better.

 

[passedpawn]

Partial pressures do not move gasses into the fermentor unless you don't have an airlock on it. The airlock works as a one-way valve allowing gas to exit and not enter.

Yes, that's what I meant.

But technically, given enough time the gasses will move through the airlock. It's slow, how slow I dont' know, but you can't stop physics.