[narcout]

I want to preface this post by saying I think it has zero practical relevance to homebrewing.  It's just a thought experiment, or maybe knowledge for the sake of knowledge.

I have heard many different people say that oxygen will not enter the headspace of a flask of starter wort that is actively fermenting because it's under positive pressure from the CO2 outgassing.

However, we know from Kunze (page 611-612 in the 5th edition of TB&M) that oxygen can diffuse through a crown capped bottle of carbonated beer: "Even with the crown cork, however, oxygen uptake through the compound can generally be expected, just as surrounding smells can be imparted with time into the beer flavour during storage of bottled beer - despite the difference in pressure between bottle interior and surrounding air."

If oxygen can enter a capped bottle of beer under positive CO2 pressure, wouldn't it also flow into the headspace of a flask of starter wort that is covered with a piece of aluminum foil, even if the starter is actively outgassing CO2?

[hopfenundmalz]

Dalton’s Law explains the diffusion into the bottled beer, so I understand that.

I’m not sure what the introduction of gas flow does. Will the O2 have enough partial pressure to overcome the flow of CO2 out. My Physics and Fluids courses were a long time ago. I hope someone can give an answer.

[erockrph]

I envision this as being similar to dropping colored water into the middle of a flowing stream versus a calm pond. While diffusion is possible, the rate is so slow that the effects of the flow should more than cancel it out.

Of course this is just conjecture until someone has the math to back it up.

[a10t2]

I’m not sure what the introduction of gas flow does. Will the O2 have enough partial pressure to overcome the flow of CO2 out. My Physics and Fluids courses were a long time ago. I hope someone can give an answer.

Just trying to put rough numbers to it:

Assuming a 1.5 L wort volume at 8°P and 80% ADF gives ~79 g fermentable extract and ~10 g (5 L) CO2 evolved. If that escapes through a 1 mm radius orifice over 24 hours the average flow velocity is 19 m/s (42 mph).

Intuitively, I don't think much air could fight that bulk velocity, but that really only applies for a window of a few hours around high krausen. Before and after, some oxygen has to get in. Finding the inflection point might be tricky (offhand, I don't think it could be solved numerically) but that should be the basic pattern for a well-covered starter.

[Robert]

I know you're talking about a narrow-necked flask, but don't we have some practical indication that the production of CO₂ doesn't effectively counter O₂ ingress in that open fermentation facilitates precisely that ingress? 

This illustration doesn't directly address the continuous introduction of more of the denser gas from below (though my intuition says that adding gas doesn't alter partial pressures as described by Dalton's Law as we are at near atmospheric pressure throughout the system,) but I found this video that visually demonstrates the otherwise difficult-to-intuit operation of our gas laws, Henry's in particular.  It should put to bed the notion of a "CO₂ blanket" and maybe facilitate your thought experiment.  It's cool anyway.  The rapidity with which diffusion occurs may be the relevant point here.

https://m.youtube.com/watch?v=_oLPBnhOCjM

It has indeed been a long time since I missed too much of what was going on in class.

[klickitat jim]

No science in my head. But I can picture some gas seeping through a concrete dam into a reservoir upstream. I can't envision it leaping up a waterfall to the head waters.

[stpug]

Oxygen Ingress...

... I think it has zero practical relevance to homebrewing....

Then why care even a little bit?  With ZERO practical relevance, it's a complete non-issue, right? ...

but you would be wrong

[narcout]

Oxygen Ingress...

... I think it has zero practical relevance to homebrewing....

Then why care even a little bit?  With ZERO practical relevance, it's a complete non-issue, right? ...

but you would be wrong

I wasn't referring to oxygen ingress in general, only the very specific case I outlined above regarding starters which are past the lag phase and are already outgassing CO2. 

Maybe I'm misinterpreting your post though.  Are you saying that oxygen will still work its way into a starter flask that is actively outgassing CO2? 

I'm not asking in bad faith, just trying to figure out why O2 gets into a capped bottle of beer that's under CO2 pressure but it's often said it will not enter into an outgassing starter flask.  I don't really understand the difference between these two scenarios.