Back suck and cold crashing and physics

Ward Chillington

Ward Chillington

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So while driving home the other night I think I had a brewing epiphany so help me out here smarter than me types.

With carbon dioxide being heavier than air, why are we so concerned with air getting in touch with the beer? Given a gallon's worth of CO2 in the bucket or carboy headspace and you're going to get about a quart of air in suck back when you cold crash things to get a 3:1 ratio over the previous pure CO2 space...is that enough of a diluted mixture to make a difference when there's 3 layers of CO2 over top of the beer?
 
The problem is that these gases don’t stratify as some think. The air induced into the fermenter will mix and even though co2 is heavier than air, oxygen comes into contact with the beer. It’s a myth that a layer of co2 protects the beer, if oxygen gets into the fermentation chamber, it makes it to the beer and staling begins.
 
You are on the right track.

The air (21% oxygen) does mix with the co2 a bit, as the "layers" are somewhat ephemeral or delicate. But, not a whole lot of oxygen gets to the liquid.

If any yeast is introduced, such as to bottle-condition, that will scavenge all the oxygen.

Don't worry much about it, really.
 
When cold-crashing with an airlock, I'll empty the airlock of sanitizer and fill it with vodka before I put it in the fridge. You have to keep an eye on it because it'll empty several times during the cooling period but sucking in vodka rather than oxygen is preferable as far as I'm concerned. Since it's only a few ounces per 5 gallons, there's no significant impact on flavor or ABV.
 
Gases of roughly similar similar weights mix. O2 and CO2 are roughly similar. For a gas to be heavy enough to stop O2 mixing it needs to be much, much heavier than CO2. Those gases you don't want anywhere near your beer. Also mixing of gases isn't an immediate thing, especially at lower temperatures. You'll get very little atmosphere entering the fermenter if the lid is open for a short amount of time, but if it's open, even a small amount for a reasonable period of time, it will mix in.

How important is the amount that comes in from cold crashing? No idea. It will oxidise your beer to some degree. Maybe not so much that you can taste the difference immediately, but it's worth avoiding if you can.
 
Just brew a hoppy beer and open the fermentor or dry hop once fermentation is finished as I've done in past you go hmmm I'm sure I put more hops in this than what I'm smelling.

That brew balloon looks like a simple idea then no need for any o2 into fermentor at all.
Cheap insurance on a fresher product.
 
HighVoltageMan! said:
The problem is that these gases don’t stratify as some think. The air induced into the fermenter will mix and even though co2 is heavier than air, oxygen comes into contact with the beer. It’s a myth that a layer of co2 protects the beer, if oxygen gets into the fermentation chamber, it makes it to the beer and staling begins.
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Simple thought experiment: Does air stratify? Even radon, as heavy as it is, will diffuse into air if disturbed.
 
Nosybear said:
Simple thought experiment: Does air stratify? Even radon, as heavy as it is, will diffuse into air if disturbed.
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This is the $100 k question. I believe it does stratify. Take dry ice and Put it in a sink. the co2 will fill the Sink. but is there O2 in there too? most likely.

I think ultimately the practice if purging with co 2 or using the balloon idea is cheap insurance
 
Thanks for the feedback! It sounds like the difference in the gases ' composition are close enough that the mixing is indeed likely to happen...to what degree and conditions. ( ex: cold being heavier than ambient air) sounds like issues up for debate.
 
I am sure the debate could be solved. We don't have the equipment to solve it lol
 
Minbari said:
I am sure the debate could be solved. We don't have the equipment to solve it lol
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Easy experiment! Take a vessel full of ordinary air. Put some carbonated beer in the bottom, let it stand a day or so. Take a lighter, light it, lower it into the vessel and see where it goes out. If it doesn't, there's no stratification.
 
Nosybear said:
Easy experiment! Take a vessel full of ordinary air. Put some carbonated beer in the bottom, let it stand a day or so. Take a lighter, light it, lower it into the vessel and see where it goes out. If it doesn't, there's no stratification.
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Would work as long as it's covered tightly as soon as the beer is poured and not disturbed. I always use the lighter trick to test for CO2 coverage in carboys.:)
 
J A said:
Would work as long as it's covered tightly as soon as the beer is poured and not disturbed. I always use the lighter trick to test for CO2 coverage in carboys.:)
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Likewise. That's how I thought of it. When filling a bucket with CO2, i can observe the gas filling it. I just think it would mix over time.
 
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Nosybear said:
Likewise. That's how I thought of it. When filling a bucket with CO2, i can observe the gas filling it. I just think it would mix over time.
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Yep. One of the aspects of cold-crashing that's not taken into consideration is that fact that it's not just the thermal contraction of the liquid and CO2 that's causing the vacuum to pull in the O2 but the fact that the CO2 is being absorbed by the liquid as it cools. All that is bound to cause a convection of molecules that will pull the O2 deeper into the mix.
 
My last few brews I have been cold crashing in the keg to mitigate O2 exposure. I use a floating dip tube to keep the liquid post clear.
 
Good read @HighVoltageMan! ..thanks. I gotta go back and read it again with a clear head to look for how long "eventually" is.
 
HighVoltageMan! said:
The reason the c02 fills the sink is due to thermal forces. Colder gases will sink, once it warms it will begin to mix.

Here's an interesting read:

https://beerandwinejournal.com/can-co2-form-a-blanket/
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Did anyone read the "side bar" article on Lake Nyos near Cameroon?
The landslide that caused the lake to erupt stored co2 which rushed down the valley and killed thousands of people and livestock WTF mind blown.
Cheers for the link ;)
 
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