[lupulus]

Then the notion of molecular weights came into the equation -- CO2 being just slightly heavier the O2.  The general recommendation was that CO2 should be used to displace the O2 with very low pressures -- no blasts of the gas but instead slow introduction into the carboy for  30+ seconds.  High pressure introductions would serve to merely mix gas because of the small differences in molecular weight but low pressure introductions of CO2 would gradually displace the O2 and sink to cover the liquid in the carboy. 

No, that is a bit of homebrew dogma that keeps coming up but really needs to die. Because gas molecules are always in motion they mix quite quickly and do not settle out according to their density. This is a good thing though because if it weren't so, we on earth would all be dead.

It actually does happen to a certain extent, despite the thermal motion. It all depends on the magnitude of the density difference. Hydrogen and Helium released into the atmosphere rise and are lost into space. Cold gases are more dense so that can exaggerate the effect. Anyone who has worked in cryogenics knows that cold, dense gases can settle to the bottom of a container or room and create a suffocation hazard. Here is an article about how Roman priests used a CO2 blanket to kill animals at the so-called Gates of Hell:
https://www.smithsonianmag.com/smart-news/how-roman-gates-hell-worked-180968197/

I agree that it is very hard to generate a CO2 blanket in a homebrew setting, but that doesn't mean that there is no such thing.

It doesn't have to be 100% CO2.

A 10% concentration of CO2 can be fatal.

From the link.
Concentrations of more than 10% carbon dioxide may cause convulsions, coma, and death [1, 15]. CO2 levels of more than 30% act rapidly leading to loss of consciousness in seconds. This would explain why victims of accidental intoxications often do not act to resolve the situation (open a door, etc.


https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5380556/#:~:text=Carbon%20dioxide%20does%20not%20only,well%2C%20such%20as%20dry%20ice.

High CO2 in the system is called hypercapnia.
Lots of info if you search for it.

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[Cliffs]

This subject came up during my homebrew club meeting last night.  Several of us dry hop while there is still a minor degree of fermentation still going on.  I use a Tilt and I can generally see when the gravity drop is starting to go straight line.  Dry hopping at this stage means that the yeast will consume the oxygen added while dry hopping. I think that it helps. There is also the possibility of biotransformation, but I'm not familiar with that process.

could dry hopping after primary while adding a small amount of sugar also accomplish this

[erockrph]

It's possible that dry hopping did trigger a small.amount of fermentation.

https://www.brewersassociation.org/educational-publications/hop-creep-technical-brief/

Possible, but very unlikely.  It doesn't happen nearly as often as people seem to think. I tried to make it happej on 3 differwnt batches and I couldn't.

Out of curiosity, do you remember what yeast strains you used? I'm thinking something that doesn't eat maltotriose (like Winsor) would give the best chance of seeing a positive result.

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[Saccharomyces]

You are not obsessed about O2 after fermentation is completed. After fermentation, O2 is one of the mortal enemies of beer. Heat and light are two others.

It is not a problem as long as there is yeast in suspension.  Here is yet another example of homebrewing dogma.

[goose]

Then the notion of molecular weights came into the equation -- CO2 being just slightly heavier the O2.  The general recommendation was that CO2 should be used to displace the O2 with very low pressures -- no blasts of the gas but instead slow introduction into the carboy for  30+ seconds.  High pressure introductions would serve to merely mix gas because of the small differences in molecular weight but low pressure introductions of CO2 would gradually displace the O2 and sink to cover the liquid in the carboy. 

No, that is a bit of homebrew dogma that keeps coming up but really needs to die. Because gas molecules are always in motion they mix quite quickly and do not settle out according to their density. This is a good thing though because if it weren't so, we on earth would all be dead.

It actually does happen to a certain extent, despite the thermal motion. It all depends on the magnitude of the density difference. Hydrogen and Helium released into the atmosphere rise and are lost into space. Cold gases are more dense so that can exaggerate the effect. Anyone who has worked in cryogenics knows that cold, dense gases can settle to the bottom of a container or room and create a suffocation hazard. Here is an article about how Roman priests used a CO2 blanket to kill animals at the so-called Gates of Hell:
https://www.smithsonianmag.com/smart-news/how-roman-gates-hell-worked-180968197/

I agree that it is very hard to generate a CO2 blanket in a homebrew setting, but that doesn't mean that there is no such thing.

Introducing CO2 into a carboy that has air in it will increase the concentration of the gas in the carboy, but with the weights of the gases (O2 and CO2), they will mix according to Dalton's Law of partial pressures.  If you fill the vessel with say sanitizer and then carefully push it out with CO2, the CO2 concentration will be higher but there will still be some oxygen left in the vessel due to the disolved oxygen in the sanitizer.  So it is impossible to get rid of all the O2 unless you mix the sanitizer in a total CO2 environment (not recommended and potentially fatal, as Jeff mentioned).

I agreed that a lot of homebrewers are way too hung up on disolved O2 in finished beer, but that is their gig and if it works for them that's cool.  Personally, I just purge my carboys by blasting CO2 into them for about 30 seconds after sanitizing them and then fill them with finished beer when I do highly dry hopped styles so I don't plug the racking arm on my conicalsand and my in-line screen  with hop debris by trying to directly keg from the conical with these beers.  It works well for me and I do not get the off flavor of oxygen staling until the beer gets really old and even then staling is very low (the beers are normally long gone before this happens).  I keg my lagers and dark ales directly from my conical.

[denny]

It's possible that dry hopping did trigger a small.amount of fermentation.

https://www.brewersassociation.org/educational-publications/hop-creep-technical-brief/

Possible, but very unlikely.  It doesn't happen nearly as often as people seem to think. I tried to make it happej on 3 differwnt batches and I couldn't.

Out of curiosity, do you remember what yeast strains you used? I'm thinking something that doesn't eat maltotriose (like Winsor) would give the best chance of seeing a positive result.

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I know that 1450 was one of them. But Shellhammer's original research was done with a light lager IIRC.  Also, IIRC, it was done with a finished beer.

[BrewBama]

You are not obsessed about O2 after fermentation is completed. After fermentation, O2 is one of the mortal enemies of beer. Heat and light are two others.

It is not a problem as long as there is yeast in suspension.  Here is yet another example of homebrewing dogma.

I agree. I guess I should ‘clarify’ () my statement of “after fermentation” to mean [after the beer had dropped bright and been taken off the yeast].

Dr Bamforth explained that if the beer is oxidized it can be “run through yeast” to clean it up. Therefore, we know oxygen is consumed by yeast.

No dogma intended.  ...but I stand by my (clarified) statement: “[after the beer had dropped bright and been taken off the yeast], O2 is one of the mortal enemies of beer. Heat and light are two others.”


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[erockrph]

It's possible that dry hopping did trigger a small.amount of fermentation.

https://www.brewersassociation.org/educational-publications/hop-creep-technical-brief/

Possible, but very unlikely.  It doesn't happen nearly as often as people seem to think. I tried to make it happej on 3 differwnt batches and I couldn't.

Out of curiosity, do you remember what yeast strains you used? I'm thinking something that doesn't eat maltotriose (like Winsor) would give the best chance of seeing a positive result.

Sent from my SM-G975U using Tapatalk

I know that 1450 was one of them. But Shellhammer's original research was done with a light lager IIRC.  Also, IIRC, it was done with a finished beer.

I recently read something posted by Stan Heironymous stating that the term "dry hop" may have come from this effect. It is believed that British brewers used this term because adding hops to a finished beer led to the beer drying out further. That's what got me thinking of Winsor.

[denny]

It's possible that dry hopping did trigger a small.amount of fermentation.

https://www.brewersassociation.org/educational-publications/hop-creep-technical-brief/

Possible, but very unlikely.  It doesn't happen nearly as often as people seem to think. I tried to make it happej on 3 differwnt batches and I couldn't.

Out of curiosity, do you remember what yeast strains you used? I'm thinking something that doesn't eat maltotriose (like Winsor) would give the best chance of seeing a positive result.

Sent from my SM-G975U using Tapatalk

I know that 1450 was one of them. But Shellhammer's original research was done with a light lager IIRC.  Also, IIRC, it was done with a finished beer.

I recently read something posted by Stan Heironymous stating that the term "dry hop" may have come from this effect. It is believed that British brewers used this term because adding hops to a finished beer led to the beer drying out further. That's what got me thinking of Winsor.

I think it may be hops with seeds have more diastatic power and British hops often have seeds.

[Richard]

You are not obsessed about O2 after fermentation is completed. After fermentation, O2 is one of the mortal enemies of beer. Heat and light are two others.

It is not a problem as long as there is yeast in suspension.  Here is yet another example of homebrewing dogma.

I have heard conflicting statement about the ability of yeast to consume oxygen post-fermentation. The issue for me is oxygen exposure at bottling time. After a cold crash most of the yeast has dropped out but there is still enough to carbonate the beer once the priming sugar has been added. That seems to me to be an indication that there is an adequate supply of healthy yeast left, but I have read several places that the yeast may be healthy enough to carbonate the beer but aren't healthy enough to consume any dissolved oxygen so some fresh dried yeast should be added at bottling time. That doesn't really make any sense to me.

What are your thoughts, Mark? You seem to indicate that any yeast that can carbonate can consume oxygen, which makes sense to me.

[majorvices]

This subject came up during my homebrew club meeting last night.  Several of us dry hop while there is still a minor degree of fermentation still going on.  I use a Tilt and I can generally see when the gravity drop is starting to go straight line.  Dry hopping at this stage means that the yeast will consume the oxygen added while dry hopping. I think that it helps. There is also the possibility of biotransformation, but I'm not familiar with that process.

could dry hopping after primary while adding a small amount of sugar also accomplish this

I think it is worth experimenting with.

The main problem with dry hopping and introducing O2 IME is diacetyl or drop off in hop aroma. While I do agree with some others that it doesn't always happen it happens enough to be concerned about it. The "diacetyl" part of the equation could also be attributed to "hop creep". 

[Saccharomyces]

What are your thoughts, Mark? You seem to indicate that any yeast that can carbonate can consume oxygen, which makes sense to me.

Yeast cells always consume O₂ post fermentation if the ethanol level is not high enough to kills the cells off.   The main benefit that bottle conditioning brings is that it gives quiescent yeast cells a new carbon source to consume, so they effectively go through a second fermentation cycle, eliminating any O₂ pickup. Now, adding O₂ to finished beer that contains viable cells and no carbon source other than ethanol results in the yeast cells under a diauxic shift where they can consume ethanol aerobically, reducing ethanol to its precursors (Google "yeast diauxic shift ethanol").  The effect is usually minuscule on carefully handed beer, but the threat from using a secondary fermenter to dry hop has become way overblown amateur brewer dogma.  Adding a small amount of priming sugar in a 1 to 2% boiled solution to a secondary or keg will pretty much end the discussion on O₂ pickup because it will eliminate diauxic shift to using ethanol as a carbon source while the yeast cells are scrubbing O₂.  As a community, we need to reference science before creating dogma.  We are creating brewing dogma faster than it can be dispelled.  I recommend reading my blog entries entitled "Carbon Credits" (https://www.experimentalbrew.com/blogs/saccharomyces/carbon-credits) and "Have You Seen Ester?" (https://www.experimentalbrew.com/blogs/saccharomyces/have-you-seen-ester) to anyone who wants to understand how yeast cells consume carbon and produce metabolic byproducts.