[Mr_Beer]

Because I am not an expert, my inclination is to ensure that beer in the secondary is ‘up to the top’ of the carboy capacity.  This is because I am concerned about the dreaded oxygen exposure for the beer and the resulting side effects. 

Recently I tried to dry hop an IPA style beer in the secondary.  Lucky for me I had it in a laundry tub because it started to overflow the carboy capacity by a lot. 

So, my conclusion is that something is happening with the beer and the hops during the dry hopping process.

This time around I put the beer in a secondary container that had about 1 gallon of unused headspace.  Lots of stuff happened but the manifestation was that the airlock started bubbling soon after the hop additions.

My question is related to what is inside the carboy – did the dry hops start fermentation again and the resulting CO2 displaced the oxygen?

Put differently, am I obsessing about the O2 and its consequences?

[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.

In fact, I dry hop in the primary 3 days after fermentation has begun. The idea is any O2 I potentially add by opening the fermenter to add the hops will be consumed by the active yeast.

[mabrungard]

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.

[Slowbrew]

I believe what you are seeing is CO2 being driven out of suspension by the addition of lots of nucleation sites, like the hops.  The surface of the hop material gives CO2 a ton of places to collect and make bubbles which then rise and bubble out of the airlock.  Eventually, equilibrium is established again and everything calms down.

I saw this same thing for the first time when making my first batch of wine, 20+ years ago.  I could have sworn that moving the wine to a secondary somehow restarted fermentation.  My son just made a 12 gallon batch of apple cider and learned the hard way about overfilling the fermenter. 

O2 pickup after ferment is a big deal but a little headspace can save some big messes.  It's a bit of a compromise.

Paul

[narvin]

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

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

[Mr_Beer]

Thanks to all who have responded.  As usual, this is the place for knowledgeable answers.

Let me digress a skosh...
I have a CO2 tank with a regulator.  Based on prior discussion in this group the notion was that if I introduce the CO2 into the carboy it would displace the O2 in the available headspace.  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. 

So I have done that for many batches -- and it seems to work with both beer and wine.  But in those cases fermentation has stopped and the amount of headspace was small. 

Along came dry hopping and the seeming expansion of material in the carboy.  I switched to a different carboy with a larger opening -- a Big Mouth Bubbler.  It has  more headspace and a larger opening to facilitate removal of the spent hop bags used during dry hopping.  Hence my question.

Based on the answers, it seems that depending on when the dry hop introduction is done, fermentation actually might pick up a skosh -- hence the apparent activity after hop introduction.  References in the answers essentially confirm this behavior which is the information I was seeking. 

The combination of CO2 introduction with the additional CO2 from the minor fermentation restart has provided me with some assurance. 

Thanks for all who took the time to respond.

[narvin]

That's one reason I don't like the big mouth bubbler... harder to purge the headspace and to push out with CO2, which I can do easily with a standard plastic carboy and a carboy cap.  But yes, a little CO2 purging is good insurance.

[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.

[denny]

IMO, O2 can be a problem but homebrewers are way too hung up on it.

[Bilsch]

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.

[narvin]

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.

Interesting, good to know.  I don't wait long enough to find out 

IMO, O2 can be a problem but homebrewers are way too hung up on it.

I agree, and actually I think the biggest impact on my beer has been spunding vs force carbonating.  The rest of my process seems like it was  "good enough" to not notice much of a difference, but this has noticeably increased the shelf life and freshness of my kegs.

[Steve Ruch]

IMO, O2 can be a problem but homebrewers are way too hung up on it.

I've never worried about it and have never had an oxidized beer in 22 years.

[denny]

IMO, O2 can be a problem but homebrewers are way too hung up on it.

I've never worried about it and have never had an oxidized beer in 22 years.

I think it's important to distinguish what homebrewers are often told arer signs of oxidation,  like wet cardboard,  and an overall diminution of a beer' s qualities, like malt flavor and hop aroma.  Those are generally more minor and harder to pick out.  And IMO that makes it more of a personal choice if and how to deal with it, since it won't necessarily ruin a beer.

[Richard]

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.

[hopfenundmalz]

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.