half-full carboy - oxygen?

[thunderaxe]

i want to brew a half-batch (~2.5 gallons) but only have a 5 gallon carboy. so far i've been fermenting in mr beer kegs which has been going fine but i'm ending up with a muted hop flavour no matter how much i dry hop and i suspect it might be from oxygen ingress since they don't really have airlocks, they just allow the CO2 to escape through the threads in the lid.

if i just fill the big glass carboy half-full, i know the top half will be full of oxygen at first, but will that be quickly driven out by the CO2 generated by the fermentation, thus creating a nice deoxygenated environment for my beer?

 

[MapleGroveAleworks]

I wouldn't recommend this as regular practice. I am of the belief that oxygen is present in headspace no matter what phase of fermentation.

If you're going to regularly brew this size batch I would recommend purchasing a smaller fermenter.

 

[brandonlovesbeer]

I’m confused about your question. Is it the head space size or the lack of hip character in a massively hopped beer?

If it’s head Space then you have nothing to worry about. During fermentation that space will fil with CO2.
CO2 is heavier than O2 and will fall under the O2 to create a nice blanket. While CO2 being produced by the fermenting beer begins to gain in volume, the O2 will be forced out the top. Basic chemistry and physics.

If it’s the lack of hop character then that’s an easy answer with a more complicated fix.
It’s a combination of your water chemistry and needing to add some salts to the brewing process.
Research the forums on salt additions for good hoppy beers.

But FYI. The head space has nothing to do with it.

 

[ESBrewer]

I think part of the o2 is consumed by yeast and some of it will be pushed away through the airlock. I don't believe in a nice layer of co2 in the presence of oxygen as gases tend to mix easily and the fermenting wort produces heat that causes more motion. Large head space may be a problem if you need to open the fermenter, especially when the fermentation has ceased. But the initial air shoud be just fine.

 

[mr_stout]

I brewed 3 gallons in a 6 gallon carboy with no problem. If you want more hop flavor select a hop with high AA levels and add your hops earlier in the boil to increase the IBU levels. How much and which hop did you add to your brew?

 

[myndflyte]

I don't think you have to worry about your headspace or using a Mr. Beer keg. I think if you want to dial in hop character, you may have to dive into water chemistry and adjusting your water profile to help bring it out.

 

[balrog]

If you have no airlock for MrBeerKeg, do you have airlock for carboy?
And the Ideal Gas Law will always bring O2 through the "CO2 blanket", and that's why esp winemakers fill carboys to the neck for less gas/liquid interface surface area--but that's weeks and weeks in carboy timeframe.

 

[chris000]

rough estimation: same amount of alcohol and CO2; so say 5%alc. and CO2 is about 1.9kg/m3. that comes to around 250l CO2 from your little batch; so do you think you can purdge the 10l headspace?
answer for yourself...

 

[chris000]

If you have no airlock for MrBeerKeg, do you have airlock for carboy?
And the Ideal Gas Law will always bring O2 through the "CO2 blanket", and that's why esp winemakers fill carboys to the neck for less gas/liquid interface surface area--but that's weeks and weeks in carboy timeframe.

I think you talk about the kinetic theroy of gases. Besides that dilution is so high that it is a mute point, one should then also not forget the effusion of lighter gases.
filling the wine demijohn to the brim is important for conditioning when no CO2 is produced and wine filled into a container full with air.

 

[balrog]

When Life gets in my way, beer will sit for 3 or even 4 weeks in the fermenter and CO2 has long since stopped being produced. That was what I was thinking about.

 

[thunderaxe]

the airlock will prevent oxygen from entering, so i'm not worried about convection etc, just wanted to make sure the fermentation would produce enough CO2 to purge the top half of the carboy and @chris000 answered that perfectly. thank you all!

 

[chris000]

When Life gets in my way, beer will sit for 3 or even 4 weeks in the fermenter and CO2 has long since stopped being produced. That was what I was thinking about.

so? while it has produced CO2, it purged the headspace and sits then underneath. you might need to degas the wine if you store it cold, but that's not bad for the wine and even better for the beer.
I have currently a little experiment running with some wine. One demijohn the classical way, one not even half full; purged with CO2. can't wait for a side-by-side comparison