How does headspace relate to carbination level (when using priming sugar)?

[AZ_Brew_Dude]

At the risk of asking a dumb question, can someone explain HOW carbonation works? I'm talking about when using priming sugar, not forced carbonation. The reason I'm asking is because I just bottled my first batch of beer two weeks ago - being overly paranoid about the bottles exploding, I left even more headroom than I probably should've. Seems like the accepted amount is 1"? Mine was more like 1.5 - 1.75".

I was hoping that by having more headspace there'd be slightly less carbonation in the beer, and that would eliminate the risk of bottle bombs. But then I read in another post that increasing the headspace actually increases the carbonation as well?!?

When does the carbonation actually start? Does the expelled CO2 first fill all available headspace, and only after that point does the beer itself begin to carbonate since the CO2 has nowhere else to go?

This actually first came up in the Beginner's forum, but I thought I might get a better explaination over here. Thanks in advance for the information!

 

[Brewsmith]

Carbonation is mostly a factor of the amount of priming sugar added. You need some headroom because the yeast will use it while fermenting the priming sugar. More headrom will mean slightly less carbonation because the CO2 is just filling the space and being compressed, rather than disolving into solution, but it would be fairly minimal, unless you have the bottle only half full. Use the proper amount of priming sugar for the style and you'll be fine, assuming you're not bottling beer that never finished primary fermentation in the first place. That's a whole other list of threads...

 

[Funkenjaeger]

People quote a 1" recommended headspace all the time, but have you ever measured where 1" is on a bottle? It's hardly below the rim that the capper grabs onto. Yet whenever you look at pictures of people's bottled homebrew it's always further down the neck, more like commercial bottles are filled - which happens to be around 1.5"-2". I get the impression that people are just repeating what they've been told, rather than actually measuring what they do.

If you have very little headspace you can have poor carbonation, as the yeast will pressurize the bottle quickly and go inactive. If you have too much headspace, as counterintuitive as it seems, you can have overcarbonation. I don't really know how to explain it but I've experienced it firsthand. On my first batch, I filled the bottles pretty unevenly due to inexperience, and the ones that were underfilled tended to be overcarbed. I've also heard people say that if you dramatically underfill (like halfway) you can end up with bottle bombs.

Regardless, the way to prevent bottle bombs is NOT to purposefully screw up the fill level. You should be preventing it by making sure your beer is fully fermented before you bottle it - changing the fill level is at best a band-aid fix to the real problem, and probably an unreliable solution at best. Ferment properly, use the right amount of priming sugar, and fill your bottles properly, and you should never have to worry about bottle bombs unless you have an infection, in which case there's not much you could have done to stop them anyway.

 

[Evan!]

I think that the underfilling-overcarb is caused by the excess oxygen that allows the yeast to produce more CO2. I might be off-base, but I believe that's what I read.

This only works to a point, like a bell curve. If you underfill dramatically (like halfway), there is so much headspace that even the excess oxygen is not enough to produce enough co2 that it pressurizes the headspace to get carbonation. I've done half-bottles before...usually it's the very last little bit of beer in the bottling bucket that I don't wanna waste. So I fill the bottle as much as I can and cap it. A couple weeks later, I uncap it, and there's a pretty forceful hiss---but very little (if any) carbonation in the beer.

 

[Funkenjaeger]

I think that the underfilling-overcarb is caused by the excess oxygen that allows the yeast to produce more CO2. I might be off-base, but I believe that's what I read.

Sounds like a logical theory. I know I've read explanations for it before, such as that one, I just can't seem to find them when the question comes up - and it does come up a lot, since it's quite counterintuitive that less beer with more beer would result in more carbonation.

 

[AZ_Brew_Dude]

OK, thanks - those posts were all good information. So it sounds like I did it about right then - either fill all the way to the lip of the bottle, or slightly below... and once the bottle filler is removed you'll have somewhere between 1-2" headspace, and this should adequately carbonate the beer without the risk of creating bottle bombs? Thanks!