Bottling level and controlling carbonation

[andyhat]

I recently watched a video by the guys from the American Home Brewer's Society (can't find the link) about bottling which stated that the same primed beer will carbonate more in a bottle if you fill it less (ie. in a standard 12 oz bottle, if it's filled with 11 oz of beer, it will have MORE carbonation than the same bottle filled with 12 oz of beer).

This seems pretty counter intuitive to me but I was wondering if this is true (or maybe he misspoke?) and if so, can someone explain why. I would think that more beer would produce more C02 and result in a higher carbonation... but maybe the amount of C02 produced is marginally less and with less liquid to dissolve into, it gets more carbonation.

Anyway, any help understanding this would be great!

EDIT: He must have misspoke because I have found this on BYO.com

Headspace. Another factor that will affect the level of carbonation in your bottle-conditioned beer is the amount of headspace you allow in the neck of the bottle above the beer. The CO2 from the bottle fermentation fills the headspace as well as going into the beer, so the more headspace you leave, the more CO2 ends up there. For a given amount of priming sugar, the greater the headspace, the lower the carbonation.

The ideal fill height is a matter of preference. High fills help to eliminate air from the headspace, helping you avoid oxidation. Lower fill levels give some protection against gushing and allow some leeway in carbonating. Sometimes over-carbonating can be corrected by cooling the beer as much as possible, gently prying the cap to release the pressure in the headspace, and then resealing the cap. This won’t work if there is no headspace.

 

[shanecb]

I really don't see why it would have more carbonation if it was filled with less beer, at all, unless someone can correct me.

Less beer would mean less overall CO2 produced, as well as there being more of a headspace to fill before the CO2 starts to enter the beer.

 

[andyhat]

Yeah. Thanks. I just read some more on this. In physics, Henry's law (in this instance) supports the fact that the amount of CO2 pressure in the beer is proportional to the pressure of CO2 in the headspace. So to me, it follows that more beer = more CO2 in the same total volume which means higher pressure and more bubbles...