- Some headspace is required, so there is some compressible gas to deal with volume changes in the beer due to temperature fluctuations (the liquid beer is relatively uncompressible).
- Too much headspace has some potential drawbacks: wasted space, potentially too much oxygen (which in theory could damage the beer).
- The exact amount of headspace (above the minimum required) does not affect the final level of carbonation that is achieved (although it may affect the rate of carbonation).
No change in priming rate is required due to headspace differences that may occur in bottles vs. cans.
The exact amount of headspace is not a factor with regard to final carbonation levels. Using the filler volume as the amount of headspace in both bottles and cans seems reasonable.
picobrewery.com has this to say about headspace:
Some brewers claim that proper carbonation will not occur without some
headspace, and will overcarbonate with excess headspace. As it turns
out, it is not true; it violates the concept of equilibrium
thermodynamics. The Homebrew Digest back in 1996 describes some
experiments run by Steve Alexander and Al Korzonas that prove this
out. Check out http://hbd.org/hbd/archive/1938.html#1938-17 for
example. As long as the beer still has some viable yeast, carbonation
depends mainly on the amount of priming sugar, not on the headspace.
Bottles with extra headspace will reach full carbonation faster,
however. This is because the extra oxygen in the headspace allows the
yeast a brief reproductive phase.
Despite these fairly well run experiments, people expect quite a bit
of headspace. This is particularly evident in competitions, where up
to 1½ inches is commonly allowed. In part, this may be due to the fact
that most of the bottle fillers on the market leave extra room when
they are pulled out of the bottle.
In my opinion, 1½ inches is way too much. Generally, good brewing
practice calls for minimizing oxygenation during bottling, so you
ought to keep the fill level high. . Any oxygen not consumed by the
yeast will go into unwanted oxidation processes. These can degrade the
malt flavor, resulting in a cardboard-like flavor, and can degrade hop
aroma. The greatest risk in underfilling bottles is the risk of
developing acetaldehyde. Acetaldehyde is the result of direct
oxidation of ethanol, and has a foul solvent-like aroma and flavor
that can make an otherwise good beer undrinkable. Acetaldehyde will
invariably form if the fill level is below the shoulder of the bottle.
Another risk of underfilling relates to bottle detonation. If the beer
is incompletely fermented or overprimed, the bottles are much more
prone to dangerous fragmentation.
If your filler leaves too much headspace, you ought to top it up
before capping. Another trick is to leave the caps loose for half an
hour or so. As CO2 escapes from the beer it will tend to flush oxygen
So, should you fill the bottle all the way up to the brim? No, since
beer expands when allowed to warm up. Expanding beer creates
unimaginable pressure (far more than overpriming) that will either
break bottles, or at least will cause leakage out of the cap. The
amount of expansion will depend on how stable you keep your storage
temperature. Don't forget to add a bit for unexpected heat, such as
leaving beer in your car, for example. I had one nasty experience
where a bottle burst all over the floor of my car. Wow, what a stink!
I personally fill my bottles quite high. However, I've often gotten
comments about high fills in competition even if I've left a half an
inch. Thermal expansion requires about 1/8" (1/4" for 22 oz bottles)
so I try to keep the fill between ¼" and ½".
Here are some of the relevant passages from the experiment mentioned above:
Dave Miller's books suggest that bottle fermentation of priming sugars
is a quick process taking several days, but that much more time is
required to get the CO2 produced into solution thus carbonating the
beverage. I speculated that Miller was probably right, and that the
amount of space at the top of a bottle, the head space, should effect
the rate at which CO2 equilibrium is reached. Several people posted
that this was unlikely since, from their experience, bottles opened
early have neither excessive gas pressure nor adequate carbonation.
My conclusion is that the underfilled bottle clearly completed bottle
fermentation and cleared more quickly than either the normal or
overfilled bottles. The overfilled bottles took longer than normal
(and aren't finished yet!).
Also note that the normally filled bottles did not complete
fermentation nearly as quickly as the bottle with fermentation lock.
For some reason the priming fermentation itself takes longer when
One explanation of the increased rate of priming fermentation with
increasing headspace is that more oxygen is included in the headspace
which it turn effects the fermentation rate. This argument requires
that someone explain why the normal fill bottle take so much longer to
ferment than one with a fermentation lock attached. Since the amount
of headspace isn't vastly different and obviously most of the oxygen
in the headspace is expelled thru the fermentation lock. Perhaps
significant oxygen passes backward thru the fermentation lock !
The other explanation is that the normal fill bottle has higher
average pressure throughout the priming fermentation than an
underfilled bottle and that the pressure SOMEHOW reduces the
activity of the yeast. Osmotic pressure?
Final Thoughts on Oxygen
The following is based on my personal reasoning and observations; is not based on any measurement of oxygen or oxidation.
In my mind, one of the benefits of bottle/can conditioning is that the process should remove all oxygen from the beer within a week or so. When active yeast has oxygen available, the oxygen is consumed. This means that any oxygen that is either dissolved in the beer during racking/priming/filling or present in the headspace of the packaged beer will be consumed as the priming sugar is eaten by the yeast to produce the desired carbonation.
My personal experience has been that I do not observe any of the symptoms of excess oxygen and the resulting oxidation in any of my bottle-conditioned beers, even if they have been stored for many months.