There are brewing standards that apply to leaving a headspace in the bottle and they are there for a good reason. Yet, there are also some myths associated with this which could teach you a bad habit, so let me explain why this matters.
But first in short, how much headspace should you leave when bottling beer?
You should leave 1 inch to 1 and 1/2 inches of headspace when bottling beer. This is both the homebrewing community’s and commercial breweries’ standard as this headspace allows for optimal use of space in a bottle without sacrificing the quality of the beer, yet it’s possible to leave less headspace than this without serious consequences.
In reality, you could just follow this ratio and not ever question why this is the standard practice and you wouldn’t miss anything. However, if you are curious let’s see what happens within the bottle and how the headspace affects certain factors.
What is the headspace of a bottle?
The headspace of a bottle is the empty space from the inner side of a cap to the first contact with liquid in a bottle. As you can see in the picture below, there is a standard between an inch to 1 and 1/2. The lower the headspace in a bottle after the standard, the more impactful will the consequences be in regarding carbonation, oxidation, sound when opening a bottle, and overflowing.
Why does the headspace in a bottle matter?
Risk of oxidation
Oxidation is the worst enemy of any homebrew which can destroy the brew over time while it conditions and the primary fault of oxidation comes from bad brewing practices.
These bad practices which could lead to oxidation of a brew start with storing of ingredients such as grains and hops, bad sealing or improper airlock use during fermentation, racking from one container to another, bad secondary fermentation control, and finally poor bottling technique.
Perhaps you are already rocking it and there’s no oxidation in your brews but I recommend that you read more about different ways how your brew is at risk of oxidation during the process of brewing, so you can better your method and make better brews in the future (as it can only help).
Okay, back to the bottling and risk of oxidation.
Some oxygen will be introduced when you pour beer through a siphon or a bottling wand (better) and depending on the technique and gear it will be less or more oxygen introduction, but leaving too much headspace won’t push the oxygen out and instead, it will get trapped in the bottle when you cap it.
A big headspace simply means too much extra oxygen, so be sure to leave only a small headspace as suggested above.
When you transfer the beer into the bottles, as the liquid fills the bottle, the air is pushed away and with minimal disturbance, only a portion of the oxygen from the air will be diluted with the beer (which is the reason you should do it slowly and not cause splashing).
When there’s a lot of oxygen that reacts with your beer, it will slowly ruin it during conditioning and create off-tastes associated with wet cardboard. (It won’t happen instantly, so if you commit this mistake you better drink it ASAP).
Headspace affects carbonation (Co2 must get trapped)
The other reason why it’s important to respect the mentioned headspace is due to the dilution of Co2 within the beer.
Leaving too much headspace will make the Co2 fill the empty space and thus it won’t dilute with the beer and stay trapped when you open the bottle.
You may have measured the priming sugar perfectly, and the yeast got fed and you achieved the desired levels of Co2 and yet when you open the bottle the beer ends up flat.
You may wonder how you did something wrong with the measurements and question your whole methodology, but in reality, all that happened is that some Co2 immediately escaped the bottle when you removed the cap as it wasn’t diluted and trapped in the beer.
When Co2 is produced it naturally dilutes itself with the liquid, yet when we cool the homebrew before the serving we additionally contract the Co2, so when we open the bottle and serve it, Co2 doesn’t escape, but the brew remains sparky for longer which also affects the head retention.
The gas expands in temperature, but it contracts in the cold! Always chill your beer before serving it.
Can there be not enough headspace?
The only downside of leaving too less of a headspace is the beer won’t make the satisfying “pshhhh” sound when you crack it and it may be a bit less convenient to pour it in a glass, but yet it’s not a big deal.
The only instance when not enough headspace would make some tiny sense in kicking the cap out is if you were to quickly warm up previously highly (Co2 3+ level) fridged beer really fast due to compression levels within the bottle caused by expansion of gas. However, this would need to be an extreme case where you chill the beer and then leave it in the sun for 10 minutes with a cap on. I mean, such cases don’t happen and you always crack the beer and start drinking it before it warms up to the level where it tastes disgusting.
Commercial beers always leave noticeable headspace because that’s how their manufacturing process works. The machine bottles and caps the beer under given standards which reduces the odds of beer overflowing, and they save beer they can sell for more (why a chips bag always seems full, but only 1/4 actually contains chips, the rest is the air).
It won’t make the bottle crack (MYTH)
Although the amount of liquid (in this case homebrew) does increase the pressure within a bottle, the only real danger to bottle bombs comes from Co2 volume and the bottle’s ability to handle pressure.
The headspace doesn’t play a big role in this instance as one more sip of beer trapped in a bottle won’t make any difference.
Co2 volume, on other hand, is directly correlated with bottle bombs, so you should be careful how much priming sugar is in the bottle and what bottle type you use.
The Co2 volume in the bottle isn’t changed by the fact that there’s an inch more or less of headspace as it will always be the same. Moreover, the empty space the headspace creates isn’t needed, as Co2 gets absorbed in the homebrew easily and this is what we want.