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Is the volume of gas at the top of a beer bottle the same for all beers, and how is it optimised?

Ron Dippold
San Diego, California, US

I am a brewer, so this is dear to my heart. The unfilled area at the top of any container is known as headspace or ullage. In a beer bottle, it serves a couple of purposes. It prevents beer from squirting out when you put a cap on. It prevents the beer from breaking the cap seal and leaking out if the beer gets hot and the liquid expands. Since the space ends up mostly containing carbon dioxide, if filled properly, this protects the beer from oxygen leaking in through the cap seal 鈥� oxygen is very bad for beer. You don鈥檛 want too much headspace: gas is more explosive than liquid, which can cause pressure to build up enough for the bottle to break. Too much headspace also exposes your beer to more oxygen during filling, and, well, it means less beer! You鈥檙e looking for about 2 centimetres at the top of a bottle, which is about 7 per cent of the volume of a typical 12-ounce, or third-of-a-litre, bottle.

If you are bottling commercially, industrial bottling lines make it easy. The machines used push a 鈥渃ounterflow鈥� or 鈥渋sobaric鈥� filling nozzle onto several empty bottles at once. This temporarily seals them, with tubes going to the bottom of the bottles. The tubes suck the air out of the bottle. They then pressurise it with CO₂ and/or nitrogen. They may do this twice, at which point, hopefully, all the oxygen is gone!

You're looking for a gap of 2 centimetres at the top of a bottle, about 7 per cent of the volume of a typical 12-ounce container

Beer around 5掳C then flows down the tubes into the bottles, filling each to the desired level from the bottom up. Because the beer is cold and the pressure in the bottle is the same as the pressure in the filling tank, there is no foaming. Once full, the nozzles are retracted and the bottles quickly capped. With a decent bottling line, you will have a very uniform fill. However, if the bottles heat up or are shaken at any point after this step, CO₂ can come out of suspension in the beer and go into the headspace. Some CO₂ will go back into the beer, if given enough time and rest, but this can give you some variation, even if the bottle was filled perfectly. If you are homebrewing, you can also use a counterflow filler, though they are usually 100 per cent manual.

Most commercial beer is filtered and pasteurised, removing and killing yeast. It is 鈥渄ead鈥� and will produce no more CO₂. It is also force-carbonated, which means high-pressure CO₂ is applied to the beer, and the degree of carbonation is very controlled.

However, most homebrew beers aren鈥檛 pasteurised or filtered. In an unpasteurised beer, the yeast is still alive and slowly continues to turn grain sugar in the beer into alcohol, also producing CO₂, which creates a little more headspace. Homebrew bottlers often leave carbonation entirely up to the yeast in the bottle, so CO₂ levels and headspace can be wildly variable. If you seriously misjudge things, you can have boxes of exploding bottles.

To answer this question 鈥� or ask a new one 鈥� email lastword@newscientist.com.

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