Why are beer bottles usually made of brown glass?

Light sets off a reaction in beer in which riboflavin captures energy and hop compounds are converted into a strong-smelling sulphur compound.

That compound is called 3-methyl-2-butene-1-thiol. The nose picks it up even at very low levels, which explains why a bottle that has stood in the sun for an afternoon smells different straight away.

Researchers at Kvasny Prumysl described in 2022 how riboflavin works as a photosensitiser and drew up an index to measure how well a package protects against it. In 2024 follow-up research followed in which tasting panels and optical measurements were set side by side.

Brown glass lets the least light through in the blue and ultraviolet part of the spectrum, precisely the part that sets off the reaction in beer.

Green glass blocks part of that light, but less than brown. Clear glass blocks even less of it and lets the reaction proceed almost unhindered.

Breweries that choose green or clear glass anyway do so to show off the beer. Some of them use hop extract from which the sensitive compounds have been removed, so the reaction has nothing left to act on.

A can lets no light through and thereby offers complete protection, whereas even brown glass lets a small part through.

The same applies to a cardboard outer box or a closed keg. Beer that is kept in the dark cannot develop a light-struck flavor, whatever packaging surrounds it.

Cans are also lighter and do not break, which makes a difference on a terrace or at a party. Anyone who associates cans mainly with lager will nowadays also find speciality beer in cans.

Beer belongs in the dark, cool and upright, because light is, next to temperature, the main cause of flavour loss in the bottle.

A cellar, a closed cupboard or the box the beer came in will do. What you want to avoid is a shelf in the sun, a windowsill or lighting that shines on the glass day and night.

For beer in clear or green glass that weighs more heavily than for brown glass. If such a bottle stands in the light for more than a few days, the chance of light-struck flavour is real.

On a beer label the alcohol percentage, contents, ingredients and shelf life are mandatory, but nothing is stated about the light sensitivity of the packaging.

Regulation (EU) No 1169/2011 sets out what information a consumer must receive. The colour of the glass falls outside it, and there is also no mandatory instruction to store beer in the dark.

The colour of the bottle is therefore the only signal you can read yourself. Brown means the most protection, green less, clear the least.