Beer is usually packaged in brown bottles. Apparently this is because if it is packaged in clear glass, sunlight can damage its flavour. But how does sunlight damage beer? What chemistry occurs?
鈥� I鈥檓 a student who has recently developed an obsession with home-brewed beer, so I hope I can offer some insight.
Nearly all beer contains hops. This provides the bitter taste and also acts as a preservative (some natural deodorants even use hops for its antimicrobial action).
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Hops contain , which provide part of the bitter flavour 鈥� and this is where the problem with light arises. When UV light hits these compounds they decompose, leading to the creation of free radicals that react with sulphur-containing amino acids. The product is a thiol 鈥� the sulphur analogue of alcohols 鈥� and this leads to a 鈥渟kunky鈥� flavour, so called for obvious reasons.
鈥淯V light causes beer to develop thiols 鈥� the sulphur analogue of alcohols 鈥� causing a skunky flavour鈥�
To protect against this, manufacturers use brown bottles that block out some of the UV rays. Clear and green bottles offer far less protection and so are more prone to develop a skunky flavour.
Try leaving a glass of beer out in the sun for even 10 minutes and compare it with one not in direct sunlight, and you will get an idea of what skunky beer tastes like.
Robert Law, Duffield, Derbyshire, UK
鈥� Visible and UV light causes a reaction in which riboflavin (vitamin B2) acts as a catalyst to break down bitter compounds called isohumulones that come from hops. This creates free radicals. One of these, , can then take a thiol group from a sulphur-containing amino acid to become 3-methylbut-2-ene-1-thiol. This compound is similar to two of the sulphurous compounds in a skunk鈥檚 spray, namely 2-butene-1-thiol and 3-methyl-1-butanethiol. Brown bottles cut out most of the harmful wavelengths of light so beer does not become 鈥渟kunked鈥�.
Eric Kvaalen, Les Essarts-le-Roi, France