Drunk as a skunk

No doubt you’ve been dying to find out what the teaser image from the other day was all about. Well, this week on Lost in Science we performed an experiment – a very responsible experiment – to find out the effect of light on beer, and how different colour beer bottles can help or hinder.

Spectrum of sunlight filtered by clear, green, blue and brown glass (click to embiggen)
Spectrum of sunlight filtered by clear, green, blue and brown glass (Image from Australian Brews News)

Beer is made from grains – usually barley – fermented by yeast. But the familiar, bitter flavour comes from hops, the flowers of which were originally added to help stop the beer going off. In particular, the bitterness comes from three compounds called isohumulones, or iso-α-acids. And when isohumulones are exposed to light, they react with proteins in the hops to form foul-smelling sulfur compounds.

This process was first identified by chemists from the University of North Carolina and Ghent University in Belgium, who exposed beer to a laser (Burns CS, Heyerick A, de Keukeleire D & Forbes MDE 2001, “Mechanism for formation of the lightstruck flavor in beer revealed by time-resolved electron paramagnetic resonance”, Chemistry – A European Journal, vol. 7, issue 9, pp. 4553-4561).

They found that the laser light caused the isohumulones to break down into free radicals, which then react with sulfur proteins to form a compound called 3-methylbut-2-ene-1-thiol (later studies showed that this reaction is usually catalysed by riboflavin, also known as vitamin B2).

This chemical is very similar to other sulfurous thiols – also known as mercaptans – that are found in skunk spray. And for this reason, beer that’s exposed to light takes on a distinctive, skunky odour.

We decided to test this in Lost in Science’s first ever on-air experiment. This involved four bottles of Corona – perhaps the most famous beer that comes in clear glass bottles. The four bottles were treated to be different colours: one completely encased in aluminium foil to block out all light, one covered in layers of red, yellow and green cellophane (to make brown), one covered only in green cellophane, and one left clear.

4 bottles of beer of different colours - silver, brown green and clear
4 bottles of beer of different colours - silver, brown green and clear

These bottles were then left out in the sunlight – well, what passes for sunlight in Melbourne.

When the bottles were opened, it was no surprise that we found the clear bottle to be disgustingly skunky and the silver bottle to be unaffected. But the coloured bottles were more interesting, and unexpected.

The reaction of isohumulones when exposed to light is more intense at higher energies, i.e. blue-green into the ultraviolet. So the green bottle should be stinkier than the brown bottle. Also, since beer is kind of brownish itself, you’d expect that the only colours it’s not absorbing are the same that the brown glass is not absorbing, so it should be safer under brown.

Whatever the logic, it’s accepted wisdom among brewers that beer in green bottles is more susceptible to being damaged by light (see this very informative article by Australian Brews News).

But this is not what we found. After at least 2 days in the “sun”, both the brown and green bottles seemed unaffected. Why is this?

One possibility is that green cellophane actually has a different absorption spectrum to green glass. The figure at the top of this post, taken from that same article at Australian Brews News, shows the spectrum of sunlight passing through bottles of various colours. This appears to show that green glass lets through a substantial amount of light at ultraviolet wavelengths – about the same as blue glass. And indeed, the laser experiment mentioned earlier used light of wavelength 308 nm, well into the ultraviolet region of the spectrum.

But although green cellophane may look the same colour as green glass (green, obviously), that’s no guarantee that it’s letting through the same amount of ultraviolet light, which of course we can’t see.

So what’s needed from us is further experimentation, using beer in actual green bottles. This is an arduous responsibility, but one the Lost in Science team is up to.

Stuart and Beth from Lost in Science testing light-struck beer
Stuart and Beth from Lost in Science testing light-struck beer (Image from my mobile phone)

A couple of final notes: some beers that come in clear glass bottles, such as Carlton Cold, use a modified hops extract that doesn’t contain the light-sensitive isohumulones. So they’re not susceptible to skunkiness; Corona though, as we’ve shown, is susceptible and should either be kept in the dark or should be drunk quickly.

And in case you’re wondering, no, the slice of lemon or lime doesn’t do anything to remove the skunky odour. Thiols break down quicker in alkaline conditions, so the acidic citrus juice won’t help. There are many theories as to why people add lime, but perhaps the most likely is simply to add flavour to the otherwise weak tasting beverage.

By all means, continue to add lime to your Corona, but consider switching to a good, honest, light-proof brown-bottled beer instead.


3 thoughts on “Drunk as a skunk

  1. Stuart & Beth,

    Nice job on the experiment! I suspect that you are correct regarding the absorption properties of the coloured cellophane v glass. They are not at all similar on a molecular level, so it should be no suprise that they absorb light in a very different way.

    I am the author of the article on the Brews News website and am glad to have now discovered your program and blog.

    Keep up the great work!

    Chris Bible

  2. Great experiment guys, well done! Please let us know if you’re ever up for more beer and science, they are a great match. Bubbles, bitterness, the mechanics of foam, the science of taste…all of them can be found in a beer glass. And let’s not forget that at least one Australian astrochemist is on a mission to find all of the necessary chemicals to make beer exist elsewhere in the galaxy! http://www.spacevidcast.com/2009/08/18/beer-in-space/

    Keep up the great work.


    1. Wow, space beer! We love space stories on Lost in Science, so it’s always great to have more ammunition. As for more beer science, I definitely intend to keep it coming. Currently working on a follow-up experiment using my own home brew…

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