Saturday, 25 April 2009
Drunk as a skunk?
Not on this shite you won't be
Recently I got round to something I've been meaning to do some time and had a proper controlled experiment on the effects of light strike. Light strike is when the action of light on hop compounds in beer causes an unpleasant taste to develop.
But that's not nearly nerdy enough so I'll go on about it in more detail:
The isomerised-alpha acids (iso-humulones) from hops, which are responsible for most of the bitterness in beer, are very sensitive to the effects of light, particularly the 350-500 nm wavelength range. The light causes a side chain on the isomerised alpha acid molecule to be cleaved, forming a highly reactive radical. The radical can then react with sulphur containing compounds to form 3-methyl-2-butene-1-thiol or MBT. This has an extremely unpleasant taste, and a flavour threshold as low as one ng/l. It is also the same chemical made by skunks, hence the American term 'skunked' for light struck beer.
That makes it clearer doesn't it?
The wavelengths of light that causes this reaction are able to penetrate clear and green glass bottles but are absorbed by brown glass. So be wary of beer not in brown bottles. The effects can be avoided by using hop compounds that are resistant to the effects of light strike: reduced isomerised hop extracts. But wouldn't you rather have beer made with whole hops not a chemical extract? I know I would.
Anyway, back to the experiment.
Having heard all about light strike and the horrors of MBT but never having been sprayed by the anal glands of a skunk I wasn't sure exactly what the taste was. But for a man of my scientific ability an easy solution was at hand. I took some of the last of my homebrewed American style hoppy IPA, put it in a clear glass and left it in the sun for an hour. Being a scientist I also poured out a small control sample which I put in a cardboard box.
The light had a huge and obvious effect on the beer. It had developed an unpleasant rubbery stench and the taste was far worse. It reminded me of the tang found in cheap French lager from green bottles but much stronger. It was almost undrinkable, and I'm one who hates beer going to waste. The control beer on the other hand was not at its best from having been sat in a cardboard box but I had no problem getting it down. So if you don't like beer that tastes like its comes from the anal glands of a skunk keep your beer safe from light.