Ultraviolet light reacts with certain hop compounds to produce a sulphurous smell.   The compound formed is 3-methyl-2-butene-1-thiol (MBT) and humans are very sensitive to it and can detect this taste in the order of parts per trillion.   This process is called skunking.   The precursor to this is isohumulone and the ultraviolet light gives it the energy to react with the sulphur contained in some amino acids.

The obvious way to reduce this is to prevent light falling onto the beer.   Barrels, kegs and cans are ideal as they let no light through at all.   Brown bottles offer a reasonable barrier to light whereas green bottles are less good.   Clear, colourless bottles are obviously the worst barriers to light of all.

Using solvent extraction the substances responsible for producing the off-flavour can be removed enabling a clear glass bottle to be used with less risk of skunking.