Distinctive flavour compounds with the aroma of skunk. Produced by the photolysis of a side chain of iso a acids and the subsequent reaction of this with sulphur containing thiol radicals to produce 3-methyl-2-butene-1-thiol. Exposing beer to light, particularly by using clear glass bottles, will initiate this reaction although specially treated hop oil, Tetra hop, will stabilise the iso a acid.