Cyclic organic compounds present in wort and often possessing distinctive flavours. Aromatic alcohol phenols are produced by yeast fermentation and may give beer a floral character above 200 mg per litre. Other phenols are less attractive. Chlorophenols produced by the reaction of phenol with chlorine are very pungently medicinal at very low levels. Spicy, clove like flavours are produced by the decarboxylation of the phenol ferulic acid from malt into 4-vinyl guaiacol. This reaction is catalysed by wild yeasts but not brewing yeasts and indicates possible contamination of yeast or beer. It is, however, a desirable feature of wheat beers which thus require an appropriate yeast for an authentic production.