Beer brewed according to a specific import process was found to have a characteristic off-flavour. The results of chemical and sensory analysis of pilot and production beer indicated that the compounds responsible for the off-flavour were dimethylsulphide, acetic acid, methional and ortho-amino acetophenone. The brewing process was found to be the major cause of off-flavour production particularly the low fermentation temperature and high pitching rate employed. Fermentation at 8oC produced dimethyl sulphide, acetic acid and methional at 70 ppb, 170 ppm and 0.35 ppm respectively. When the fermentation was carried out at 12oC the corresponding values for these compounds were 50 ppb, 145 ppm and 0.15 ppm. Since initial screening implicated sulphur containing compounds, methional, methionine, cystine and sulphate were added to wort and the wort fermented using domestic and imported yeast. It was found that the imported yeast, but not the domestic yeast, metabolised methionine to methional. Although the malt used was also implicated in the production of off-flavour the major cause was the low fermentation temperature employed.
Keywords : beer dimethylsulphide fermentation gas chromatography off-flavour