Reducing power of hop cultivars and beer ageing

Since lipid auto-oxidation during wort boiling is a determining for the app earance of staling flavour in aged beers, we have investigated the reducing power of hops added in the boiling kettle. An assay based on the inhibitio n of linoleic acid oxidation in the presence of an initiator [2,2'-azobis(2 -amidino-propane) dihydrochloride = AAPH] enabled us to distinguish hop var ieties and conditionings. Large differences in hop flavanoid contents expla ined the higher antioxidant activity of low-cr-acid samples versus bitter v arieties and CO2 hop extracts. As expected, adding hop pellets to the kettl e effectively increased the overall reducing activity of wort. Supercritica l CO2 hop extracts had no significant effect due to their extremely low lev el of polyphenols. The concentration of the very well-known marker of beer ageing, trans-2-nonenal, was lower in boiled wort exhibiting a better reduc ing power. The AAPH reducing power test applied to hops or worts was thus e fficient to predict the nonenal synthesis during boiling. Hop varieties and conditionings emerged from this work as key-parameters for improving the r educing power of wort and the flavour stability of the final product. (C) 2 001 Elsevier Science Ltd. All rights reserved.