Age-induced decomposition of iso-a-acids, the main bittering principles of beer, determines the consistency of the beer bitter taste. In this study, the profiles of iso-a-acids in selected highquality top-fermented and lager beers were monitored by quantitative high-performance liquid chromatography at various time intervals during ageing. Tlte degradation of the iso-a-acids as a function of time is represented by the ratio, in percentage, of the sum of the concentrations of trans-isocohumulone and trans-isohumulone to the sum of the concentrations of cis-isocohumulone and cis-isohumulone. Tfiis parameter is relevant with respect to the evaluation of bitterness deterioration in aged beers. Trans-iso-a-acids having a shelf half-life of less than one year proved to be significantly less stable than cis-iso-a-acids, but it appears feasible to counteract degradation if a stiitable beer matrix is available. Tlte fate of the trans-iso-a-acids in particular adversely affects beer bitterness consistency. In addition to using hop products containing low amounts of trans-iso-a-acids, brewers may profit of the remarkable stability of tetrahydroiso-aacids, even on prolonged storage, for the production of consistently bitter beers.KeyWords: Hops, iso-a-acids, tetrahydwiso-a-acid$, beer bitterness, beer ageing, HPLC analysis. INTRODUCTION associated with beer ageing, because such compounds, in particular /nms-2-nonenal, are considered responsibleOrganoleptic consistency of beer is of paramount for the so-called cardboard or staling offimportance to the modern brewing industry2. Raw navour3.6.io.2<>3:>.42.43. An array of enzymic and chemical materials are in general well characterised and reactions are supposed to be involved in the formation technological innovations continuously lead to of ageing carbonyls, including melanoidin-catalysed improvements in the brewing process, yet reproducible oxidation of higher alcohols1516, oxidative reactions of production of flavour-stable beers remains a challenging iso-a-acids4151617'2"-34, Strecker-type degradation of amino task for a brewer. In particular, lager beers are liable to acids61516 '28-11'32,auto-oxidation1516-27'32,photo-oxidationS7;w, undergo rapid taste and flavour changes that can readily and enzymic degradation7827^7 of unsaturated fatty acids, be tracked. Beer deterioration is an extremely complex aldol-type condensations of short-chain aldehydes1516, process, while, moreover, various beer types age in ancj secondary auto-oxidation of aIdehydes15l6--»2. different ways2. Importantly, the relative significance of these various pathways is not clear and even the nature of precursorsIn particular, the formation of volatile unsaturated for the carbonyls has not been unequivocally established43, carbonyl compounds with 6 to 10 carbon atoms has beenAmong the non-volatiles in beer, the iso-a-acids are *To whom correspondence should be addressed. pre-eminent as the main bitter principles. The iso-ct- Age-induced decomposition of iso-a-acids acids comprise 6 major components, ...