Alterations of the Profiles of Iso-α-Acids During Beer Ageing, Marked Instability of Trans-Iso-α-Acids and Implications for Beer Bitterness Consistency in Relation to Tetrahydroiso-α-Acids

Abstract

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, ...

Analytical-Sensory Determination of Potential Flavour Deficiencies of Light Beers

“…; Alltech Associates, Deerfield, USA). Chromatographic conditions were as described by De Cooman et al 6 . For quantification of iso-α-acids, tetrahydroiso-α-acids and dihydroiso-α-acids, the international calibration standards ICS-I2, ICS-T2, and ICS-R2, respectively (Labor Veritas, Zürich, Switzerland) were used as external standards.…”

“…Therefore, 10 mL of beer was mixed with 200 µL of Carrez-1 (106 g K 4 Fe(CN) 6 .3H 2 O dissolved in 1,000 mL demineralised water) and 200 µL of Carrez-2 (220 g Zn(CH 3 COO) 2 .2H 2 O and 30 mL of glacial acetic acid, made up to 1,000 mL with demineralised water). After centrifugation at 11,000 g for 5 minutes, the samples were ready for manual injection into the HPLC apparatus through a Rheodyne 7125 injection valve, equipped with a 20 µL sample loop.…”

Analytical and Sensory Assessment of Hoppy Aroma and Bitterness of Conventionally Hopped and Advanced Hopped Pilsner Beers

“…Quantitative analysis of iso-α-acids in beer was performed by HPLC of beer extracts on a Hitachi Liquid Chromatograph (Merck, Darmstadt, Germany), according to De Cooman et al 4 The HPLC consists of a programmable HPLC pump (L-7100) with a quaternary low pressure gradient system, a diode array detector (L-7450A), an interface module (D-7000), a solvent degasser (L-7612), an autosampler (L-7200), and an Alltima 5 µm C18 column (150 mm × 4.6 mm i.d. ; Alltech Associates, Deerfield, USA).…”

“…; Alltech Associates, Deerfield, USA). A dicyclohexylamine-iso-α-acids ICS-I1 complex (66.5% (w/w) iso-α-acids) (Labor Veritas, Zürich, Switzerland) was used for quantitative determination of iso-α-acids (additional details on HPLC analysis of iso-α-acids are given in the references 4,17 ).…”

“…This definitely holds true when applying advanced bittering preparations such as (reduced) isomerised extracts containing iso-α-acids, dihydroiso-α-acids or tetrahydroiso-α-acids 6 . When combined with appropriate analytical techniques (e.g., HPLC) for quantitative and qualitative evaluation of these advanced products and adequate dosing technologies, the brewer is currently able to prepare beers with a reproducible bitterness and an improved bitterness stability on storage 4 .…”