On the Autoxidation of Bitter-Tasting Iso-α-acids in Beer

Abstract

The iso-alpha-acids, the major contributor to bitter beer taste, is well-known to strongly degrade during beer aging. The storage of beer in brown glass bottles revealed a strong depletion of the trans-configured isomers in a highly specific manner, whereas the corresponding cis-iso-alpha-acids seemed to be hardly affected. In comparison, storage of beer in polyethylene terephthalate bottles, which are known to be permeable to oxygen, induced a drastic degradation of both isomers independent of their cis/trans configuration. To investigate the chemical transformation of iso-alpha-acids under oxidative storage conditions, suitable model experiments were performed, and the reaction products that formed were identified as previously not reported hydroperoxy- and hydroxyl-allo-iso-alpha-acids by means of one-/two-dimensional NMR and liquid chromatography-mass spectrometry experiments; for example, cis- and trans-configured hydroperoxy-alloisohumulone as well as the corresponding hydroxy-alloisohumulones were generated upon oxidation of cis- and trans-isohumulone independent of their cis/trans configuration. In addition, the oxidation products formed from the various iso-alpha-acid congeners were quantitatively determined in a series of beer samples stored under defined conditions. For the first time, these data help to understand the molecular mechanism involved in the autoxidative degradation of iso-alpha-acids in beer.

Quantitative Sensomics Profiling of Hop-Derived Bitter Compounds Throughout a Full-Scale Beer Manufacturing Process

“…Whereas the formation of these tricyclohumols 37-39 from the iso-R-acids 6-8 were confirmed to be transspecific (14), the humulinic acids 40-45 ( Figure 2, 3), the alloisohumulonehydroxides 46-51, and the corresponding peroxides 52-57 were found as oxidation products of both, the cis-and trans-iso-R-acids 6-11 (Figure 3), in particular, when beer samples were accessible to air oxygen (15). *To whom correspondence should be addressed.…”

“…This is well in line with the limited water solubility of these β-acids counteracting their extractability from the hop material during wort boiling and favoring their precipitation upon cooling of the final wort (29,30). Although degradation reactions of β-acids (15)(16)(17) during an authentic brewing process are not described in literature, about 20% of the starting amount of β-acids added with the hops was found to be oxidized to give 445.9 g cohulupone (18), 247.3 g hulupone (19), and 126.1 g adhulupone (20) per gyle in the final wort (Table 3). However, the initial concentration of these hulupones in wort decreased to a lower level of 51.2-147.5 g/gyle in the finished beer VII, corresponding to a utilization rate of 4.8-10.4% (Table 3).…”

Detecting Beer Intake by Unique Metabolite Patterns

“…This is attributed not only to the low hops content of the test beer but also to formation of several oxidative degradation products of IAAs 32, 33 underlining their instability. Tentatively identified degradation products of trans-IAA in urine were tri-and tetra-cyclohumenes and tri-cyclohumols 14,34 whereas, mono-and di- 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 31 hydroxylated humulones and humulinones appeared as oxygen-dependent degradation products of both cis-and trans-IAAs 33 . Indeed, only tricyclohumols were detected in low hops beer (MSt2), which has also been shown previously to be one of the major degradation products 34 .…”

Study of energy-saving wort boiling systems for Czech beer production.

“…Jejich hořkost, změny v průběhu stárnutí piva, autooxidační reakce a další vlastnosti jsou dobře popsány (Jaskula et al ., 2010;Hughes a Simpson, 1996) . Byly také zpracovány kinetické studie izomerační reakce za různých podmínek chmelovaru (Intelmann a Hofmann, 2010) . Izomerace je dynamickým procesem tvorby a následné částečné degradace iso--α-kyselin, který závisí zejména na teplotě, době varu a proudění v pánvi .…”

“…Their bitterness changes during beer aging, autooxidative reactions and other properties are well described (Jaskula et al ., 2010;Hughes and Simpson, 1996) . Kinetic studies of the isomerization reaction under various wort boiling conditions were also carried out (Intelmann and Hofmann, 2010) Isomerization is a dynamic process of iso-α-acids creation and subsequent partial degradation, which depends on temperature, time and flow conditions in the boiling kettle . Thermal degradation of iso-α-acids leads to the formation of undesirable sensory compounds (Malowicki and Shellhammer, 2005) .…”