The bitter acids in hops (Humulus lupulus L.) and beer, such as α-, β-, and iso-α-acids, are known to affect beer quality and display various physiological effects. However, these compounds readily oxidize, and the effect of the oxides on the properties of beer or their potential health benefits are not well understood. In this study, we developed a simple preparative method for the bitter acid oxide fraction derived from hops and designated the constituents as matured hop bitter acids (MHBA). HPLC-PDA-ESI/HRMS and MS(2) revealed that MHBA are primarily composed of α-acid-derived oxides, which possess a common β-tricarbonyl moiety in their structures similar to α-, β-, and iso-α-acids. We also developed a quantitative analytical method of whole MHBA by HPLC, which showed high precision and reproducibility. Using our newly developed method, the concentration of whole MHBA in several commercial beers was evaluated. Our results will promote the study of bitter acid oxides.

The resins from hops (Humulus lupulus L.), which add the bitter taste to beer, are classified into two main sub-fractions, namely, soft and hard resins. α- and β-Acids in soft resin and their transformation during the wort boiling process are well-studied; however, other constituents in resins, especially hard resin, have been unidentified. In this study, we identified humulinones and hulupones as soft-resin components, in addition to 4'-hydroxyallohumulinones and tricyclooxyisohumulones A and B as hard-resin components. These compounds are all oxidation products derived from α- or β-acids. We also investigated compositional changes in the hard resin during the wort boiling process, which has a significant effect on the taste of the beer, by using model boiling experiments. The major changes were identified to be isomerization of 4'-hydroxyallohumulinones into 4'-hydroxyallo-cis-humulinones, followed by decomposition into cis-oxyhumulinic acids. These findings will be helpful in systematically evaluating and optimizing the effect of the hard resin on beer quality.

The transformation of α-acids [in hops (Humulus lupulus L.)] to iso-α-acids (in beer) during the brewing process is well known, but the occurrence and structure of the oxidized α-acids during hop storage are not well documented. Because an understanding of these oxidized compounds is essential to optimize the effects of oxidized hops on the quality of beer, we investigated the autoxidation products of humulone (a representative congener of α-acids) using a simplified autoxidation model. Among the oxidation products, tricyclooxyisohumulones A (1) and B (2), tricycloperoxyisohumulone A (3), deisopropyltricycloisohumulone (4), and the hemiacetal 5 of tricycloperoxyhumulone A (5') were isolated, and their structures were elucidated for the first time. The occurrence of compounds 1-4 in stored hops was verified using LC/MS/MS analysis. We also monitored the levels of compounds 1-4 during hop storage using LC/MS/MS analysis.

The bitter taste of beer originates from resins in hops (Humulus lupulus L.), which are classified into two subtypes (soft and hard). Whereas the nature and reactivity of soft-resin-derived compounds, such as α-, β-, and iso-α-acids, are well studied, there is only a little information on the compounds in hard resin. For this work, hard resin was prepared from stored hops and investigated for its compositional changes in an experimental model of beer aging. The hard resin contained a series of α-acid oxides. Among them, 4'-hydroxyallohumulinones were unstable under beer storage conditions, and their transformation induced primary compositional changes of the hard resin during beer aging. The chemical structures of the products, including novel polycyclic compounds scorpiohumulinols A and B and dicyclohumulinols A and B, were determined by HRMS and NMR analyses. These compounds were proposed to be produced via proton-catalyzed cyclization reactions of 4'-hydroxyallohumulinones. Furthermore, they were more stable than their precursor 4'-hydroxyallohumulinones during prolonged storage periods.

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