A new quantification method for hop-derived bitter compounds in beer was developed. By means of LC-MS/MS operating in the multiple reaction monitoring mode, a total of 26 hop-derived bitter compounds, namely, the post-, co-, n-, ad-, pre-, and adpre-congeners of iso-alpha-acids, alpha-acids, and beta-acids, as well as the prenylflavonoid isoxanthohumol and the chalcone xanthohumol, could be simultaneously detected for the first time in a single HPLC run in authentic beer samples without any cleanup procedures. To compensate for the effect of coextracted matrix components in LC-MS/MS analysis, the so-called ECHO technique was applied for the first time as a suitable strategy for the quantitative analysis of the hop-derived bitter compounds in fresh and stored beer. On the basis of quantitative data, the remarkable instability of alpha-acids and trans-iso-alpha-acids was confirmed, and it was observed that the degradation of trans-iso-alpha-acids during the storage of beer is not dependent from the nature of the alkanoyl side chain of the congeners. In contrast, an increase of the concentrations of beta-acids and of the prenylflavonoid isoxanthohumol as well as of the chalcone xanthohumol during the storage of beer was observed.

In order to study the role of different haplotypes of taste receptor genes in food choice, it is necessary to first identify the cognate hTAS2R bitter taste receptors for the key bitter compounds in food products of our daily diet. In order to identify the candidate receptors mediating the bitter taste of hop-containing beverages such as beer, we transiently transfected plasmids encoding the 25 human TAS2Rs into human embryonic kidney 293T cells, stably expressing the chimeric G-protein G16gust44. Thereby, we coupled the activation of hTAS2R receptors to the release of Ca 2+ from intracellular stores. The transfected cells were loaded with a calcium-sensitive fluorescence dye and challenged by 15 hop-derived compounds, including α-acids, β-acids, trans/cis-iso-α-acids, isoxanthohumol, xanthohumol, and 8-prenylnaringenin. Depending on their chemical structure, all these compounds activated various combinations of the three bitter taste receptors hTAS2R1, hTAS2R14, and hTAS2R40 with distinct threshold concentrations and EC 50 values. Notably, this is the first time that an agonist for hTAS2R40 is reported. The threshold concentrations and EC 50 values obtained from the taste receptor assays were much lower than those determined by human psychophysical experiments, even though the rank order of potency for the various compounds was similar in both experiments. Thus, the subjects perceived the bitterness of the investigated compounds at higher concentrations than those predicted by the results of the in vitro experiments. These differences were shown to be due, at least in part, to interactions of the bitter substances with the oral mucosa.

Besides undesirable changes in the attractive aroma, a significant decrease in the intensity of the bitterness as well as a change of the taste into a lingering, harsh bitterness has long been known as a shelf-life limiting factor of beer. Multiple studies have demonstrated that the aging of beer induces a decrease of the total amount of cis- and trans-iso-alpha-acids, the well-known bitter principles of beer. Although the trans-iso-alpha-acids exclusively, not the cis-iso-alpha-acids, were found to be degraded upon storage of beer, the key transformation products formed exclusively from the trans isomers in beer are not known. In the present study, suitable model experiments followed by LC-MS/MS and sophisticated NMR spectroscopic experiments, including the measurement of residual dipolar couplings (RDCs) in gel-based alignment media as well as a novel broadband and B(1)-field-compensated incredible natural abundance double-quantum transfer experiment (INADEQUATE) pulse sequence, enabled the identification of a series of previously unknown trans-specific iso-alpha-acid transformation products, namely, tricyclocohumol, tricyclocohumene, isotricyclocohumene, tetracyclocohumol, and epitetracyclocohumol, respectively. HPLC-MS/MS analysis of these compounds, which exhibit the aforementioned harsh lingering bitter taste and have threshold concentrations ranging from 5 to 70 micromol L(-1), confirmed their generation during aging of beer and, for the first time, explained the storage-induced changes of the beer's bitter taste on a molecular level.

No abstract

Thermal treatment of the hop beta-acid colupulone under wort boiling conditions, followed by LC-TOF-MS and 1D/2D NMR spectroscopy, revealed cohulupone, hulupinic acid, nortricyclocolupone, two tricyclocolupone epimers, two dehydrotricyclocolupone epimers, two hydroxytricyclocolupone epimers, and two hydroperoxytricyclocolupone epimers as the major bitter-tasting beta-acid transformation products. Among these compounds, the chemical structures of the hydroxy- as well as the hydroperoxytricyclocolupone epimers have not previously been confirmed by 1D/2D NMR experiments. Depending on their chemical structure, these compounds showed rather low recognition thresholds ranging from 7.9 to 90.3 micromol/L. The lowest thresholds of 7.9 and 14.7 micromol/L were found for cohulupone, imparting a short-lasting, iso-alpha-acid-like bitter impression, and for hydroxytricyclocolupone, exhibiting a long-lasting, lingering, and harsh bitterness perceived on the posterior tongue and throat. Furthermore, HPLC-ESI-MS/MS analysis allowed for the first time a simultaneous detection and quantitation of these bitter-tasting beta-acid transformation products in a range of commercial beer samples without any sample cleanup. Depending on the type of beer, these studies revealed remarkable differences in the concentrations of the individual beta-acid transformation products.

For the first time, quantitative LC-MS/MS profiling of 56 hop-derived sensometabolites contributing to the bitter taste of beer revealed a comprehensive insight into the transformation of individual bitter compounds during storage of beer. The proton-catalyzed cyclization of trans-iso-α-acids was identified to be the quantitatively predominant reaction leading to lingering, harsh bitter tasting tri- and tetracyclic compounds such as, e.g. the cocongeners tricyclocohumol, tricyclocohumene, isotricyclocohumene, tetracyclocohumol, and epitetracyclocohumol, accumulating in beer during storage with increasing time and temperature. The key role of these transformation products in storage-induced trans-iso-α-acid degradation was verified for the first time by multivariate statistics and hierarchical cluster analysis of the sensomics data obtained for a series of commercial beer samples stored under controlled conditions. The present study offers the scientific basis for a knowledge-based extension of the shelf life of the desirable beer's bitter taste and the delay of the onset of the less preferred harsh bitter aftertaste by controlling the initial pH value of the beer and by keeping the temperature as low as possible during storage of the final beverage.

Although the complex taste profile of beer is well accepted to be reflected by the molecular blueprint of its sensometabolites, the knowledge available on the process-induced transformation of hop-derived phytochemicals into key sensometabolites during beer manufacturing is far from comprehensive. The objective of the present investigation was, therefore, to develop and apply a suitable HPLC-MS/MS method for the simultaneous and comprehensive quantitative monitoring of a total of 69 hop-derived sensometabolites in selected intermediary products throughout a full-scale beer manufacturing process. After data normalization, the individual sensometabolites were arranged into different clusters by means of agglomerative hierarchical analysis and visualized using a sensomics heatmap to verify the structure-specific reaction routes proposed for their formation during the beer brewing process.

Hongarebushi, Japanese dried skipjack tuna and a high quality ingredient of Japanese dashi, was investigated for its taste active composition. The recent investigation focused on a debittered fish fraction, which revealed a strong umami and salt impact accompanied with a pleasant and pronounced sourness. Whereas the umami and salt tastes could be correlated to monosodium glutamate (MSG), ribonucleotides, and mineral salts, the pleasant sourness was not exclusively induced by organic acids. The essential compound imparting the sour orosensation, persistence, and mouthfulness of the debittered skipjack tuna extract was investigated, and omission experiments emphasized the impact of N-acetylglutamic acid (NAG) on the overall taste sensation of the debittered fish extract. This metabolite, which is known to be present as a minor constituent in animal- and plant-derived foods, was quantified in this study for the first time in seafood, soybean products, dried shiitake mushrooms, and dried fish in notable amounts. Furthermore, it was described for the first time as an essential taste contributor to the nonvolatile profile of a foodstuff, in this case of a debittered extract of hongarebushi.