A Review of Cyclic and Oxidative Bitter Derivatives of Alpha, Iso-Alpha and Beta-Hop Acids

Quantification of Hop-Derived Bitter Compounds in Beer Using Liquid Chromatography Mass Spectrometry

Hops (Humulus lupulus L.) are essential raw materials for beer brewing, and the major contributors to beer bitterness are isohumulones (iso-α-acids) and humulinones. In recent years, many breweries have focused on the production of hop-forward beer styles by adding hops after or during the cold fermentation stage, which will tend to release humulinones or other hop-derived bitter compounds. In this study, a LC-MS/MS method was developed for quantification of 60 hop-derived bitter compounds in 25 min. Reverse-phase chromatography with an alkaline methanol/acetonitrile (70:30) mobile phase was used for the separation. The quantitative range was 0.053-3912 ng/mL with correlation coefficient r > 0.99, and the LOQ were 0.26 and 0.053 ng/mL for iso-α-acids and humulinones. Precision (RSD < 5.0%) and accuracy (recovery 86.3%-118.1%) were both satisfactory. The abundance of hop-derived bitter compounds in the dry-hopped beer (Double-India Pale Ale) and the nondry-hopped beer (Vienna Lager) were monitored throughout the fermentation and storage stages, and the formation of oxidation and cyclization products showed difference profiles between these two beers. The quantification results reveal how hop-derived bitter compounds change throughout the brewing process, as well as the influence of hops and brewing techniques on beer bitterness.

Changes in beer bitterness level during the beer production process

Beer has been enjoyed by consumers for years. Today, hops are inextricably associated with this beverage. Although they have been the subject of research for decades, knowledge of their bittering components and interactions during the beer production process is still incomplete. Current literature clearly indicates that the bitterness experienced in beer comes from a much wider range of compounds than just iso-α-acids. Although compounds that can be classified into β-acids, humulinones, hulupones, hard resins, and polyphenols are characterized by lower levels of bitterness and are present in hops in lower quantities than α-acids, they might determine, together with them, the final level of bitterness in beer. Unlike α-acids, the influence of compounds from these groups, their transformations, changes in their content during the beer production process and factors that affect their final concentration in beer have not yet been thoroughly studied. In case of α-acids, it is known that factors, such as chemical composition of wort, its extract and pH, amount of hops added and α-acids’ content, boiling time, and temperature at which hops were added influence the level of bitterness. This phenomenon is further complicated when dry hopping is used. Due to the presence of humulinones, polyphenols, and α-acids, a relatively simple spectrophotometric determination of IBU can give erroneous results. IBU determination, especially in dry-hopped beers, should be coupled with HPLC analysis, taking into account appropriate bitterness coefficients.

Determination of α- and β-acids in hops by liquid chromatography or electromigration techniques: A critical review

Hop plays an essential role in brewing beer and its study and analysis is of paramount importance. - and -acids are considered two of the most important hop components. While -acids are associated with the bitter flavor, -acids have antimicrobial effects. This work aims to critically review the published analytical methods for - and -acids determination in hops employing separation methods in liquid medium: liquid chromatography (LC) and capillary electrophoresis (CE). The types of hop samples, the optimized protocols to extract the hop acids, and the main instrumental conditions for both LC and CE techniques are highlighted and discussed. Specific and critical aspects of the - and β-acids separation by LC and CE and some challenges in this field are raised. Several key aspects discussed in this review may be of practical importance for brewers, whether in the microbrewery or industry and for researchers in the brewing field.

Impact of Non-Saccharomyces Wine Yeast Strains on Improving Healthy Characteristics and the Sensory Profile of Beer in Sequential Fermentation

The use of non-Saccharomyces yeasts in brewing is a useful tool for developing new products to meet the growing consumer demand for innovative products. Non-Saccharomyces yeasts can be used both in single and in mixed fermentations with Saccharomyces cerevisiae, as they are able to improve the sensory profile of beers, and they can be used to obtain functional beers (with a low ethanol content and melatonin production). The aim of this study was to evaluate this capacity in eight non-Saccharomyces strains isolated from Madrid agriculture. For this purpose, single fermentations were carried out with non-Saccharomyces strains and sequential fermentations with non-Saccharomyces and the commercial strain SafAle S-04. The Wickerhamomyces anomalus strain CLI 1028 was selected in pure culture for brewing beer with a low ethanol content (1.25% (v/v)) for its fruity and phenolic flavours and the absence of wort flavours. The best-evaluated strains in sequential fermentation were CLI 3 (Hanseniaspora vineae) and CLI 457 (Metschnikowia pulcherrima), due to their fruity notes as well as their superior bitterness, body, and balance. Volatile compounds and melatonin production were analysed by GC and HPLC, respectively. The beers were sensory-analysed by a trained panel. The results of the study show the potential of non-Saccharomyces strains in the production of low-alcohol beers, and as a flavour enhancement in sequential fermentation.

Impact of Hop Freshness on Dry Hopped Beer Quality

The hop plant is seasonal, but beer production continues throughout the whole year. The quality of hops begins to decrease immediately after harvesting; therefore, maintaining the highest possible quality is important. A good indicator of hop freshness is the hop storage index (HSI). In this study, three different varieties of hops with five different HSI values, from 0.3 to 0.7, were used for brewing with the dry hopping technique. The main goal was to evaluate the impact of the HSI value on beer quality in terms of hop aroma and bitterness. Alpha acids, iso-alpha acids, humulinones, bitterness units and hop aroma compounds were chemically analysed. Sensorial analysis was also conducted on all samples. Decreases in the intensity and quality of hop aroma were detected with increasing HSI. The quality of bitterness was also reduced. High HSI also led to undesirable gushing. Beers brewed with hops with HSI values greater than 0.4 had deviations in aroma and bitterness when compared with beers brewed with fresh hops.

Anti-Inflammatory Markers of Hops Cultivars (Humulus lupulus L.) Evaluated by Untargeted Metabolomics Strategy

Hops (Humulus lupulus L.) are edible flowers commonly used to add flavour and aroma to beer, besides they have rich chemical diversity and medicinal potential. In this work, an ex vivo anti-inflammatory assay via the LPS-induced signalling pathway and metabolomics approaches were performed to evaluate the ability of hops to inhibit the production of prostaglandin E2 (PGE2) inflammatory mediator and analyze which metabolites produced by the nine different hop cultivars are potential anti-inflammatory markers. Columbus, Chinook and Hallertau Mittelfrüh hop cultivars yielded extracts with PGE2 release inhibition rates of 86.7, 92.5 and 73.5 %, respectively. According to the multivariate statistical analysis, the majority of the metabolites correlated with the activity were prenylated phloroglucinol and phenolic homologs. These results suggest promissory anti-inflammatory hop metabolites.

The Influence of Hop Phenolic Compounds on Dry Hopping Beer Quality

Background: The article considers the phenolic hop compounds’ effect on the quality indicators of finished beer. The topic under consideration is relevant since it touches on the beer matrix colloidal stability when compounds with potential destabilizing activity are introduced into it from the outside. Methods: The industrial beer samples’ quality was assessed by industry-accepted methods and using instrumental analysis methods (high-performance liquid chromatography methods—HPLC). The obtained statistical data were processed by the Statistics program (Microsoft Corporation, Redmond, WA, USA, 2006). Results: The study made it possible to make assumptions about the functional dependence of the iso-α-bitter resins and isoxanthohumol content in beer samples. Mathematical analysis indicate interactions between protein molecules and different malted grain and hop compounds are involved in beer structure, in contrast to dry hopped beer, where iso-a-bitter resins, protein, and coloring compounds were significant, with a lower coefficient of determination. The main role of rutin in the descriptor hop bitterness has been established in kettle beer hopping technology, and catechin in dry beer hopping technology, respectively. The important role of soluble nitrogen and β-glucan dextrins in the perception of sensory descriptors of various technologies’ beers, as well as phenolic compounds in relation to the formation of bitterness and astringency of beer of classical technology and cold hopping, has been shown. Conclusions: The obtained mathematical relationships allow predicting the resulting beer quality and also make it possible to create the desired flavor profiles.

Sensory evaluation, chemical structures, and threshold concentrations of bitter-tasting compounds in common foodstuffs derived from plants and maillard reaction: A review

The bitterness of foodstuffs is often associated with toxicity, which negatively influences product acceptability. However, bitter compounds have many benefits, and a slight bitter taste is sometimes favored. In this review, we summarize the methods used to isolate and evaluate the taste of bitter compounds in different foods. The chemical structures and threshold concentrations of these compounds are also recapped. Although the structures and thresholds of many bitter compounds have been confirmed, further studies are needed to develop detailed bitter-masking strategies and establish the relation between functional groups (hetero-cyclic substituents and bonding types) and taste quality. Furthermore, a comprehensive bitterness database and chemometric data must be provided in order to quickly assess the bitterness of unfamiliar products.

Radical Intermediates in the Degradation of Hop Acids

Radical formation in isohumulones was investigated under different types of stress, including temperature, transition metal ions, and hydrogen peroxide. Including dihydroisohumulones and tetrahydroisohumulones, as relevant analogues, allowed us to evaluate critical functionalities in radical formation. Using spin-trapping methodology with 5,5-dimethyl-1-pyrroline N-oxide and N-tert-butyl-α-phenylnitrone as relevant traps, followed by simulation of corresponding spin adducts, identification of incipient radicals was attempted. The isohexenoyl side chain in isohumulones, but not present in dihydro- and tetrahydroisohumulones, was most sensitive to radical formation. Kinetic profiles further demonstrated that radical formation in this moiety was accelerated in the presence of ferrous ions. Reactivity of parent six-membered-ring humulones in radical formation was different, as scavenging of free radical species was more important. Lupulones, despite similarity with humulones, showed a different behavior with an obvious radical decay pathway during ageing, mainly ascribed to radical formation on the ring structure. Quantification of final spin adducts allowed us to determine absolute importance of the different degradation pathways. Eventually, mechanisms are presented explaining why isohumulones are more prone to radical processes in (aut)oxidation and thermal decay than close relatives such as dihydroisohumulones.

Ultra-Micro-Scale-Fractionation (UMSF) as a Powerful Tool for Bioactive Molecules Discovery

Herein is detailed the development and validation of an ultra-micro-scale-fractionation (UMSF) technique for the discovery of plant-based, bioactive molecules, coupling the advantages of ultra-performance liquid chromatography mass spectrometry (UPLC-MS) separations with microtiter plate-based bioassay screens. This novel one-step approach simultaneously uses UPLC to collect chemical profile information, while performing high-resolution fractionation, greatly improving workflow compared to methods relying on high-performance liquid chromatography (HPLC), solid phase extraction or flash systems for chromatographic separations. Using the UMSF technique, researchers are able to utilize smaller quantities of starting materials, reduce solvent consumption during fractionation, reduce laborious solvent dry down times, replace costly single-use solid-phase-extraction cartridges with reusable analytical-sale UPLC columns, reduce fractionation times to less than 10 min, while simultaneously generating chemical profile data of active fractions and enjoying superior chromatographic resolution. Using this technique, individual bioactive components can be readily purified, identified, and bioassayed in one step from crude extracts, thereby eliminating ambiguous synergistic effects often reported in plant-based natural products research. A successful case-study is presented illustrating the versatility of this technique in identifying lupulone as the principal cytotoxic component from hops (Humulus lupulus L.), using a brine shrimp (Artemia franciscana) model. These results confirm and expand upon previous cell-based bioassay studies using a more complex, multicellular organism, and add to our understanding of structure-function activity relationships for secondary metabolites in hops and the Cannabaceae plant family.