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Why Humulinones are Key Bitter Constituents Only After Dry Hopping: Comparison With Other Belgian Styles
Abstract
Although long renowned worldwide for its unique dry-hopped (DH) Trappist beer, Belgium did not develop this process for other brands until the last decade. Twenty-one commercial Belgian DH beers were investigated and compared with a few other typical Belgian beers whose production involves either late hopping or aged hop addition (Gueuze). Bitterness was determined by spectrophotometric measurements (isooctane extraction) and by reversed phase high performance liquid chromatographic with UV detector (RP-HPLC-UV) (simultaneous quantitation of humulones, cis-/trans-isohumulones, reduced isohumulones, humulinones, and hulupones). In dry-hopped Belgian beers, humulinones (found at concentrations up to 13.3 mg/L) were estimated to be responsible for up to 28% of their bitterness. As humulinones revealed to be gradually lost through boiling (22%), clarification (5%), and fermentation (14%), non-dry-hopped (NDH) beers often displayed levels below 1.7 mg/L. Even in Gueuze beers for which old, humulinone-containing hops are used, no humulinone was found. Contrary to humulones, which were detected up to 7.2 mg/L in DH beers, hulupones were found at less than 3 mg/L in all Belgian beer styles. Humulinones were not produced in the boiling wort from humulones (in contrast to hulupones, readily synthesized from lupulones) but were significantly solubilized from hop thanks to their hydrophilicity. Yet, while the co-form accounted for about 50% of the humulones, the n-form prevailed for humulinones. Some humulinone degradation products were evidenced by RP-HPLC-MS/MS, and as suggested by their retention time (RT), should be more polar than their precursors. Bottle refermentation emerged as an additional critical step of humulinone loss, explaining the low levels found even in some strongly DH beers.
... The values found by the authors are higher than previously believed, which indicate that these compounds may have a much more important role than previously thought. Eventually, hulupones are oxidized to unbitter hulupic acid [3,4,6,[16][17][18][19][20][21][22]. ...
... However, even with this analysis, factors, such as content of alcohol, polyphenols, and essential oils, together with pH, carbonation level, and water chemistry, among others, alter the bitterness perceived by the consumer. So far, no internationally accepted method has been established that fully captures the actual level of bitterness experienced [21,23]. ...
... Thus, these compounds may largely account for the perceived bitterness among these beers. Ferreira et al. [21] suggest that humulinones may account for up to 28% of the bitterness in dry-hopped beers. Because hopping during the boil typically uses smaller amounts of hops than dry hopping, and these compounds are found in hops in small amounts, humulinones are only found in significant amounts in dryhopped beers. ...
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.
... Values of trans-isohumulones, below 0.5 mg/L (C, D and G), can only be obtained with a high boiling temperature or by using pre-isomerized hops [35] (Figure 5c). Tetrahydro-isohumulones (tetra, often used for foam stabilization) were detected at an average of 3.0 mg/L in half of the fresh samples (A, B, C, H, I, and J) while humulones remained under 1.5 mg/L (even undetectable in 7 NABLABs) in contrast to dry hopped beers, which displayed in a recent paper [41] concentrations between 1.1 and 7.2 mg/L. In the same way, as expected [41], cis-humulinones were found only as traces (0.2-0.9 mg/L) except in the three dry hopped samples (2.7-7.0 mg/L in B, C, and D) and beers E and K, for which aged hops has probably been used [42] (Figure 5d). ...
... Tetrahydro-isohumulones (tetra, often used for foam stabilization) were detected at an average of 3.0 mg/L in half of the fresh samples (A, B, C, H, I, and J) while humulones remained under 1.5 mg/L (even undetectable in 7 NABLABs) in contrast to dry hopped beers, which displayed in a recent paper [41] concentrations between 1.1 and 7.2 mg/L. In the same way, as expected [41], cis-humulinones were found only as traces (0.2-0.9 mg/L) except in the three dry hopped samples (2.7-7.0 mg/L in B, C, and D) and beers E and K, for which aged hops has probably been used [42] (Figure 5d). Hulupones, which can either be produced from hop lupulones during wort boiling or solubilized from hops [41], also remained under 1.7 mg/L. ...
... In the same way, as expected [41], cis-humulinones were found only as traces (0.2-0.9 mg/L) except in the three dry hopped samples (2.7-7.0 mg/L in B, C, and D) and beers E and K, for which aged hops has probably been used [42] (Figure 5d). Hulupones, which can either be produced from hop lupulones during wort boiling or solubilized from hops [41], also remained under 1.7 mg/L. The BU values significantly dropped during storage (only 39% remaining in beer G) ( Table 2). ...
Nowadays, non-alcoholic (NAB) and low-alcoholic beers (LAB) still significantly suffer from staling defects when fresh, partially due to absence of ethanol as antioxidant. In the current work, the fate of flavan-3-ols (monomers, dimers, and trimers) and bitter compounds (isohumulones, humulinones, etc.) of 11 commercial NABLABs available on the Belgian market was monitored through one year of aging at 20 °C in the dark. Fresh NABLABs contained variable flavan-3-ols and bitter compounds levels (between 3.0–10.0 mg/L and 8.0–39.0 mg/L, respectively), depending on different technological processes used. Chill haze and color were also investigated as potential oxidation markers of fresh and aged beers. Surprisingly, contrary to conventional beers, the oligomers’ concentration (dimer and trimer procyanidins) exhibited a strong correlation (R2 = 0.95) with chill haze before aging, suggesting prematured oxidation of the samples. After a year of storage, significant degradation occurred as for regular dry hopped beers (process very sensitive to oxidation), only 27% remaining for flavan-3-ol dimers and an average 16% for trans-isohumulones. Oxidation risk appears here as the main weakness of NABLABs, which could be probably improved by spiking very efficient antioxidants.
... Such difference may be related to the ingredients and brewing process employed by different monasteries and manufacturers. For example, humulinones, a group of humulone derivatives originating from the hop soft resins, are found at concentrations of up to 13.3 mg/L in some Belgian unique dry-hopped Trappist beers, and are estimated to be responsible for up to 28% of their bitterness (Ferreira et al., 2018). While for other typical nondry-hopped Belgian beers, the contents of humulinones are often below 1.7 mg/L (Ferreira et al., 2018). ...
... For example, humulinones, a group of humulone derivatives originating from the hop soft resins, are found at concentrations of up to 13.3 mg/L in some Belgian unique dry-hopped Trappist beers, and are estimated to be responsible for up to 28% of their bitterness (Ferreira et al., 2018). While for other typical nondry-hopped Belgian beers, the contents of humulinones are often below 1.7 mg/L (Ferreira et al., 2018). Several exceptional samples that fall in the region of the opposite chemical composition of beer samples and the strong resemblance between the Trappist and non-Trappist beers, only a little information can be found about the actual difference between the two classes. ...
... These Belgian monastic style ales possess quite similar appearances and fluorescent properties to that of certified Trappist beers, and hence are rather difficult to be distinguished solely by fluorescence spectroscopy. It has been revealed that some Abbey beers, which also adopt the same dry hopping process as the typical Trappist beers, contain similar levels of humulinones and bitterness (Ferreira et al., 2018). ...
Front-face synchronous fluorescence spectroscopy (FFSFS) was applied for the rapid and noninvasive recognition of Belgian and Netherlandish Trappist beers against non-Trappist beers. The front-face synchronous fluorescence spectra at wavelength intervals (??) of 30 and 60 nm for 80 bottles of beer, including 41 Trappist and 39 non-Trap-pist beers, were acquired in a 5 × 10 mm fused-quartz cuvette settled in a traditional right-angle sample compartment. The discrimination model was constructed by either principal component analysis (PCA) combined with linear discriminant analysis (LDA) or partial least squares-discriminant analysis (PLS-DA). Both PCA–LDA and PLS-DA models were validated by full (leave-one-out) cross-validation and k-fold cross-validation (k = 5). The PCA–LDA model presents reliable discrimination performance, with the cross-validated sensitivity (true positive rate) and specificity (true negative rate) in the range of 82.9–85.4% and 71.8–76.9%, respectively. The misclassification mainly occurs to a small portion of ambiguous Trappist and non-Trappist samples such as Abbey beers, which are rather similar to Trappist beers.
... The cis and trans co-and n-isohumulones and tetrahydroisohumulones were analyzed in red and brown Flemish beers by HPLC-UV. [18] Beer samples devoid of yeast were degassed by shaking and diluted twice in methanol. After 15 min, the mixture was filtered through a Chromatil polyester filter (0.45 µm, Macherey-Nagel, Düren, Germany). ...
... Similar measurements recently performed on three gueuze beers have shown the total absence of isohumulones in them. [18] The results for red/brown ales confirmed the low bitterness of these acidic beers, which was not surprising given the choice of old hops mainly to limit Gram + lactic bacteria during fermentation. In LG, 1.5 mg/L tetrahydroisohumulones (eluting at 28.4, 30.8 , and 31.6 min) had been added, most probably to improve its foam stability. ...
Red and brown Flemish sour beers form a distinct class of Belgian beers obtained by mixed (yeast/lactic bacteria) microbial fermentation and often resulting from blending a 1-to-2-year-old beer with a younger one to obtain a balance between acidic character and sweetness. A detailed composition in volatiles (phenols, lactones, esters, alcohols, acids, …) of three beers representative of the red and brown subcategories is presented. GC data were obtained after different extraction procedures, including solvent-assisted flavor evaporation (SAFE) and headspace. The first results showed the influence of Brettanomyces yeast on the phenol and ester contents. An efficient Brettanomyces activity in the red sour beers (especially in Rodenbach Vintage) was observed, favored by long maturation in wooden casks. This was organoleptically perceived by the horsey flavors brought by 4-ethylguaiacol and 4-ethylphenol, and the solvent-like ethyl acetate through esterase activity. The brown Flemish sour beer (produced in stainless steel fermenters) showed significantly more unreduced 4-vinylguaiacol and 4-vinylphenol, although traces of 4-ethylguaiacol and 4-ethylphenol were also detected (most probably here issued from torrefied malts, as suggested by the opposite substituted phenol/guaiacol ratio).
... Broadly, dry hopping is defined as the cold extraction (4-20°C) of nonvolatile and volatile chemicals from hops into an alcoholic solution (Fig. 2), thereby avoiding isohumulone formation (23). However, dry hopping has been shown to modify beer bitterness, and recently several studies have identified humulinones (oxidized -acids) as important drivers of beer bitterness in dry-hopped beers (7,(24)(25)(26). Overall, these "aroma" hop additions allow brewers to impart hop aroma while reducing the hop's bitter contribution. ...
... Although the main function of dry hopping is to add aroma to beer, a lot of unintended changes occur in hoppy beers due to the nonvolatile extractions at the extreme hopping rates that brewers are using. Although isohumulones have been identified as the main driver of bitterness in kettle-hopped beers, the oxidation products of humulones (called humulinones) have been identified as important drivers of bitterness in dry-hopped beers (7,25,26). Humulinones have been shown to be ~2/3 as bitter as isohumulones (24) and are present in freshly baled, aged, or improperly stored hops at varying levels (0.1-0.5 w/w%) ( Fig. 1) (25,37). Even though the mechanism of humulinone formation is still unknown, the oxidation of humulones in aerobic storage can occur as quickly as a few days and is likely to be the main driver of humulinone formation (25). ...
The considerations that constitute hop quality have been changing in recent years as brewers use greater and greater quantities of hops for aroma additions to increase aroma perception while striving to keep total bitterness low and consistent. Several analytical drivers of overall hop aroma intensity and quality have been identified in various studies, but these drivers are often specific to varieties and/or hopping technique. In fact, today's high dry-hopping rates lead to a lot of unintended consequences in beer flavor and stability due to the extraction of nonvolatile components from hops. Decisions involving hop quality should be aligned with the type of hop additions brewers use to impart aroma and flavor to beer. As more is learned about these analytical drivers of dry-hop beer quality from both bitterness and aroma perspectives, brewers can use this information to produce more consistent and sustainable hop-forward beers.
... For a long time, dry hopping was not considered to contribute to bitterness, since no isomerisation of alpha acids occurs. However, this idea has been revisited recently by researchers who have investigated the influence of dry hopping on beer bitterness [4][5][6]. For example, Parkin and Shellhammer [4] reported that bitterness in dry hopped beers is also derived from humulinones and polyphenols. ...
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.
... According to Algazzali et al. [8] it seems that humulinones are almost 66% as bitter as isoalpha-acids contributing significantly to the beer bitterness profile. Ferreira et al. [10] concluded that after dry hopping, humulinones were responsible for up to 28% of beer bitterness, being a good marker for the quality of beer. Their effect is less significant for the beer obtained through the traditional hoping process due to the fact that these components are highly degraded during boiling and fermentation when they are absorbed into yeast. ...
The production of heavily hopped beers, such as Indian Pale Ale (IPA) styles, has been gaining momentum in recent years in the Central European markets. To this end, the dry hopping process is becoming increasingly popular, mostly in microbreweries, but also with larger manufacturers. In our research, we investigated the dissolution rate of the main volatile component of hops, β-myrcene with a modified dry hopping method. Following the primary fermentation, we applied the dry hopping process, where the weighed hops were chopped and blended into a container with 0.5 L of beer and later added to the young beer. During the dry hopping process, we determined various important parameters of the beer, and we repeated the same measurements for the bottled beer. In the first 96 h of the dry hopping process, we monitored the concentration of β-myrcene so that we managed to determine the dissolution rate constant (k = 0.1946 h-1). The β-myrcene concentration stabilizes after 44 h in the fermenter. At the same time, measurements were conducted for bitterness, pH, CO2 and alcohol content, extract and density during the process. Our experiment demonstrates that a new method of dry hopping provides a much higher concentration of β-myrcene (215 μg/L) than other methods indicated in former studies in the field. A health and safety assessment of β-myrcene was also made and we determined what the safe amount of β-myrcene ingested with IPA beer is. Our modified process was successful, we were able to determine the dissolution rate of β-myrcene, and the recommended daily intake of IPA beer with particular reference to β-myrcene.
... These metabolites are essential oils (mostly terpenes, sesquiterpenes and their oxygenated analogous, terpenoids and sesquiterpenoids), phenolic compounds, alpha-acids and beta-acids (Cattoor et al., 2013). Hop cones are used in beer as it provides components that improve microbiological stability, foam, aromas, flavor and bitterness (Schmidt & Biendl, 2017;Silva Ferreira et al., 2018). Beer bitterness comes mostly from the isomerization reaction of hop alpha acids (AA); these molecules have low solubility in water and during the boiling process they are converted into iso-alpha acids (IAA), a soluble molecule that provides the characteristic beer bitterness (Kunze, 2004). ...
highly hopped and dry hopped beer production and consumption has increased steadily for the last twenty years, following the rise of craft breweries in the United States of America and the trend they set over the world. This overuse of hops offers some environmental, technological, and economic challenges for the beer industry. Researchers have been studying several ways to make beer more ecofriendly by promoting reuse of spent hops and increasing extraction yields. Dry hopping is a size-dependent process, making it feasible for craft breweries with their small scale production, but quite a challenge for larger tanks and breweries. Based on the literature and industrial experience from brewers, the aim of this research was to analyze and discuss different dry hopping methodologies for the brewing industry and how the parameters affect the final product. In order to shed light on this trending topic and to better aid brewers in choosing the most suitable, efficient, and environmental-friendly dry hopping process for their brewery, this work approaches the main variables that promote aroma transfer from hops into beer and how to optimize it.
... The Dalgliesh plot [9] describes aromatic changes occurring in lager beers during storage. A linear decrease in bitterness (degradation of isohumulones and/or humulinones) coincides with an increase in sweet aroma and toffee flavor, together with the well-known cardboard taint (caused by trans-2-nonenal) [10][11][12][13] and ribes off-flavor (a catty smell linked to the presence of 3-sulfanyl-3-methylbutyl formate) [14,15]. Aging of specialty beers is even more complex, with defects such as Madeira off-flavor [16], phenolic perception [17], a change in hoppy aromas [18], and a detected ether taint [19]. ...
The aromatic complexity of craft beers, together with some particular practices (use of small vessels, dry hopping, etc.), can cause more oxidation associated with pre-maturated colloidal instability, Madeira off-flavors, bitterness decrease, and aroma loss. As bottle refermentation is widely used in Belgian craft beers, the aim of the present work is to assess how this practice might impact their flavor. In fresh beers, key flavors were evidenced by four complementary techniques: short-chain fatty acids determination, esters analysis, XAD-2 extract olfactometry, and overall sensory analysis. In almost all of the fresh beers, isovaleric acid was the sole fatty acid found above its sensory threshold. Selected samples were further analyzed through natural aging at 20 °C. The presence of yeast in the bottle minimized the trans-2-nonenal released from Schiff bases and proved less deleterious than suggested by previous studies with regard to fatty acid release and ester decrease through aging. Furthermore, according to the yeast species selected, some interesting terpenols and phenols were produced from glucosides during storage.
... Broadly, dry hopping is defined as the cold extraction (4-20°C) of nonvolatile and volatile chemicals from hops into an alcoholic solution (Fig. 2), thereby avoiding isohumulone formation (36). However, dry hopping has been shown to modify beer bitterness, and recently several studies have identified humulinones (oxidized -acids) as important drivers of beer bitterness in dry-hopped beers (3,(37)(38)(39). Overall, these "aroma" hop additions allow brewers to impart hop aroma while reducing the hop's bitter contribution. ...
Recently, the evolution in the production of hop-forward beer styles has brewers reexamining their hop usage throughout the brewing process and is resulting in a coincidental change in the definition of hop quality. The increased use of aroma hop additions and the demand for aroma hop varieties have shifted brewers' focus toward hop oil content and composition instead of, or in addition to, organoleptic properties and increased humu-lone (alpha-acid) content. This focus on new aspects of hop quality has led to a reexamination of the agricultural and processing methods that potentially impact aroma hop quality. It is essential for hop quality parameters to be based on the planned use of hops throughout the brewing process (i.e., kettle , late, or dry hopping) but also constrained by realistic expectations of what growers can achieve given the relatively tight harvest window for hops, the ever-changing demands by brewers for unique varieties, and climatic challenges that hop farmers face from year to year.
... Although, iso-humolones have been identified as the main driver of bitterness in "kettle-hopped" beers, the oxidation products of humulones (called humulinones) have been identified as important drivers of bitterness in dry-hopped beers. 3,22,123 Humulinones have been shown to be ~2/3 as bitter as isohumulones 124 and are present in freshly baled, aged, or improperly stored hops at varying levels (0.1-0.5 w/w %) ( Figure 3) 22,54 . Even though the mechanism of humulinone formation is still unknown, the oxidation of humulones in aerobic storage can occur as quickly as a few days and is likely to be the main driver of humulinone formation. ...
Please use this link for free access to my PhD dissertation: https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/kk91fr75p
Abstract: Dry-hopping is the addition of hops to the wort on the cold side of the brewing process. Unlike standard hop additions, its main purpose is not to produce a characteristic bitterness but to extract as much of the hop essential oils as possible, which are largely lost in the standard hopping process. When dry-hopped, it is possible to obtain a beer with an aroma that is difficult to achieve when hops are used on the hot side of the brewing process. As a result, this process has become very popular in recent years, particularly in beers that belong to the ‘craft beer revolution’ trend. In addition, the usefulness of this process is increasing with the development of new hop varieties with unique aromas. This article presents the main components of hops, focusing on those extracted during the process. Changes in the composition of beer bittering compounds and essential oils resulting from this process are discussed. This paper presents the current state of the knowledge on the factors affecting the degree of extraction, such as hop dosage, the time, and temperature of the process. Issues such as process-related physicochemical changes, hop creep, low flavor stability, haze formation, and green flavor are also discussed.
Fluorescence spectroscopy combined with parallel factor analysis (PARAFAC) has successfully been applied for the analysis of food and beverages containing numerous autofluorescent compounds. For the decomposition of such data, it is crucial to establish the PARAFAC model complexity. This is not a trivial matter, especially when the sample complexity increases. Diagnostics are available for assisting the choice of the number of PARAFAC components, such as the core consistency. In this short communication, we show that when it comes to real (complex) data, the core consistency is too conservative and other diagnostic tools must be taken into account. We emphasize that it is imperative to inspect the PARAFAC excitation and emission loadings and assess whether these are chemically meaningful.
Hop is a multifunctional specie; however, a large part of its production is destined for the beer market. In Brazil, all hop demand is imported, which has aroused interest in national production. Genetic material and cultivation systems can influence hops production. Thus, this study evaluated morphological and productive performance of hop varieties grown under organic and conventional management in the central-west region of São Paulo, Brazil. The hop yard was installed in the experimental area of the FCA/UNESP (São Paulo State University, School of Agriculture) - Botucatu, in November 2018, the data were collected in second year of production. A randomized block design was adopted, in 2 x 5 Split-Plot, being the main factor: cultivation systems (organic and conventional), and secondary factor: hop varieties (Columbus, Chinook, Nugget, Cascade, and HallertauMittelfrüeh), with four blocks and four plants per plot. Crop-specific morphological and productive parameters were evaluated. The results showed no significant influence of cultivation systems in morphological parameters. Regarding the varieties, Chinook, Cascade, and Nugget stand out for cone length. Columbus, Chinook and Cascade showed higher yields when grown in organic system, obtaining 1100.66, 1088.27 and 940.40 g of cones per plant, respectively. While, HallertauMittelfrüeh variety was the least productive in both systems, in organic system with 160.50 g per plant production, and 267.84 g per plant in conventional system, and, also showing lower cycle compared to the others.
Dry hopping imparts distinct aromas but also a series of non-volatile compounds suspected of causing flavor and physical instability during beer storage. In this work, color, chill haze, total polyphenols, total flavanoids, and flavan-3-ol monomers (catechin and epicatechin) and oligomers (procyanidin dimers and trimers) were monitored in five commercial pale-colored Belgian dry-hopped beers over 24 months of storage at 20 °C in the dark. Fresh dry-hopped beers contained unusually high levels of flavan-3-ol monomers (up to 6.6 mg/L) and oligomers (up to 14.1 and 10.2 mg/L dimers and trimers, respectively). The increase in color intensity during storage (up to 6.4°EBC) correlated with fresh beer monomer levels, while the oligomer content correlated with chill haze formation (up to 25.7°EBC). The evolution of these two physical attributes also correlated with the level of total polyphenols in the fresh beers. In a pilot-scale production, kettle hopping was shown to impart either monomers (early) or oligomers (late), while dry hopping promoted efficient extraction of both monomers and dimers (extraction yields of 62 and 74%, respectively). Dry hopping thus plays an important role in color and chill haze increase.
The aim of the present work was to compare levels of short chain fatty acids, esters, terpenoids and polyfunctional thiols in (mostly bottle-refermented) commercial Belgian dry-hopped beers before and after 2 years of storage at 20 °C (the usual best-before date in Belgium). Among the hop-derived volatiles, the terpenoids linalool and geraniol, the polyfunctional thiols 3SHol, 3SHA and 3S4MPol, and the esters ethyl isobutyrate, ethyl isovalerate and ethyl heptanoate (up to 499, 53, 0.2, 2, 3, 84, 63, and 19 µg/L, respectively) were found above their sensory thresholds in most fresh dry-hopped beers. The fermentation-derived esters reached concentrations similar to those previously reported for non-dry-hopped beers, with ethyl hexanoate and isoamyl acetate (up to 0.4 and 3.9 mg/L, respectively) often above their sensory thresholds. Except ethyl isovalerate (more than 85% still present), most hop odorants and fermentation esters showed degradation over the 2-year storage period: only 45%–70% of linalool, geraniol, and ethyl hexanoate and even less than 40% for polyfunctional thiols, ethyl isobutyrate, and ethyl heptanoate initial concentrations were detected after storage. How the dry-hopping process affects this degradation was further investigated in model media. Fermentation esters proved to be more strongly impacted in dry-hopped than in non-dry-hopped beers because of hop esterase activity. In addition to being aware of the need to avoid hop esterases, craft brewers are here advised to use bottle refermentation for its ability to regenerate some flavors and consume packaged oxygen. No deleterious effect of yeast, such as short chain fatty acid excretion, was evidenced.
The evolution of isohumulones, main bitter compound of beers, through storage and its bitter impact has been studied for many years. With the uprising of highly hopped and, especially, dry-hopped beers during the last decade, other compounds such as humulones, cis-humulinones and hulupones have been shown to participate in both the sensory and measured bitterness of these beers. However, the fate of these compounds through aging is still unclear. The present work evaluated, by means of spectrophotometric and RP-HPLC-UV analyses, the evolution of bitterness units (BU), cis- and trans-isohumulones, humulones, cis-humulinones, hulupones, and tetrahydro-isohumulones in 21 Belgian dry-hopped beers after two years of storage. Besides the expected degradation of trans-isohumulones, it was observed an average decrease of 91% of humulones and 73% of cis-humulinones. Together, these results indicate a decrease of 18 – 43% of the calculated bitterness intensity of dry-hopped Belgian beers. The losses of cis-humulinones in dry-hopped beers bring seven new oxidation compounds, some of them very similar to those issued from trans-isohumulones. The bitterness impact of these compounds is yet to be determined.
Background
Hops are mostly processed for the extraction of compounds conferring aroma and bitterness to beer. A more controllable addition of these properties can be achieved when solvent hop extracts are produced and added in the desired brewing stage.
Scope and approach
This review presents a survey on the research and patents on emerging extraction technologies and the importance of the solvent and operational conditions on the composition of hop extracts, destined for the brewing industry, but also for the production of novel ingredients with potential for nutraceuticals.
Key findings and conclusions
For the preparation of extracts, the efficiency of conventional solvents has been progressively improved by the assistance with intensified technologies and particularly by the use of greener solvents, a trend where this industry has been pioneer. The design of multistage multiproduct processes for the integral utilization of hops following a biorefinery approach is also addressed.
Humulinone and hulupone extracts were prepared for evaluating their bitterness intensity in beer. Previously established methods for oxidizing α-and β-Acids followed by preparative liquid chromatography resulted in humulinone and hulupone extracts of 93.5 and 92.7% purity, respectively, as measured by HPLC. The humulinone and hulupone extracts were dosed into unhopped lager over a range of concentrations from 8 to 40 mg/L. Similarly, purified iso-α-Acids were dosed into unhopped beer at concentrations ranging from 6 to 30 mg/L. A nine-member trained panel scaled the bitterness intensity of all samples in five replicated testing sessions. Humulinones were found to be 66% as bitter as isoα-Acids (±13%), and hulupones were found to be 84% as bitter as iso-α-Acids (±10%). The results of this study suggest that humulinones and hulupones are more bitter than was previously suspected.
The sensory bitterness intensity of the major hop bitter acids has been shown to vary. The cis- and trans-isomers of isocohumulone and isohumulone were isolated and purified by preparative high-performance liquid chromatography. These compounds were tested by seven tasters over a concentration range of 16.6-166 μM in a model phosphate-buffered system and compared with quinine sulfate (13.1-131 μM). More than 1,200 two-glass taste tests were carried out. Over these concentration ranges, the sensory attributes of these substances obeyed the Stevens' psychophysical power law. cis-Isohumulone was the most bitter hop acid studied while trans-isocohumulone was the least bitter. Further testing in an unhopped beer by six tasters demonstrated similar differences in the bitterness intensities of hop bitter acids. These differences were partially masked by the background flavor of the beer.
Hop aroma components, which mainly comprise terpenoids, contribute to the character of beers. However, pretreatments are necessary before analyzing these components because of their trace levels and complicated matrixes. Here, the stir bar-sorptive extraction (SBSE) method was used to detect and quantify many terpenoids simultaneously from small samples. This simple technique showed low coefficients of variation, high accuracy, and low detection limits. An investigation of the behavior of terpenoids identified two distinct patterns of decreasing concentration during wort boiling. The first, which was seen in myrcene and linalool, involved a rapid decrease that was best fitted by a quadratic curve. The second, which was observed in beta-eudesmol, humulene, humulene epoxide I, beta-farnesene, caryophyllene, and geraniol, involved a gentle linear decrease. Conversely, the concentration of beta-damascenone increased after boiling. As the aroma composition depended on the hop variety, we also examined the relationship between terpenoid content and sensory analysis in beer.
In this article, a detailed study on hop alpha-acid isomerization kinetics is presented. Because of the complex wort matrix and interfering interactions occurring during real wort boiling (i.e., trub formation and alpha-acids/iso-alpha-acids complexation), this investigation on alpha-acid isomerization kinetics was performed in aqueous buffer solution as a function of time (0-90 min) and heating temperature (80-100 degrees C). Rate constants and activation energies for the formation of individual iso-alpha-acids were determined. It was found that iso-alpha-acid formation follows first-order kinetics and Arrhenius behavior. Differences in activation energies for the formation of trans- and cis-isomers were noticed, the activation energy for the formation of trans-iso-alpha-acids being approximately 9 kJmol (-1) lower.
The range of different nonvolatile constituents extracted from hops in highly hopped beers suggests that isohumulones may not be the sole contributor to beers' bitterness. Among brewers producing hop-forward beer styles there is concern that the Bitterness Unit (BU) is no longer an accurate predictor of beer bitterness. This study examined factors within the beer matrix that influence sensory bitterness perception in highly hopped beers. Over 120 commercial beers were evaluated using sensory and instrumental techniques. Chemical analysis consisted of the BU via spectrophotometry, hop acids via HPLC, total polyphenols via spectrophotometry, and alcohol content plus real extract via an Alcolyzer. Sensory analysis was conducted over two studies, and the beers' overall bitterness intensity were rated using a 0-20 scale. This study identified that the BU measurement predicts sensory bitterness with a nonlinear response, and it proposed an alternative approach to predicting bitterness based on isohumulones, humulinones, and ethanol concentrations. The study also revealed the importance of oxidized hop acids, humulinones, as a significant contributor to beer bitterness intensity.
The dual-purpose hop varieties Amarillo, Citra, Hallertau Blanc, Mosaic, and Sorachi Ace were recently shown to contain unusually high amounts of some discriminating terpenoids, polyfunctional thiols, and precursors of the latter (cysteine and glutathione adducts). The present work aimed to investigate the terpenol glucoside fraction in hops and its potential contribution to beer after a dry hopping process. Terpenols were quantified by stir-bar sorptive extraction GC-MS in five pilot monovarietal dry-hopped beers. In all of them, linalool and geraniol were found above their sensory thresholds (72-178 and 7-57 μg/L, respectively, for a threshold of 8 μg/L for linalool and 4 μg/L for geraniol). β-Citronellol also exceeded its threshold when the Amarillo, Citra, or Sorachi Ace cultivars were used. The hop glucoside potential was analyzed by GC-MS after enzymatic degradation. A relative hydrolysis efficiency factor was applied to our data to take into account that the commercial P-glucosidase releases octan-1-ol, used here as an internal standard, 2.8 times more efficiently than geraniol. β-Glucosidase treatment caused the release of linalool, α-terpineol, β-citronellol, and geraniol from all five dual-purpose cultivars, but in much lower amounts than the corresponding free terpenols (0.6-28.6 mg/kg of aglycons versus 7.8-109.2 mg/kg of free forms). Further quantitative analyses focusing on more traditional aromatic and bitter hops are now needed to compare their glucoside fractions with those here investigated.
Monovarietal dry-hopped beers were produced with the dual-purpose hop cultivars Amarillo, Hallertau Blanc, and Mosaic. The grapefruit-like 3-sulfanyl-4-methylpentan-1-ol was found in all three beers at concentrations much higher than expected on the basis of the free thiol content in hop. Even cysteinylated precursors proved unable to explain our results. As observed in wine, the occurrence of S-glutathione precursors was therefore suspected in hop. The analytical standards of S-3-(4-methyl-1-hydroxypentyl)glutathione, never described before, and of S-3-(1-hydroxyhexyl)glutathione, previously evidenced in grapes, were chemically synthesized. An optimized extraction of glutathionylated precursors was then applied to Amarillo, Hallertau Blanc, and Mosaic hop samples. HPLC-ESI(+)MS/MS revealed, for the first time, the occurrence of S-3-(1-hydroxyhexyl)glutathione and S-3-(4-methyl-1-hydroxypentyl)glutathione in hop, at levels well above those reported for their cysteinylated counterparts. S-3-(1-Hydroxyhexyl)glutathione emerged in all cases as the major adduct in hop. Yet, although 3-sulfanylhexan-1-ol seems relatively ubiquitous in free, cysteinylated, and glutathionylated forms, the glutathione adduct of 3-sulfanyl-4-methylpentan-1-ol, never evidenced in other plants up to now, was found only in the Hallertau Blanc variety.
Brewing Materials and Processes: A Practical Approach to Beer Excellence presents a novel methodology on what goes into beer and the results of the process. From adjuncts to yeast, and from foam to chemometrics, this unique approach puts quality at its foundation, revealing how the right combination builds to a great beer. Based on years of both academic and industrial research and application, the book includes contributions from around the world with a shared focus on quality assurance and control. Each chapter addresses the measurement tools and approaches available, along with the nature and significance of the specifications applied. In its entirety, the book represents a comprehensive description on how to address quality performance in brewing operations. Understanding how the grain, hops, water, gases, worts, and other contributing elements establish the framework for quality is the core of ultimate quality achievement. The book is ideal for users in corporate R&D, researchers, students, highly-skilled small-scale brewers, and those seeking an understanding on how the parts impact the whole in beer production, providing them with an ideal companion to complement Beer: A Quality Perspective. Focuses on the practical approach to delivering beer quality, beginning with raw ingredients. Includes an analytical perspective for each element, giving the reader insights into its role and impact on overall quality. Provides a hands-on reference work for daily use. Presents an essential volume in brewing education that addresses areas only lightly covered elsewhere.
Thirty-four commercial lager beers were analysed for their hop bitter acid, phenolic acid and polyphenol contents. Based on analytical data, it was evident that the beers had been produced using a range of different raw materials and hopping practices. Principal Components Analysis was used to select a sub-set of 10 beers that contained diverse concentrations of the analysed bitter compounds. These beers were appraised sensorially to determine the impacts of varying hop acid and polyphenolic profiles on perceived bitterness character. Beers high in polyphenol and hop acid contents were perceived as having ‘harsh’ and ‘progressive’ bitterness, whilst beers that had evidently been conventionally hopped were ‘sharp’ and ‘instant’ in their bitterness. Beers containing light-stable hop products (tetrahydro-iso-α-acids) were perceived as ‘diminishing’, ‘rounded’ and ‘acidic’ in bitterness. The hopping strategy adopted by brewers impacts on the nature, temporal profile and intensity of bitterness perception in beer.
Free terpenoids and both free and bound polyfunctional thiols were investigated in five selected dual-purpose hop cultivars. Surprisingly, the dual-purpose Sorachi Ace variety was found to contain higher amounts of farnesene (2101 mg/kg) than aromatic hops such as Saaz but only traces of 3-methylbutylisobutyrate, a compound that usually distinguishes all bitter varieties. All five cultivars investigated here showed an exceptional citrus-like potential explained by either monoterpenic alcohols or polyfunctional thiols. Among the monoterpenic alcohols, β-citronellol at concentrations above 7 mg/kg distinguished Amarillo, Citra, Hallertau Blanc, Mosaic, and Sorachi Ace from Nelson Sauvin and Tomahawk, two previously investigated dual-purpose hops, while linalool (312 mg/kg) and geraniol (211 mg/kg) remained good discriminating compounds for Nelson Sauvin and Tomahawk, respectively. Regarding polyfunctional thiols, higher amounts of 3-sulfanylhexyl acetate (27 μg/kg) characterized the Citra variety. Free 4-sulfanyl-4-methylpentan-2-one proved discriminant for Sorachi Ace, while the bound form is predominant in Nelson Sauvin. On the other hand, an S-conjugate of 3-sulfanylhexan-1-ol was found in Sorachi Ace at levels not far from those previously reported for Cascade, although the free form was undetected here. Both free and bound grapefruit-like 3-sulfanyl-4-methylpentan-1-ol (never evidenced before the present work) emerged as discriminating compounds for the Hallertau Blanc variety. The apotryptophanase assay also allowed us to evidence for the first time an S-conjugate of 2-sulfanylethan-1-ol.
A procedure for extracting hop acids from beer or wort was developed. A solid-phase extraction procedure employing a 3.0-ml Bakerbond spe* octyl column was used to absorb the hop compounds. After the concen-trated hop compounds were washed, they were desorbed with methanol. UV measurement of either international bitterness units or high-per-formance liquid chromatography (HPLC) was used to determine the amount of hop acids present. UV analysis could be correlated with the ASBC-recommended procedure for bitterness units. HPLC analysis al-lowed quantitation of hulupones, humulinic acid, isohumulones, humu-lones, lupulones, p-isohumulones, and tetrahydro-isohumulones. Recov-ery of isohumulones is better than 90%, and recovery of other specific hop compounds is better than 75%. A necessary first step in the analysis of hop bitter acids is their separation from the many interfering compounds naturally present in the beer or wort matrix (2). Liquid-liquid extraction procedures have been the mainstay for this separation but usually are cumbersome and require large volumes of solvent (5). Solid-phase extraction procedures have gained in popularity over the last 10 years and have, to a great extent, replaced the conventional liquid-liquid extraction procedures (6). The Institute of Brewing, in a recently completed collaborative study on the estimation of iso-a-acids in beer by high-performance liquid chromatography (HPLC), used an octadecyl solid-phase extraction column (200 mg X 3 ml) for sample preparation. Although the results obtained by the HPLC method were not precise enough to permit adoption as a recommended method, the HPLC method was suggested as an alternative method to the bitterness units procedure because of its advantage in measuring bitterness in terms of iso-a-acid content (4). The purpose of this research was to develop an improved extraction procedure for the analysis of hop bittering compounds. The octyl column was tested for its suitability for this extraction.
A procedure relying on high performance liquid chromatography for the estimation of iso-alpha-acids in beer has been collaboratively tested by the IOB Analysis Committee. In addition the trial samples were analysed by the IOB Recommended Method for the measurement of bitterness (BU). It was judged that the results obtained by the HPLC method were not sufficiently precise to permit adoption as a Recommended Method. However, since the method has the advantage of measuring bitterness in terms of iso-alpha-acids, it is suggested as an alternative to that of the Recommended Method. The iso-alpha-acids are absorbed from beer on to a C18 Bond Elut column and then selectively desorbed prior to isocratic analysis by HPLC using an eluting solvent of methanol/water/phosphoric acid/tetrabutylammonium hydroxide and a C18 radialpak cartridge. For both methods the repeatability values (r95) were not dependent upon mean concentration (m) whereas the reproducibility values (R95) were dependent upon concentration. The values of (r53) and (R95) obtained were 2.11 and (1.38 + 0.134 m) over the concentration range 13.8 to 34.0 mg/litre for the HPLC procedure and 1.20 and (0.76 + 0.122 m) over the concentration range 15.4 to 38.6 BU for the Recommended Method.
The iso-α-acids and their chemically-modified variants play a disproportionately large role in the final quality of beer. Here, fundamental aspects of two of these quality issues — foam and bitterness — are discussed. A common feature of both issues is the dependence on the hydrophobic character of the hop compounds on both bitterness potency and ability to stabilise foams. Thus the isocohumulones appear significantly less bitter than the other, more hydrophobic hop compounds. Also apparent were the differences in bitterness between the cis- and the trans-isomers, with the former being the more potent. Also described are the differences in the partitioning of the cis- and trans-iso-α-acids into beer foam. The trans-isomers are enriched in foam relative to their cis-counterparts and may account for the observed enrichment of cis-isomers in the final beer relative to the common ratios observed upstream.
It is recommended that the bitterness of beer be estimated by a modified Brenner method and that, as from 1st September, 1968, the results be expressed in E.B.C. bitterness units.
Although hop technology has been a substantial part of brewing science for the last 130 years, we are still far from claiming to know everything about hops. As hops are considered primarily as a flavour ingredient for beer, with the added benefit of having anti-microbial effects, hop research is focused on hops as a bittering agent, as an aroma contributor and as a preservative. Newer fields in hop research are directed toward the relevance of hops in flavour stability, brewing process utilisation, the technological benefits of hops in brewing as well as hops as a source of various substances with many health benefits. However the more we find out about the so-called “spirit of beer” the more questions emerge that demand answers. While hop research was only an ancillary research field for decades, during the last ten years more universities and breweries have determined that hops must play a meaningful role in their research efforts. This article gives an overview of the up-to-date knowledge on hop aroma, hop derived bitterness, and the role of hops in flavour stability as well as light stability. Hop research is a wide field, therefore in this review only selected topics are reviewed. Other research areas such as hops utilisation, the antifoam potential of hops, or the advances in knowledge pertaining to the physiological valuable substances of hops go beyond the scope of this article.
Rapid methods for the determination of the sum of isohumulone, isocohumulone and isoadhumulone in bear have been developed. Rapid Method I consists essentially of the extraction of the iso-compounds from acidified beer with a measured quantity of 2,2,4-trimethylpentane followed by spectrophotometric measurement of the concentration of iso-compounds in an aliquot of the extract. The apparent values given by this method are approximately 30% higher than countercurrent distribution values, owing to background absorption, but can be reduced to real values by means of a regression equation obtained from the relationship between the values obtained by the countercurrent and rapid methods. This procedure should be adequate for most routine purposes. Where higher accuracy is desirable, Rapid Method II may be used. This is basically the same as Method I, but employs a phase separation to eliminate most of the substances which contribute to background absorption. Values obtained by Method II and by countercurrent distribution are in close agreement.
Almost all of the bitter substances which are lost during pilot-scale brewery fermentations using yeast N.C.Y.C. 240 can be recovered unchanged. The largest losses of bitter substances are associated with the dirty head formed during the first 18 hours of a fermentation. Hop utilization can be improved by folding this dirty head back into the fermenting wort. This process is most effective when carried out after the ethanol concentration of the wort has reached about 1%, but before the bitter substances have had the opportunity to undergo extensive oxidative degradation. Losses of bitter substances are lower when 25% of a malt grist is replaced by wheat flour. There is no consistent relationship between losses of bitter substances and either the assimilable or total nitrogen of wort. With increase in wort O.G. there is increasing loss of bitter substances, such losses being principally associated with the dirty head.
Le suivi de tanks de garde d’une production industrielle montre que les arômes du houblon se solubilisent dans la bière dès les deux premières semaines du processus de houblonnage à cru (cas du linalool, du myrcène et de l’acide 3-méthylbutanoïque). Par contre, trois semaines sont nécessaires pour la production de molécules odorantes issues de glycosides du houblon (4-vinylsyringol, citronellol, béta-damascénone). De nombreux arômes soufrés (thiols polyfonctionnels) aux odeurs de fruits exotiques sont également libérés. Le procédé de houblonnage à cru se révèle donc bien plus complexe qu’une simple solubilisation des huiles essentielles du houblon, surtout si la période de maturation est prolongée.
Three new humulone derivatives have been isolated and identified as: 3,4-dihydroxy-2-(3-methyl-2-butenyl)-4-(4-methyl-3-pentenoyl)-2-cyclopentenone (6);4-ethanoyl-3,4-dihydroxy-2-(3-methyl-2-butenyl)-2-cyclopentenone (7) and 3,4-dihydroxy-2-(3-methyl-2-butenyl)-2-cyclopentenone (8), respectively. They arise by deacylation of anti-isohumulone (3a), which is formed from humulone (1a) following an isomerization with ring contraction in opposite direction than the usual one producing isohumulones (2a).
α-Acids and β-acids, two main components of hop resin, are known to be susceptible to oxygen and degraded during hop storage, although the oxidation products in stored hops have not been fully identified. In this study, we developed a high-performance liquid chromatography (HPLC) analysis method suitable for separation and quantification of the oxidation products. This HPLC analysis clearly proved, for the first time, that humulinones and hulupones are major products in oxidized hops. We are also the first to identify novel 4'-hydroxy-allohumulinones, suggested to be oxidative products of humulinones, by means of NMR spectroscopy and high-resolution mass spectrometry. Using the developed analytical method, changes in α- and β-acids and their oxidation products during hop storage were clearly revealed for the first time.
Two American hop varieties, Washington-grown Cascade and Idaho-grown Hallertauer mittelfrüh, were chosen for this aging study and its effect on beer flavor quality. Prior to brewing, hops were refrigerated at 27°F (fresh), while one portion of the hops from the same bale was aged at 90°F for 2 weeks (aged I) and another portion for 9 weeks (aged II). Samples at each brewing stage were taken for chemical profile analysis. The finished beers were also submitted for flavor profile evaluations. Geraniol, linalool, and citronellol are mainly responsible for the floral/citrus note, while the oxidation products of α-humulene, especially the humulenol II and humulene diepoxides, contribute to the herbal/spicy note in beer. A grapefruit-like fruity flavor is also detected in beers brewed with extensively aged Cascade and Hallertauer hops. The survival of various hop-derived aroma compounds is also discussed.
Forty-one thiols, mainly β-sulfanylalkyl acetates, β-sulfanylalkyl alcohols, and β-sulfanylalkyl carbonyls, were recently evidenced in hop. In a beer hopped with the Tomahawk cultivar, most of them were found at higher levels than expected. The aim of the present work was to investigate the polyfunctional thiols in beers hopped with different varieties. A few thiols proved not to come only from hop (mainly 2-sulfanylethyl acetate, μg/L levels, and 1-sulfanylpentan-3-one and 1-sulfanylpentan-3-ol, ng/L levels, internal standard (IST) equivalents). The thiol profile of Saaz-hopped beer proved similar to that of the reference beer produced without hop. A high level of 3-sulfanyloctan-1-ol emerged as an indicator of the use of Tomahawk hop (140 ng/L, IST equivalents; FD (flavor dilution) = 65536). In both Cascade- and Tomahawk-hopped beers, 3-sulfanylhexan-1-ol and 3-sulfanylheptan-1-ol were smelled at high flavor dilutions, although only for the latter, significant amounts of the unreduced 3-sulfanylheptanal were found in hop. As already claimed for hop authentication, 3-sulfanyl-4-methylpentan-1-ol remains a good marker of Nelson Sauvin-hopped beers (548 ng/L, IST equivalents; FD = 65536), together with 4-sulfanyl-4-methylpentan-2-one (128 ng/L, FD = 4096). As illustrated by the huge production occurring during fermentation, accurate prediction of hop varietal impact requires quantitating thiol adducts in hop. S-3-(1-Hydroxyhexyl)cysteine was evidenced here for the first time in Cascade hop.
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.
The rate of isomerization of alpha acids to iso-alpha acids (the compounds contributing bitter taste to beer) was determined across a range of temperatures (90-130 degrees C) to characterize the rate at which iso-alpha acids are formed during kettle boiling. Multiple 12 mL stainless steel vessels were utilized to heat samples (alpha acids in a pH 5.2 buffered aqueous solution) at given temperatures, for varying lengths of time. Concentrations of alpha acids and iso-alpha acids were quantified by high-pressure liquid chromatography (HPLC). The isomerization reaction was found to be first order, with reaction rate varying as a function of temperature. Rate constants were experimentally determined to be k1 = (7.9 x 10(11)) e(-11858/T) for the isomerization reaction of alpha acids to iso-alpha acids, and k2 = (4.1 x 10(12)) e(-12994/T) for the subsequent loss of iso-alpha acids to uncharacterized degradation products. Activation energy was experimentally determined to be 98.6 kJ per mole for isomerization, and 108.0 kJ per mole for degradation. Losses of iso-alpha acids to degradation products were pronounced for cases in which boiling was continued beyond two half-lives of alpha-acid concentration.
Presented at the 36th EBC Congress
- M Biendl
Humulinone Formation in Hops and Hop Pellets and Its Implications for Dry Hopped Beers
- J P Maye
- R Smith
- J Leker
Dry Hopping and Its Effects on the International Bitterness Unit Test and Beer Bitterness
- J P Maye
- R Smith
How to Analyze Dry Hopped Beer? Presented at the 36th EBC Congress
- M Biendl
Brewing: Science and Practice
- D E Briggs
- C A Boulton
- P A Brookers
- R Stevens