How Does the Flavor in Hops Extract Come About?

Hops (Humulus lupulus L.) are perennial herbaceous plants with separate male and female plants [1], and are one of the most important raw materials in beer brewing. Due to its antibacterial and preservative properties, ability to maintain foam stability, and significant influence on beer flavor, it is often referred to as the “soul of beer” [2–3]. Hops extracts contain a rich composition of flavor compounds [4], and over 400 components have been identified in hops essential oil to date, with some studies predicting that the number of compounds could exceed 1,000 [5]. Meanwhile, different brewing processes can lead to significant variations in hop flavor in beer [6]; yeast-mediated biotransformation of hop flavor compounds during beer brewing also influences hop flavor in beer [7]. Therefore, in-depth research on hop flavor is crucial for enhancing beer product flavor.

Although there are also reviews on hops in China [8‒10], these primarily focus on the analysis of hop extract components and the interpretation of their pharmacological effects, with limited research on hop flavor and its analytical techniques, and there remains a gap compared to international cutting-edge research. Therefore, this paper provides a detailed description of the composition of hop flavor compounds, reviews the current research directions in hop flavor studies using bibliometric analysis, discusses existing hop flavor research techniques, proposes the application of quantum chemical calculations in hop flavor research, and outlines the future development trends of hops, aiming to enhance the precise use of hops, promote the development of the hop industry, and provide theoretical references for beer flavor regulation.

1 Composition of hop flavor compounds and research progress

The hop cone (i.e., the inflorescence) consists of the pedicel, bracts, bracteoles, and lupulin glands. Among these, the lupulin glands contain hop resin and hop essential oil, which are the key components responsible for the bitterness and aroma of beer. Given the complex composition of hop flavor compounds, scientists worldwide have conducted extensive research.

1.1 Composition of Bitter Compounds in Hops

Bitterness is a relatively unpleasant food flavor characteristic but is a crucial quality indicator for beer. As beer ages, bitterness may exhibit characteristics such as attenuation, lingering, and astringency, thereby reducing the beer's drinkability. The bitterness in beer primarily originates from the α-acids, β-acids, and polyphenols in hop extracts, as well as their derivatives formed through chemical transformations under specific conditions. These components collectively influence the bitterness of beer.

α-acids (Figure 1) are a mixture of five isomers: humulone, co-humulone, trans-humulone, trans-humulone, and trans-humulone [11]. Due to their hydrophobic structure, they are present in very low concentrations in beer and have a high threshold, so their contribution to beer bitterness is negligible. However, it can be converted into iso-α-acid (Figure 2) through protonation and ketol rearrangement, forming a five-membered ring compound with two chiral carbon atoms. Iso-α-acid has better water solubility and contributes approximately 80% of the bitterness in beer [12]. Additionally, α-acid can undergo natural oxidation to form humulone (Figure 3). Although humulone has only 66% of the bitterness of iso-α-acid [13], its higher water solubility enhances the bitterness of beer. β-acid (Figure 1) is a compound composed of five isomers: humulone, co-humulone, humulone, prähumulone, and post-humulone [14]. Due to the addition of an isoprenyl group in its structure compared to α-acid, it has stronger hydrophobicity and a higher bitterness threshold. Therefore, it does not directly contribute bitterness to beer but readily oxidizes into the more bitter humulone (Figure 4), The bitterness of humulone is 84% that of isohumulone [13], which can compensate for bitterness loss during beer storage and maintain beer bitterness stability.

Xanthohumol (Figure 1) is another important compound contributing bitterness to beer, in addition to α-acid and β-acid. However, it tends to precipitate and be lost by binding with proteins in wort or fermentation liquid, and undergo isomerization and oxidative degradation reactions, resulting in its contribution to beer bitterness being relatively insignificant. However, the isomerization of humulone can generate isohumulone (Figure 5), which has a lower bitterness profile and influences beer bitterness [15]. In addition to xanthurenic acid, hops contain other polyphenolic compounds that influence beer bitterness, such as flavonoids like quercetin and kaempferol, flavanols like catechin, and carboxylic acids like ferulic acid [16]. SHELLHAMMER et al. [17] found that beer bitterness and astringency are related to the total polyphenol content, types, polymerization degree, and molecular weight of polyphenolic compounds in beer. Additionally, for every 15–20 mg/L increase in the mass concentration of total polyphenols in beer, the bitterness value increases by one unit.

The bitterness in beer primarily originates from hops, and the compounds in hops that influence beer bitterness are diverse and can undergo chemical transformations, collectively contributing to the complex bitterness of beer. Current research on hop bittering compounds includes the types of bittering compounds, their sources, their functions, and the use of biotechnology to enhance the content of bitter acids. However, many studies on the mechanisms of bittering perception are not sufficiently detailed, and there is a lack of comprehensive, scientific, and systematic research to validate these findings. For example, an increase in hydroxyl groups in hop bittering compounds enhances astringency and masks bitterness, while an increase in carbon-carbon double bonds enhances bitterness intensity [15]. Meanwhile, the key brewing process control points that influence beer bitterness during beer production have not yet been clarified [15], making it impossible to guide precise regulation of beer bitterness during production. Therefore, it is essential to actively conduct research and verification on the mechanisms of bitterness perception, and integrate beer bitterness research with brewing processes to enhance the stability and acceptability of beer bitterness.

1.2 Composition of aromatic compounds in hops

The aroma of hops primarily originates from hop essential oils, which account for only 0.5%–3.0% of hops but are rich in various aromatic compounds. This paper summarizes some of the aromatic compounds in hops based on published literature and queries their odors in the Flavor Ingredient Library (https://www.femaflavor.org/flavor-library) (Figure 6)[18‒22]. As shown in Figure 6, the composition of hop essential oils is generally divided into three major categories: hydrocarbon compounds, oxygen-containing compounds, and sulfur-containing compounds[18];  Hydrocarbons are further divided into monoterpenes, sesquiterpenes, and alkanes; oxygen-containing compounds are divided into terpenols, sesquiterpenols, aldehydes, ketones, esters, and epoxides; sulfur-containing compounds are divided into thiols and thioethers; these compounds collectively contribute to the rich aroma of beer.

Hydrocarbon compounds constitute a significant proportion of hop essential oils and play a prominent role in hop aroma. They can generally be classified into monoterpenes, sesquiterpenes, and esters, with monoterpenes accounting for 50–70% and sesquiterpenes for 30–50% [23].  Monoterpenes and sesquiterpenes have low boiling points and poor water solubility, making them highly volatile and prone to oxidation during hop processing, storage, and beer brewing [24], and thus have a limited impact on beer aroma. Although ester compounds are present in much lower concentrations in hops compared to monoterpenes and sesquiterpenes, their high boiling points, good water solubility, and chemical stability make them more likely to be retained in beer, thereby exerting a significant influence on beer flavor.

Oxygenated compounds account for approximately 15%–25% of hop essential oils [23], composed of alcohols, aldehydes, ketones, acids, esters, and epoxy compounds. Different alcohol compounds have distinct aromas; 2-methyl-2-butene-1-ol and 2-propenol have a pungent odor;  while linalool and geraniol have floral aromas. Aldehydes and ketones are generally derived from secondary metabolites formed during aging and are used to evaluate hop freshness [25]. Aldehydes have pungent and green aromas [26], while ketones contribute to the formation of floral aromas [27].

RETTBERG et al. [8] found that small amounts of aldehyde and ketone compounds can make the flavor and aroma of hops unique, rich, and well-balanced. Acidic compounds generally originate from product spoilage and have a sour, pungent odor, which can negatively affect flavor [28]. Caryophyllene oxides are formed by the oxidation of β-caryophyllene and have unpleasant moldy odors, negatively affecting the overall aroma quality of hops. They are used to evaluate hop freshness. Compared to hydrocarbon compounds, oxides have higher boiling points, better water solubility, and more stable chemical properties, making them less prone to loss during subsequent processing and storage, and thus more likely to influence the characteristic aromas of beer.

Although sulfur compounds are present in extremely low concentrations in hop essential oils, accounting for less than 1% [23], their low sensory threshold significantly influences the overall aroma style of hops [29]. They primarily include compounds such as hydrogen sulfide, methyl mercaptan, dimethyl sulfide, and diethyl sulfide. LERMUSIEAU et al. [22] found that most sulfur-containing compounds have unpleasant odors such as foul or irritating smells, which negatively impact the overall aroma quality of hops. However, thiols are fruit sulfur compounds with fruity aromas. Some studies suggest that sulfur compounds are responsible for the unique flavor characteristics of beer.

Hops exhibit a rich array of sensory flavor characteristics, including fruity, woody, and floral aromas. This study summarizes, classifies, and grades sensory descriptions of hop aromas based on published literature [19, 30], and presents a hop aroma radar map for reference. As shown in Figure 7, hops exhibit aromas such as non-citrus fruits, floral, herbal, spicy, and citrus fruits, accompanied by off-flavors like sulfur and cheese, forming a relatively complex aroma profile.

1.3 Progress in hop flavor research

Bibliometrics employs mathematical and statistical measurement methods to evaluate and predict the current status and future trends of scientific and technological research. Currently, bibliometrics has been widely applied in fields such as computer science [31], economics [32], mathematics [33], and medicine [34]. This study selected the Peking University Core Journal Database from the China National Knowledge Infrastructure (CNKI) academic journals as the source of Chinese literature. In the CNKI database (https://www.cnki.net/), an advanced search query was used: Subject = “Hops” “Flavor,” with the source category selected as the Peking University Core Journal Database. After searching and excluding irrelevant literature, a total of 49 Chinese-language documents were obtained. The Web of Sciences Core Collection database (https://www.webofscience.com/) was selected as the source of English-language literature. A advanced search query was used: (TS=Hops or TS=flavor). After searching and excluding irrelevant literature, a total of 526 English-language literature were obtained.

As shown in Figure 8, the keywords with higher frequencies are: “beer,” “hops,” “volatile components,” “gas chromatography-mass spectrometry,” “hop essential oil,” and “dry hopping.” As shown in Figure 9, the keywords with higher frequencies are “Humulus lupulus L.,” “fermentation,” “cultivar,” “gas chromatography-mass spectrometry,” “linalool,” “yeast,” and “dry hopping.” Combining the keywords from both sources, it can be observed that aroma research in hop flavor studies currently outweighs bitterness research. In aroma research, the primary analytical technique employed is gas chromatography-mass spectrometry, with a high focus on compounds such as linalool. Additionally, studies on the influence of yeast on hop flavor compounds are also highly popular. Meanwhile, when investigating the impact of hops on beer flavor, dry hopping experiments are frequently used for validation.

2    Technological Advances in the Study of Flavor Compounds in Hops Extracts

2.1    Research Techniques for Bitter Compounds

With the development of detection and analysis techniques and sensory evaluation methods, the analysis and identification of bitter compounds in beer have become increasingly systematic and comprehensive (Table 1). The bitterness value of beer is influenced by various factors, including hop variety, addition amount, and fermentation parameters. A normal bitterness value is a prerequisite for the harmony of the beer body and flavor stability. The bitterness value method (photometric method) is the traditional method preferred by major breweries and universities, and it is widely applied in the industry. Liu Qian et al. [35] used the bitterness value method (photometric method) to determine the bitterness values of tetrahydro and hexahydro hop products produced using different manufacturing processes, effectively elucidating the relationship between hop product addition levels and bitterness values. However, the bitterness value method (photometric method) also has drawbacks such as significant errors, low precision, and poor reproducibility. Liquid chromatography has the advantages of high sensitivity, high separation efficiency, and minimal sample requirements.

Liu Zecang [18] used liquid chromatography to accurately determine the α-acid and β-acid content in hops, but liquid chromatography is unable to evaluate the bitterness quality and bitterness intensity in beer. Sensory evaluation methods include quantitative descriptive analysis (QDA), check-all-that-apply method (CATA), and rate-all-that-apply method (RATA), which can accurately reflect sample flavor and highlight sensory characteristics.

GAHR et al. [36] successfully analyzed the changes in hop bitterness in beer at different temperatures using sensory evaluation. However, sensory evaluation requires highly skilled evaluators and is subject to significant subjective bias. Nuclear magnetic resonance spectroscopy enables the quantitative analysis of complex components and the accurate identification of compound structures using small sample quantities. IKHALAYNEN et al. [37] confirmed the compound structures of bitter acids in beer using magnetic resonance spectroscopy and validated common metabolic pathways. However, this instrument is expensive and not suitable for widespread application.

Fluorescence spectroscopy is an important method for analyzing chemical components, offering advantages such as simple sample preparation and non-destructive sample detection. APPERSON et al. [38] successfully detected proteins, composite polyphenols, and isomeric α-acids in beer using fluorescence spectroscopy. However, fluorescence spectroscopy is significantly influenced by sample matrices. An electronic tongue is a method that simulates the human taste recognition system, using an array of taste sensors to detect different taste substances and generate electrical signals for analysis [41]. It can quickly reflect the overall flavor information of a sample but cannot identify specific compounds. Electrospray extraction ionization mass spectrometry has advantages such as no sample pretreatment, real-time, continuous, and online analysis. However, the instrument is expensive and not easily scalable. Each detection method has its own advantages and disadvantages. Therefore, when conducting research, we should thoroughly analyze the characteristics of each method, combine them with our expected objectives to make reasonable selections, and, when necessary, use a combination of multiple methods to comprehensively analyze bitterness.

2.2 Aroma Compound Research Techniques

Hops extracts can impart unique aromas to beer and alter beer styles. Research on hop aromas is crucial for the development of the beer industry. However, due to the complex composition of aromatic compounds in hops and their extremely low content in beer, appropriate pretreatment and analytical techniques must be selected. Table 2 lists the pretreatment techniques for hop aromatic compounds and their characteristics, while Table 3 lists the analytical techniques for hop aromatic compounds and their characteristics.

SDE is a pretreatment method that combines steam distillation with solvent extraction to simultaneously extract and concentrate flavor compounds. This method has high extraction efficiency and is effective for low-volatility and high-molecular-weight components. Wang Yanan [42] successfully extracted total flavonoids from hops using simultaneous distillation extraction. However, this method has drawbacks such as long extraction times, high temperatures, and a tendency to cause aroma degradation and distortion in the sample. SAFE is an internationally recognized sample pretreatment method that requires equipment such as a butterfly distillation apparatus, a high-vacuum pump, a precision water bath, and a liquid nitrogen tank. This method operates at low temperatures, reducing the loss of heat-sensitive volatile components in the sample [43]. Additionally, the volatile flavor compounds extracted by this method are closer to their original state in the sample, making it particularly advantageous for constructing beer flavor profiles and flavor maps. However, due to the need for concentration and dehydration steps, this method may also result in some loss of low-boiling-point, highly volatile aromatic components.

SPME is one of the most commonly used pretreatment methods for enriching volatile flavor compounds. This method involves adsorbing and concentrating flavor compounds using adsorbents on the surface of microfibers, offering advantages such as simple operation, rapid processing, no need for solvent addition, and the ability to reflect the true aroma of the sample. However, it has low extraction efficiency for weakly volatile components. SBSE is a technique developed based on SPME,  compared to SPME fibers, SBSE has a larger adsorbent surface area, achieving higher extraction efficiency, high sensitivity, and good reproducibility. However, it has limited coating options and poor extraction efficiency for polar organic compounds [44].

GC-MS has advantages such as high separation efficiency, high sensitivity, low sample consumption, fast analysis speed, and large information content, making it the most commonly used detection method. However, it has poor resolution for isomers. GC-O uses the human nose as a detector, leveraging the separation capabilities of gas chromatography to directly detect the odor characteristics of various flavor compounds. It can also be coupled with mass spectrometry to achieve the purpose of identifying the substances detected. It offers high sensitivity and the advantage of integrating with human sensory perception, but it is time-consuming, has poor reproducibility, and requires highly skilled odor evaluators.

GC-IMS is a newly developed high-sensitivity, low-detection-limit, simple-to-operate rapid detection technique that does not require a vacuum system. However, it does not respond to small alkane-type molecules [47]. Electronic noses are instruments developed in the 1990s for analyzing and detecting complex odors. They feature multiple interactive sensitive sensor arrays and detect odors by simulating the human olfactory process. Compared to artificial sensory evaluation, electronic nose technology offers advantages such as short response times, rapid detection, accuracy, good repeatability, and objective reliability. However, it can only obtain overall information about odors and  and cannot perform qualitative or quantitative analysis of specific substances [18].

Currently, cutting-edge hop flavor research has applied molecular sensory science technology. Molecular sensory science is an interdisciplinary field based on sensory evaluation, detection, and analysis technologies [48], with its core content being the qualitative and quantitative description and analysis at the molecular level, and the precise reconstruction of food flavor compounds [49].It possesses strong capabilities for concentrating volatile substances, screening and identifying compounds contributing to aroma, precisely quantifying key aromatic compounds, and accurately analyzing their aromatic contributions, as well as precisely verifying reconstructed aromatic compounds [50].

Key flavor active substance evaluation methods are applied in this technology. The odor activity value (OAV) evaluates the impact of volatile substances in food on food quality based on two dimensions: concentration and threshold [51–52]. Generally, aromatic compounds with OAV > 1 are considered to have a significant influence on the overall aroma of a product, and the higher the value, the greater the influence on aroma. Aroma extraction dilution analysis (AEDA) involves professional evaluators conducting sensory evaluations of target compounds through continuous, stepwise dilution of aroma extracts until the odor can no longer be detected, thereby determining the sensory thresholds of different flavor compounds [53]. The highest dilution value at which a flavor compound can be detected by smell is the aroma dilution factor (FD) of that substance. A higher FD value indicates greater aroma intensity [54].

SU et al. [55] analyzed the aroma characteristics of dried hops using HS-SPME-GC-MS-O and found that methyl caprylate, myrcene, trans-bergamotene, linalool, and geraniol as high-intensity aromas. SU et al. [56] used GC-MS-O and AEDA to identify aroma compounds in Cascade and Chinook hops from multiple regions of Virginia, and quantified selected aroma compounds using stable isotope dilution and standard addition methods,  A total of 33 aromatic active compounds were detected, among which myrcene, methyl caprylate, geraniol, and linalool exhibited higher FD and OVA values.

The application of molecular sensory science and technology to hop flavor research can identify key flavor-active compounds in hops, uncover key aromatic differences between different hop varieties, significantly enhance the depth of hop flavor research, and provide technical support for the precise application of hops. In the future, hop flavor analysis can adopt a combination of multiple pretreatment methods and utilize high-resolution instruments such as two-dimensional gas chromatography-time-of-flight mass spectrometry and gas chromatography-electrostatic field orbit trap mass spectrometry to enrich the detection of trace components, providing a better data foundation for subsequent analyses.

3 Conclusion

The beer industry has shifted from pursuing quantity to quality, and beer flavor is one of the most important indicators of quality. As the “soul of beer,” hops play a crucial role in contributing to and influencing beer flavor. Research on the flavor compounds of hop extracts has made significant progress, with comprehensive application of research techniques. The absolute quantification of hop flavor compounds and the verification of key flavor components have been systematically conducted, and studies on the factors influencing hop flavor in beer have also laid a solid foundation. To better meet consumers' demands for beer quality, future research can focus on the following areas.

The hop industry is developing rapidly, with many varieties emerging that have excellent agronomic traits and unique flavors. However, few studies have been conducted on the influence of terroir on hop flavor. Therefore, future research should integrate terroir factors with hop flavor studies to promote the development of the hop industry. Meanwhile, different hop varieties possess unique flavor characteristics. To clarify the differences between hops from different regions and varieties, a hop flavor profile database can be established to better support beer product development. Currently, some key flavor components in hops have been identified, but their specific metabolic pathways and regulatory mechanisms remain poorly understood. Future research should focus on elucidating the metabolic pathways of yeast in the production of hop flavor compounds, integrating these findings with beer fermentation processes, leveraging the inherent advantages of yeast, and continuously optimizing beer flavor to achieve greater control over flavor profiles. Additionally, the interactions among hop flavor compounds, the synergistic effects between hop flavor compounds and other flavor compounds in beer, as well as the sources and changes of off-flavor compounds in hops can all be studied in depth.

Many hop flavor mechanisms remain at the inferential stage, lacking comprehensive, scientific, and systematic research validation. Quantum chemistry, as a branch of theoretical chemistry, can conduct precise theoretical mechanism studies on systems, compensating for the limitations of traditional experimental methods. It has been applied in fields such as chemistry [57], medicine [58], and food science [59]. XIAO et al. [59] utilized quantum chemistry to analyze the flavor-forming mechanism of oleic acid during heating. Huang Zhangjun et al. [60] combined quantum chemistry with gas chromatography to clarify the mechanism of “acid increase and ester decrease” during the storage of baijiu and the reasons for the reduction of free molecules, providing theoretical support for the flavor stability of baijiu during storage.

Research on the flavor mechanisms of hop extracts can be conducted from two aspects: bitterness and aroma. In terms of bitterness, the mechanisms of α-acid isomerization forming iso-α-acid, β-acid oxidation forming humulone, and xanthone oxidation and xanthone isomerization forming iso-xanthone can be verified, and the new possibilities of chemical conversion of bitter substances such as α-acid, β-acid, xanthone, and other bitter compounds. In terms of aroma, known aroma mechanisms such as the oxidation of caryophyllene to β-caryophyllene oxides can be verified, and the changes in hydrocarbon compounds, oxygen-containing compounds, and sulfur-containing compounds in hops during the beer brewing process can be explored mechanistically, with traceability studies conducted on key aroma compounds in the finished beer.

In summary, future hop research can be conducted in two aspects: flavor expression and flavor mechanism, to further enhance beer flavor regulation, improve raw material utilization, and promote the sustainable development of the hop industry.

Reference

[1]CHEN XY, WANG MY, DENG CH, et al. The hops (Humulus lupulus L.) genome  contains  a mid-sized terpene  synthase  family  that  shows  wide functional and allelic diversity [J]. BMC Plant Biol, 2023, 23(1): 280.

[2]ZHOU Y, XUE L, WU ZJ, et al. Research progress on beer volatile flavor components [J]. Food Res Dev, 2021, 42(1): 210‒219.

[3]SONG WJ, LI HF, JIANG JY, et al. Study on fingerprint of hops based on high performance  liquid  chromatography  [J].  Food  Ferment  Ind,  2022, 48(22): 279‒284.

[4]EYRES  GT,  MARRIOTT PJ, DUFOUR JP. Comparison of odor-active compounds in the spicy fraction of hop (Humulus lupulus L.) essential oil from  four  different  varieties   [J].  J  Agric  Food  Chem,  2007,   55(15): 6252‒6261.

[5]ROBERTS MT, DUFOUR JP, LEWIS AC. Application of comprehensive multidimensional gas chromatography combined with time-of-flight mass spectrometry  (GC×GC-TOFMS)  for  high  resolution  analysis  of  hop essential oil [J]. J Sep Sci, 2004, 27(5‒6): 473‒478.

[6]WANG  H.  Influence  of  hop  varieties,  freshness,  recipes  and  hopping methods on beer flavor [D]. Wuxi: Jiangnan University, 2012.

[7]SVEDLUND N, EVERING  S, GIBSON B,  et al. Fruits of their labour: Biotransformation  reactions  of yeasts  during  brewery  fermentation  [J]. Appl Microbiol Biot, 2022, 106: 4929‒4944.

[8]RETTBERG N,  BIENDL M, GARBE LA. Hop aroma and hoppy beer flavor: Chemical backgrounds and analytical tools-A review [J]. J Am Soc Brew Chem, 2018, 76: 1‒20.

[9]LI X, LI S, WU YJ, et al. Research progress of hops [J]. J Jilin Med Coll,2019, 40(2): 143‒145.

[10]ZHAO YF, LIU TP, ZHAO SC, et al. Research progress on the mechanism of xanthohumol, the active ingredient of hop, in treating osteoporosis [J]. China Med Herald, 2022, 19(29): 34‒37, 45.

[11]LIU YM. Research progress on chemical constituents and pharmacological effects of hops [J]. Food Sci, 2009, 30(23): 521.

[12]KEUKELEIRE D, VINDEVOGEL J, ROMAN  S,  et al. The historyand  analytical chemistry of beer bitter acids [J]. TrAC-Trend Anal Chem, 1992, 11(8): 275‒280.

[13]ALGAZZALI V, SHELLHAMMER T. Bitterness intensity of oxidizedhop  acids: Humulinones and hulupones [J]. J Am Soc Brew Chem, 2016, 74(1): 36‒43.

[14]XIONG  HP.  Study  on  chemical  constituents  of  hops  and  purification technology of xanthohumol [D]. Hangzhou: Zhejiang University, 2005.

[15]LIU ZC, LIU YM. Bitters and hop bitters in beer [J]. China Brew, 2019, 38(1): 13‒19.

[16]DRESEL  M,  DUNKEL  A,  HOFMANN  T.  Sensomics  analysis  of key bitter compounds in the hard resin of hops (Humulus lupulus L.) and their contribution to the bitter profile of pilsner-type beer  [J].  J Agric Food Chem, 2015, 63(13): 3402‒3418.

[17]SHELLHAMMER  TH,  MCLAUGHLIN  IR,  LEDERER  C.  Bitterness modifying  properties  of  hop  polyphenols   extracted   from  spent  hop material [J]. J Am Soc Brew Chem, 2008, 66(3): 174‒183.

[18]LIU  ZC.  Basic  research  on  the  construction  of  domestic  hop  quality evaluation system [D]. Urumchi: Xinjiang University, 2020.

[19]MACHADO  JCJ,  LEHNHARDT F, MARTINS ZE,  et al.  Sensory and olfactometry  chemometrics  as  valuable  tools  forassessing  hops’  aroma impact on dry-hopped beers: A studywith wild portuguese genotypes [J].

Foods, 2021, 10: 1397‒1412.

[20]FÉCHIRA  M,  WEAVERB  G,  ROYC  C,  et  al.  Exploring  the  regional identity  of cascade  and  mosaic®  hops  grown  at  different  locations  in Oregon and Washington [J]. J Am Soc Brew Chem, 2022, 81(3): 480‒492.

[21]DUAN LL, YANG XY, JI DR, et al. Study on volatile flavor components of Saz hop wine based on electronic nose and GC-MS [J]. China Brew, 2020, 39(8): 138‒142.

[22]LERMUSIEAU  G,  COLLIN  S.  Volatile  sulfur  compounds  in  hops  and residual concentrations in beer-A review [J]. J Am Soc Brew Chem, 2003, 61(3): 109‒113.

[23]LAFONTAINE  S,  SHELLHAMMER  TH.  How  hoppy  beer  production has redefined hop quality and a discussion of agricultural and processing strategies to promote it [J]. Tech Quart, 2019, 56(1): 1‒12.

[24]SANCHEZ NB. Analytical  and sensory evaluation of hop varieties  [D].

Portland: Oregon State University, 1990.

[25]MACHADO  JCJ,  FARIA  MA,  FERREIRA  IM,  et  al.   10-Hops:  New perspectives for an old beer ingredient [J]. Nat Bever, 2019, 13: 267‒301.

[26]KISHIMOTO  T,  WANIKAWA  A, KONO K,  et al.  Comparison  of the odor-active compounds in unhopped beer and beers hopped with different hop varieties [J]. J Agric Food Chem, 2006, 54(23): 8855‒8861.

[27]ROBERT TR, WILSON RJH. Hops in handbook of brewing [M]. 2nd ed, Boca Raton, FL: Taylor & Francis, 2006.

[28]VOLLMER  DM,   LAFONTAINE  SR,  SHELLHAMMER  TH.  Aroma extract dilution analysis of beersdry-hopped with Cascade, Chinook, and Centennial [J]. J Am Soc Brew Chem, 2018, 76(3): 190‒198.

[29]GUADAGNI DG, BUTTERY RG, OKANO S. Odour thresholds of some organic compounds associatedwith food flavours  [J]. J  Sci Food Agric,

1963, 14(10): 761‒65.

[30]HOLLE AV, MUYLLE H, HAESAERT G, et al. Relevance of hop terroir for beer flavor [J]. J Inst Brew, 2021, 127(3): 238‒247.

[31]SHUKLA AK, MUHURI PK, ABRAHAM A. Abibliometric analysis and  cutting-edge overview on fuzzy techniques in big data [J]. EAAI, 2020, 92: 103625.

[32]ZABAVNIK D, VERBIČ M. Relationship between the financial and the real economy: A bibliometric analysis [J]. Int Rev Econ Finance, 2021, 75: 55‒75.

[33]YANG Y, LV K, XUE J, et al. A bibliometric analysis and visualization of fractional order research in China over two decades (2001–2020) [J]. Am J

Math, 2021. https://doi.org/10.1155/2021/7996776

[34]TAO  Z,  ZHOU   S,  YAO  R,   et  al.   COVID-19  will   stimulate  a  new coronavirus research breakthrough: A 20-year bibliometric analysis  [J]. Ann Transl Med, 2020, 8(8): 528.

[35]LIU  Q,  LIU  YQ,   LI  Z.  Study  on  the   influence  of  tetrahydric  and hexahydric hop products with different production processes on the foam and bitterness of beer [J]. Global Alcinfo, 2021, 149(21): 25‒30.

[36]GAHR  A,   FORSTER  A,   SCHULL  F,   et  al.   The   stability  of  bitter substances in beer during the ageing process kellerer [J]. Brew Sci, 2020, 73: 149‒157.

[37]IKHALAYNEN   YA,   PLYUSHCHENKO   IV,   RODIN   IA,   et   al.   P Hopomics:  Humulus  lupulus  brewing  cultivars  classification  based  on LC-MS  profiling  and  nested  feature  selection   [J].  Metabolites,  2022, 12(10): 945.

[38]APPERSON  K, LEIPER KA, MCKEOWN IP, et al. Beer fluorescence and the isolation, characterisation and silica adsorption of haze-activebeer proteins [J]. J Inst Brew, 2015, 108(2): 193‒199.

[39]WU  X, TAHARA Y, YATABE R, et al. Taste sensor: Electronic tongue with lipid membranes [J]. Anal Sci, 2020, 36(2): 147‒159.

[40]ZHU L, HU Z, GAMEZ G, et al. Simultaneous sampling of volatile and non-volatile   analytes   in   beer   for   fast   fingerprinting   by   extractive electrospray ionization mass spectrometry [J]. Anal Bioanal Chem, 2010, 398(1): 405‒413.

[41]ZHANG Y, WANG  BW,  CHEN  ZM. Application  of electronic tongue  technology in the identification of Pu’er tea [J]. J Zhejiang Agric Sci, 2023, 9: 1‒5.

[42]WANG  YN.  Continuous  extraction  of total  flavonoids  from  hops  and preparation of hop essential oil microcapsules [D]. Chongqing: Chongqing University, 2016.

[43]KANG HG, KIM HY, JEE H,  et al. Compound of Cynanchum wilfordii and    Humulus   lupulus    L.    ameliorates    menopausal    symptoms    in ovariectomized mice [J]. Reprod Sci, 2023, 30(5): 1625‒1636.

[44]PAGUET AS, SIAHA, LEFEVRE G, et al. Phytochemical characterisation and aromatic potential for brewing of wild hops  (Humulus lupulus L.) from Northern France: Towards a lead for local hop varieties [J]. Food Chem, 2023, 433: 137302.

[45]WANG Q, SUN JJ, HOU J, et al. Effects of different varieties of hops on characteristic  aroma  substances  in  beer  [J].  Farm  Prod  Process,  2021, 535(17): 5‒10, 13.

[46]HAO J, JIANG CY, CHAI Y, et al. Analysis of volatile flavor compounds in tobacco from different origin based on GC-IMS [J]. J Light Ind, 2023, 38(2): 87‒93, 117.

[47]LI FL, XIANG SM, LI J, et al. Analysis of volatile aroma components in different types of craft beer based on headspace-gas chromatography-ion mobility spectrometry [J]. J. Food Saf Qual, 2023, 14(7): 305‒314.

[48]GUAN HN, ZHAO SF, LIU DY, et al. Research progress of meat flavor based on molecular sensory science [J]. J Food Sci Technol, 2023, 9: 1‒16.

[49]GUO H X, FENG T, QI WY, et al. Effects of electron-beam irradiation on volatile  flavor  compounds   of  salmon  fillets  by  the  molecularsensory science  technique   [J].  J  Food   Sci,  2020.  https://doi.org/10.1111/1750- 3841.15541

[50]ZHAO JY, LI X, FAN TT. Identification of coffee flavor substances by molecular sensory science [J]. Food Ind, 2021, 42(12): 419‒423.

[51]WANG HL, YANG P, LIU C,  et al. Characterization of key odor-active compounds in thermal reaction beef flavoring by SGC×GC-O-MS, AEDA, DHDA, OAV and quantitative measurements  [J]. J Food Compos Anal, 2022, 114: 104805.

[52]LI J X, WU XY, BI JF, et al. Characterization of key aroma compounds of apple  slices  dried  by  hot-air  at  different  temperatures  by  GC-MS  and electronic nose [J]. Sci Technol Food Ind, 2022, 43(18): 272‒282.

[53]LIU Q, JIA JH, BAI YL, et al. Research progress of flavor substances and analysis techniques in beer [J]. China Brew, 2023, 42(8): 20‒27.

[54]TE D, WANG CM, LIU P, et al. Research progress on the application of molecular sensory science and technology in flavoring  [J].  Chin  Cond, 2021, 46(5): 198‒200.

[55]SU  X,  HURLEY  K,  XU  Z,  et  al.  Performance  of  alternative  drying techniques on hop (Humulus lupulus L.) aroma quality: An HS-SPME- GC-MS-O and chemometrics combined approach [J]. Food Chem, 2022, 381: 132289. .

[56]SU  X, YIN Y. Aroma characterization of regional cascade and chinook hops (Humulus lupulus L.) [J]. Food Chem, 2021, 364: 130410.

[57]TAN YJ, HU XW, YANG QJ,  et al. Quantum chemical  calculation of Al8O12 cluster model for adsorption mechanism of hydrogen fluoride on  alumina surface [J]. Chin J Process Eng, 2023, 9: 1‒9.

[58]LI L. Theoretical research on the application of low-vinamil materials in biomedical field [D]. Changchun: Jilin University, 2023.

[59]XIAO L, WANG S, WANG Y, et al. Density functional theory studies on the oleic acid thermal oxidation into volatile compounds [J]. Food Chem: X, 2023, 19: 100737.

[60]HUANG  ZJ,  ZHANG  WH,  ZENG  YH,  et  al.  Quantum  chemical calculation  of hydrogen bond  interaction between  main  components  in liquor storage [J]. Food Sci, 2021, 42(12): 45‒51.