What Are the Active Ingredients in Hops Flower Extract?

Hops Humulus lupulus L., also known as Hubei, snake hemp flower, fragrant snake hemp, and beer flower (Uyghur), is a perennial climbing herbaceous plant belonging to the Moraceae family and Humulus genus. It is commonly used medicinally for its unripe flower-bearing fruit spikes. It has a bitter taste, is slightly cool in nature, and is non-toxic. It possesses stomachic, digestive, sedative, and diuretic properties. Clinically, it is used to treat indigestion, abdominal distension, pulmonary tuberculosis, cystitis, neurasthenia, and insomnia [1]. It is primarily distributed in northern Xinjiang and is also cultivated in North China, Northeast China, Zhejiang, and Shandong provinces [2]. Recent studies have found that hops contain various chemical components such as resins, essential oils, polyphenols, and polysaccharides, exhibiting pharmacological activities including antitumor, antioxidant, antibacterial, and sedative-hypnotic effects.

1 Active Components of Hops Extract

1.1 Resins

Resins are one of the primary chemical components of hops, with a highly complex structural composition that remains incompletely characterized to date. According to the European Brewery Convention (EBC), they are generally classified into soft resins and hard resins based on their solubility in different organic solvents [3]. Soft resins refer to those with a resin content accounting for 10%–25% of the total hop weight [4]. Soft resin components include α-acids [5] (humulone, etc.) and β-acids [6] (lupulone, etc.) (Figure 1). α-Acids are one of the most important chemical components in hops, consisting of five isomers: humulone, co-humulone, adhumulone, humulone, and humulone, among others [5]; β-acids are another important component of soft resins in hops, consisting of lupulin, humulone, humulone, humulone, and humulone, among others [6]. Forino et al. [7] isolated 4-hydroxy-caryophyllene from the cultivated hop variety “Cascade” (Figure 2). Both α-acids and β-acids contain double bonds, making them prone to oxidation and catalytic hydrogenation reactions, which degrade hop quality (Figures 3 and 4).

Hard resins refer to the insoluble fraction of total resins in hexane and belong to the category of unidentified resins. Hard resins include both naturally occurring hard resins in hops and the oxidation products of α-acids and β-acids [8]. Chen Libin et al. [9] determined the content of hard resins by weight analysis, subtracting the amount of soft resins soluble in ethane from the total resin content.

1.2 Polyphenols

1.2.1 Phenolic acids

Phenolic acid compounds refer to a class of derivatives of p-hydroxybenzoic acid and p-hydroxybenzoic acid [10]. Wang Chaoqun et al. [11] found that the main phenolic acid compounds in hops are vanillic acid, protocatechuic acid, ferulic acid, sinapic acid, and gentisic acid. In addition, hops also contain hydroxybenzoic acid [12], cinnamic acid [13], and gallic acid in the form of glycosides.

1.2.2 Catechin compounds

Catechin compounds refer to a class of flavanol derivatives. In hops, catechin compounds primarily include catechin [15], epicatechin [14-15], gallocatechin [16], and epigallocatechin [17].

1.2.3 Proanthocyanidins

In hops, the main proanthocyanidin compounds include proanthocyanidin dimers B1, B2, B3, B4, and proanthocyanidin trimers [18-21]. Li Yushu [16] analyzed the average polymerization degree (mDP) and monomer composition of proanthocyanidins in hops using sulfur-leaching-RP-UPLC-MS, finding that the main constituent units of proanthocyanidins in hops are (+)-catechin (>70%), followed by epicatechin (>10%) and gallocatechin (<5%).

1.2.4 Flavonoids

Modern research indicates that flavonoids are a major active component in hops, classified into chalcone (Fig. 5, 6), flavone (Fig. 7), and flavanone [22] based on their parent nucleus structures. Among these, xanthohumol (Fig. 8) is a flavonoid unique to hops and has garnered significant attention. It is produced by the hop gland and is a natural isoprenylated flavonoid compound, first isolated by Power et al. [23]. To date, 48 flavonoids have been reported (Tables 1–3). Additionally, hop lactones include colorless cyanidin [35] and colorless delphinidin [35] (Figure 9).

1.3 Volatile Oils

The volatile components in hops are collectively referred to as hop essential oils, which are secreted by the hop lupulin glands and serve as an important source of aroma in hops. Hop essential oils have a complex chemical composition, primarily divided into three categories: hydrocarbons, oxygen-containing compounds, and sulfur-containing compounds, with oxygen-containing compounds being the main components of hop essential oils [35]. Kac et al. [36] analyzed the volatile components of hops from different regions using GC, identifying over 200 compounds. Li Hong et al. [37] analyzed the volatile components of hop oil using HS-GC-MS, identifying and characterizing 74 compounds.

1.4 Polysaccharides

Polysaccharides are also chemical components of hops and possess certain biological activity; however, research on polysaccharides in hops is currently limited. Qiao Xi Xi [38] analyzed hops polysaccharides and found that the monosaccharide composition of hops polysaccharides includes D-arabinose, D-galactose, L-( -) -fucose, L-rhamnose, D-mannose, D-galacturonic acid, and D-glucuronic acid, with molar ratios of 10.77:3.91:2.19:0.33:0.20:0.31:0.57.

1.5 Other

Forino et al. [7] isolated hu-mudifucol (Figure 10) and cascadone (Figure 11) from the beer flower cultivar “Cascade.”

2 Pharmacological effects

According to the “Manual of Traditional Chinese Medicinal Plants of Xinjiang,” hops have a bitter taste, are slightly cool, non-toxic, and possess stomachic, digestive, sedative, and diuretic properties. They are used for indigestion, abdominal distension, pulmonary tuberculosis, cystitis, neurasthenia, and insomnia [1]. The “Ningxia Materia Medica” records that hops have the functions of strengthening the stomach, calming the mind, anti-tuberculosis, clearing heat, promoting diuresis, and resolving phlegm and stopping cough. They are used for indigestion, hysteria, insomnia, pulmonary tuberculosis, and cough. Externally, they are used to treat scrofula and traumatic infections [39]. The “Sichuan Materia Medica” [40], “Inner Mongolia Flora” [41], and “Loess Plateau Flora” [42] also record that hops can be used medicinally, with effects including clearing heat and promoting diuresis, cooling the blood and reducing fever, and calming the mind. Over the years, research has found that hops have inhibitory effects on various bacteria and exhibit pharmacological activities such as antitumor, antioxidant, sedative-hypnotic, and estrogen-like effects.

2.1 Antibacterial

The α-(β-) acids in hops have good antibacterial effects, especially against G+ bacteria (including micrococci, staphylococci, mycobacteria, and streptococci). The results of studies on the anti-mycobacterial activity of hops indicate that ethanol extracts of hops at different concentrations (4, 8 g/L) exhibit significant inhibitory effects on both sensitive and resistant strains of Mycobacterium tuberculosis resistant to rifampicin [43]. Lian Wenqi [44] used α-acid and β-acid isolated through experimental separation to determine their inhibitory effects on Escherichia coli, Bacillus subtilis, and Staphylococcus aureus. The results showed that as the concentration of α- and β-acid solutions increased, the inhibitory effect became more pronounced, and the inhibition zone expanded.

2.2 Antioxidant

Total flavonoids in hops exhibit strong antioxidant activity [45], with reducing capacity lower than that of vitamin C at the same concentration but significantly higher than that of BHT at the same concentration [46]. Guo Miao et al. [47] isolated polyphenol extracts from spent hops (containing 489.6 mg·g^(−1) of procyanidin, flavonoids 261.8 mg·g^(−1)), and administered orally to high-fat mice at doses of 200–800 mg·kg^(−1)·d^(−1) for five weeks. The results indicated that hop polyphenols exhibit significant lipid-lowering and antioxidant effects in high-lipid mice. Qiao Xi Xi [38] employed five in vitro evaluation systems, including DPPH radical scavenging capacity, hydroxyl radical scavenging capacity, nitrite scavenging capacity, reducing power measurement, and β-carotene-linoleic acid emulsion method, to investigate the in vitro antioxidant activity of hop polysaccharides. The results indicated that hop polysaccharide extracts exhibited certain antioxidant activity in different antioxidant systems. Among these, hop polysaccharides demonstrated relatively stronger total antioxidant activity and scavenging capacity for hydroxyl radicals and nitrosyl radicals in the β-carotene-linoleic acid emulsion evaluation system.

2.3 Antitumor

Modern research has found that humulone in hops has antitumor activity. Compared with the components in water-soluble artichoke extract and citrus flavonoids in naringin, humulone can more effectively inhibit the in vitro proliferation of human liver tumor cells [48]. Miranda et al. [49] conducted in vitro antiproliferative experiments on breast cancer cells (MCCF-7), cloned cancer cells (HT-29), and ovarian cancer cells (A-2780) in vitro. The results showed that humulone exhibited a significant dose-dependent inhibitory effect on the growth of several tumor cells without causing toxicity to normal cells, with the most pronounced inhibitory effect on ovarian cancer cells, followed by breast cancer cells. Li Mingze et al. [50] found that isomatrine exhibited strong inhibitory effects on human gastric adenocarcinoma cells (SGC-7901) and human liver cancer cells (HcpG-2), with IC₅₀ values of 13.4 and 11.58 mg/L, respectively.

2.4 Sedative and hypnotic effects

Sedative and analgesic effects are one of the most traditional uses of hops. The German Commission E recommends hops for the treatment of emotional disorders, anxiety, and sleep disorders [51]. In a trial involving 184 patients with mild insomnia, under the same conditions, the combined use of hops extract (83.8 mg) and valerian extract (374 mg) resulted in significantly improved sleep quality in patients, with no rebound of insomnia [52]. Xie Wei et al. [53] investigated the sedative and hypnotic effects of a hops-containing decoction on mice. The results indicated that the decoction significantly reduced spontaneous activity in mice and significantly prolonged the sleep time induced by pentobarbital sodium, suggesting that the decoction has certain hypnotic effects.

2.5 Estrogen-like effects

With the increasing research and development of natural plant hormone substances, they are being increasingly applied to treat diseases related to hormonal abnormalities. Miligan [54] found that phenolic compounds such as humulone and isohumulone in hops exhibit estrogen-like effects, competing with 17-β estradiol for estrogen receptor binding, with an affinity 1/10 that of estradiol. Wang Jiangbi et al. [55] investigated the effects of hop water extract on ovariectomized obese rats, showing that after ovariectomy, the rats' body weight gradually increased. Oral administration of hop extract inhibited the weight gain of ovariectomized rats and improved insulin sensitivity while reducing insulin resistance.

2.6  Blood glucose-lowering effects

Recent studies have found that isohumulone [56] and humulone [57] in hops can reduce serum glucose, triglyceride, and FAA levels in KK-A(y) model mice. Liu Yumei et al. [58] investigated the therapeutic effects of hops supercritical CO₂ extract, β-acid, and hexahydro β-acid on a diabetic mouse model induced by tetrahydroquinoline replication. They found that hop supercritical CO₂ extract, β-acid, and hexahydro β-acid all had therapeutic effects on diabetic symptoms in mice, and this effect was positively correlated with the dose of the administered sample.

2.7 Antiviral

Hops also exhibit antiviral activity. Buckwold et al. [59] found that humulone inhibits the bovine viral diarrhea virus (BVDV), herpes simplex virus 1 (HSV-1), and herpes simplex virus 2 (HSV-2). Additionally, Wang et al. [60] found that humulone inhibits HIV-1. Lou et al. [61] studied the inhibitory effect of humulone on hepatitis C virus and found that humulone significantly inhibits hepatitis C virus RNA levels at 3.53 μmol/L, indicating that humulone has antiviral activity against hepatitis C virus.

3 Conclusion

Hops, as an indispensable raw material in beer production, have always attracted significant attention. However, this has also limited their application beyond brewing, neglecting their other potential uses. However, with the advancement of scientific and technological methods, research on hops has become more in-depth, gradually identifying its primary active components, such as α- and β-acids and humulone, which have been proven to possess a series of pharmacological activities, including antibacterial, antitumor, antioxidant, estrogen-like, hypoglycemic, and antiviral effects. Hops possess broad application prospects and development potential, which remain to be further explored and utilized.

References

[1]Xinjiang Military Region Logistics Department Health Bureau. Manual of Traditional Chinese Herbal Medicine in Xinjiang [M]. Urumqi: Xinjiang People's Publishing House, 1970.

[2]State Administration of Traditional Chinese Medicine. Chinese Materia Medica [M]. Shanghai: Shanghai Science and Technology Press, 2005.

[3]Zhu Enjun. Current Status of Development and Application of Hops Products at Home and Abroad [J]. Journal of Agricultural Products Processing, 2006(5): 60.

[4]Luo Huimin, Li Lin. Study on Polyphenolic Compounds and Bitter Substances in Hops [J]. Beer Technology, 2002(9): 68.

[5]Liu Yumei. Research Progress on the Chemical Composition and Pharmacological Effects of Hops [J]. Food Science, 2009, 30(23): 521.

[6]Xiong Haoping. Study on the Chemical Composition of Hops and the Purification Technology of Humulone [D]. Hangzhou: Zhejiang University, 2005.

[7]Martino Forino, Simona Pace, Giuseppina Chianese, et al. Humidifucol and bioactive prenylated polyphenols from hops (Humulus lupulus cv. “Cascade”) [J]. J Nat Prod, 2016, 79(3): 590.

[8]Liu Shaqing. Review of bitter substances in hops [J]. Beer Science and Technology, 2007 (4): 22.

[9]Chen Libin, Pan Zongjie. Effect of storage temperature on the chemical composition of granular hops [J]. Beer Science and Technology, 2004(2): 61.

[10]Lin Lin. Study on the Influence of Polyphenolic Compounds on the Non-biological Stability of Beer [D]. Qingdao: Qingdao University of Science and Technology, 2010.

[11]Wang Chaoqun, Gu Fanghong, Duan Kaihong. Determination of Polyphenolic Compounds in Different Hops [J]. Food and Fermentation Industry, 2010(12): 181.

[12]Kowalczyk D, Wieca M, Cichocka J, et al. The phenolic content and antioxidant activity of the aqueous and hydroalcoholic extracts of hops and their pellets [J]. Journal of the Institute of Brewing, 2013, 119(3): 103.

[13]Li Yongqiang.  The effect of hop addition on the formation of carbonyl compounds during beer storage [J]. Beer Science and Technology, 2011 (11): 60.

[14]Wang Xuping, Yang Lei, Yang Xiaolan, et al. Purification of hop polyphenols using macroporous resin and analysis of their composition [J]. Food Science, 2013, 34(22): 15.

[15]McMurrough, I. High-performance liquid chromatography of flavonoids in barley and hops [J]. J Chromatogr A, 1981, 218: 683.

[16]Li Yumei. Study on the original anthocyanins in beer raw materials [D]. Wuxi: Jiangnan University, 2014.

[17]Taylor A W，Elizabeth B，Kennedy J A，et al.Hop  (Humulus lupulus L. ) proanthocyanidins characterized by mass spectrometry， acid catalysis，and gel permeation chromatography. [J］.  J Agric  Food Chem，2003，51 ( 14) :4101.

[18] Mc Murrough I.High-performance liquid chromatography of fla- vonoids in barley and hops[J］.  J Chromatogr A，1981，218 : 683.

[19]Jerumanis  J. Quantitative  analysis  ofinbarley，hops  and  beer  by high-performance liquid  chromatography   ( HPLC) [J］. J  Inst Brew，1985，91 (4) : 250.

[20]Li  H J，Deinzer M L.Structural identification and distribution of  proanthocyanidins in 13 different hops[J］.  J Agric Food Chem， 2006，54( 11) : 4048.

[21]Stevens J F，Miranda  C L，Wolthers K R , et al.  Identification and in vitro biological activities of hop proanthocyanidins : inhibition of nNOS  activity  and  scavenging  of  reactive  nitrogen  species[J］.  J Agric Food Chem，2002，50( 12) : 3435.

[22] Li Jun, Cui Chengbin, Cai Bing, et al. Research progress on hop flavonoids [J]. Chinese Herbal Medicine, 2008, 39(7): 1110.

[23]Power F B，Tutin F，Rogerson H.  CXXXV. —The constituents of hops[J］.  J Chem Soc Trans，1913，103 : 1267.

[24]Sun  S  S，Watanabe  S，Saito  T.Chalcones  from  Humulus  lupulus [J］.  Phytochemistry，1989，28 (6) : 1776.

[25]Stevens J F，Ivancic M，Hsu V L，et al.Prenylflavonoids from Hu- mulus lupulus[J］.  Phytochemistry，1997，44(8) : 1575.

[26]Stevens J F，Taylor A W，Nickerson G B，et al.  Prenylflavonoid  variation in Humulus lupulus : distribution and taxonomic signifi- cance of xanthogalenol and 4 '-O-methylxanthohumol[J］.Phyto- chemistry，2000，53 (7) : 759.

[27]Zhao F，Nozawa H，Daikonnya A，et al.  Inhibitors of nitric oxide production from  hops   ( Humulus  lupulus  L. ) [J］. Biol  Pharm Bull，2003，26( 1) : 61.

[28]Chadwick  L R , Nikolic D，Burdette J E，et al.  Estrogens and Con- geners from  Spent  Hops   ( Humulus  lupulus) [J］. J  Nat  Prod， 2004，67 ( 12) : 2024.

[29]Zhao  F，Watanabe  Y ，Nozawa  H ，et al. Prenylflavonoids  and phloroglucinol derivatives from  hops  ( Humulus  lupulus) [J］.J Nat Prod，2005，68 ( 1) : 43.

[30]Zhang, W., Wang, S., Fu, C., et al. Flavonoids in hops [J]. Chinese Journal of Traditional Chinese Medicine, 2013, 38 (10): 1539.

[31]Etteldorf N, Etteldorf N, Becker H. New chalcones from hop Humulus lupulus L [J]. Z Naturforsch C, 1999, 54(7/8): 610.

[32]Li Jun. Study on the chemical constituents and antitumor activity of hops [D]. Tianjin: Tianjin University, 2007.

[33]J Hubáek，M Trojna.Papierchromatographie  der flavonol-glykoside des hopfens  (Humulus lupulus L. ) [J］.Coll  Czech  Chem Commun，1964，29 : 1259.

[34]J Hubáek.Paper chromatography of flavonoids from hops  (Humulus  lupulus  L.  ) . III. Identification  of  apigenin ，apigenin 7-glucoside ，and  quercetin  7-glucoside[J］.Coll  Czech  Chem Commun，1970，35 ( 10) : 3119.

[35]Yin Haibo, Wang Ying, Zheng Taikun, et al. Research progress on Chinese hop plants [J]. Journal of Liaoning University of Traditional Chinese Medicine, 2001, 3 (1): 60.

[36]Kac M, Kralj D. Studying biodiversity of hop (Humulus lupulus L.) accessions from the composition of their essential oils [J]. Acta Horticulturae, 1998, 476(476): 313.

[37]Li Hong, Wang Yunchuan, Zhang Wujiu, et al. HS-GC/MS analysis of hop oil [J]. Food and Fermentation Industry, 2004, 30(10): 52.

[38]Qiao Qianqian. Purification and antioxidant activity of hop polysaccharides [D]. Urumqi: Xinjiang University, 2012.

[39]Xing Shrui.  Ningxia Materia Medica [M].  Yinchuan: Ningxia People's Publishing House, 2006.

[40]Chinese Academy of Sciences.  Sichuan Materia Medica [M].  Chengdu: Sichuan People's Publishing House, 1960.

[41]Ma Yuquan.  Inner Mongolia Flora [M].  Hohhot: Inner Mongolia People's Publishing House, 1985.

[42]Fu Kunjun. Flora of the Loess Plateau [M]. Beijing: Science Press, 2000.

[43]Serkani J E, Isfahani BN, Safaei HG, et al. Evaluation of the effect of Humulus lupulus alcoholic extract on rifampin-sensitive and resistant isolates of Mycobacterium tuberculosis [J]. Res Pharm Sci, 2012, 7 (4): 235.

[44]Lian Wenqi. Optimization of the separation and extraction process of α-acid and β-acid from hops and analysis of their antibacterial activity [D]. Lanzhou: Gansu Agricultural University, 2012.

[45]Zhang Chuan, Kan Jianquan, Chen Zongdao. Research progress on the anticancer effects of bioflavonoids [J]. Chinese Food Additives, 2003 (3): 17.

[46]Cao Weiguo, Liu Zhiqin, Shao Yun, et al. Research Progress on the Pharmacological Effects of Flavonoid Compounds [J]. Journal of Northwest Plant Science, 2003, 23 (12): 2241.

[47]Guo Miao, Yang Xiaolan. Lipid-lowering and antioxidant effects of hop polyphenols on hyperlipidemic mice [J]. Food Science, 2015, 36(3): 183.

[48]Ho Y  C，Liu C H，Chen C N，et al.  Inhibitory effects of xanthohu- mol from  hops   ( Humulus  lupulus  L.  )  on  human  hepatocellular  carcinoma cell lines[J］.  Phytother Res，2008，22( 11) : 1465.

[49]Miranda  C L，Stevens J F，Helmrich A，et al.  Antiproliferative and  cytotoxic effects of prenylated flavonoids from hops  (Humulus lu- pulus )  in  human  cancer  cell lines[J］. Food  Chem  Toxicol， 1999，37 (4) : 271.

[50]Li Mingze, Tian Song, Yu Lei, et al. In vitro antitumor activity and mechanism of isoharpypin in hops [J]. Journal of Harbin University of Commerce: Natural Science Edition, 2007, 23 (1): 9.

[51]Blumenthal, M. The Complete German Commission E Monograph: Therapeutic Guide to Herbal Medicines [M]. Austin: American Botanical Council, 1998.

[52]Morin C M, Uwe K, Célyne B, et al. Valerian-hops combination and diphenhydramine for treating insomnia: a randomized placebo-controlled clinical trial [J]. Sleep, 2005, 28 (11): 1465.

[53]Xie Wei, Zhao Zhiqiang, Han Shengyin, et al. Pharmacological effects of the decoction of Mian De An: Sedative and hypnotic effects [J]. Journal of Ningxia Medical University, 1996(3):7.

[54]Milligan SR, Kalita JC, Pocock V, et al. The endocrine activities of 8-prenylnaringenin and related hop (Humulus lupulus L.) flavonoids [J]. J Clin Endocrinol Metab, 2001, 85 (12): 4912.

[55]Wang Jiangbi, Luo Rong, Tian Xuesong, et al. Effects of hops on ovariectomized obese rats [D]. Lanzhou: Lanzhou University, 2004.

[56]Yajima  H，Ikeshima  E，Shiraki  M ，et  al.Isohumulones ，bitter  acids derived from hops，activate both peroxisome proliferator-ac- tivated receptor  alpha  and  gamma  and  reduce insulin resistance  [J］.  J Biol Chem，2004，279(32) : 33456.

[57]Nozawa  H.  Xanthohumol，the chalcone from beer hops  (Humulus lupulus L. ) ，is the ligand for farnesoid X receptor and ameliorates lipid and glucose metabolism in  KK-A ( y) mice [J］.Biochem Biophys Res Commun，2005，336(3) :754.

[58] Liu, Yumei, and Tang, Jian. “Study on the effect of hop bitter acid on blood glucose regulation.” Food Science, 2009, 30(3): 256.

[59]Buckwold V E，Wilson R J H，Nalca A，et al.Antiviral activity of hop constituents against a series of DNA and  RNA viruses[J］.  Antiviral Res，2004，61 ( 1) : 57.

[60]Wang  Q，Ding  Z  H，Liu J  K，et  al.Xanthohumol ，a  novel  anti- HIV-1 agent purified from hops Humulus lupulus [J］.Antiviral  Res，2004，64(3) : 189.

[61]Lou  S，Zheng Y M，Liu S L，et al.Inhibition of hepatitis C virus replication  in vitro  by  xanthohumol，a  natural  product  present  in hops [J］.  Planta Med，2014，80(2) : 171.