Hops

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Hops are added to impart bitterness, a pleasant hoppy aroma and enhanced microbiological stability to a beer. Hop (Humulus lupulus L.) is a dioecious plant belonging to the family of cannabaceae. For brewing purposes, the unfertilized hop cones of female plants are preferably used. Dried hop cones contain up to 4% of polyphenols.

the common hop, Humulus lupulus, is a climbing herbaceous perennial vine that belongs to the family Cannabaceae. Its original name was given by the Romans, Lupulus salictarius [23]. For brewing, only the female plant flowers (pine-like cones) are used. Hops are used as a spice in the brewing process and only 200–600 g/hL of beer. Many hop varieties contribute to beer with two main characteristics: bitterness and aromas. However, some have more pronounced use for their bitterness and others for aromas [61]. Among many different compounds, resins and essential oils are the most important compounds for the brewing industry, but phenolic compounds, important for the flavor of the finished beer, are significant as well [61]. Hard resins appear as a result of oxidation and polymerization reactions of the soft resins substances. Humulones (α-acids) and lupulones (β-acids) belong to soft resins. They exhibit lower pH values because they contain a phenolic group that can release a hydrogen ion (H+) [61]. The α-acids get extracted during wort boiling when they undergo oxidative isomerization to iso-α-acids (iso-humulones), also recognized as bitter compounds of beer (Figure 3). Circa 70% of beer bitterness is a result of isomerization [61].

With only a small portion being marketed as raw hops, most of the hops produced are processed into hop pellets, hop extracts and isomerized products. Hop pellets are produced with different degrees of enrichment of lupulin (type 45 or 90, with pellets type 45 yielding 45 kg concentrated pellets from 100 kg of raw hops). For the production of hop pellets, the raw hops are dried (50 to 60 °C for 6 to 10 hr), finely ground and then pelletized.

Hops give beer its bitter taste and influences the aroma. The use of hops as the sole flavoring agent in beer began in the 14th century.

In beer, hops are added during the boil to provide bitterness, inhibit later bacterial growth, and enhance beer foam stability (Almaguer et al., 2014). Hops (Humulus lupulus L.) is a perennial vine whose female flower, hop cones, provide a source of secondary metabolites which provide the desired properties. Glandular trichomes of the hop cones produce secreted metabolic compounds: hop bitter acids, prenylated-flavonoids, and essential oils (Champagne and Boutry, 2016). Although hops’ contribution to the overall beer proteome is negligible, hops have a proteome that is involved in the biochemical process that drives hop function and relevance to beer production and quality.

Hop cones, with an inordinate extra load of leaf material, in some cases showed improvements in beer stability and in the destruction of peroxide that forms in the brewhouse operations. Green hopped beer also showed remarkable stability, judged by the sustained appearance of protein thiols, as well as from sensory evaluation over years.

Bitterness is an important property of beers and according to several authors, about 80% of bitterness originates from hops during boiling [109,110,111]. As mentioned before, the female hop cones and their soft resins rich in α-acids (cohumulone, humulone, adhumulone) and β-acids (colupulone, lupulone, adlupulone) are used in brewing. The major bitter compounds in beer are iso-α-acids [112]. The ratio of trans/cis stereoisomers for standard beers is close to 3:7 beers [113], or 68:32 in favor of the cis-compounds. The half-life of cis-compounds is approx. five years, and of trans-isomers is about a year. This makes cis-forms much more stable [114]. The isomerized α-acids are intensely bitter, and they represent the typically bitter beer taste. Their concentrations range from 15 (American lagers)–100 ppm (bitter English ales). However, bitter taste in beer is modified with residual sugars and results in pleasant bitterness for the consumer [114]. Isomerized α-acids have tensioactive properties, which stabilize the beer foam, and they act inhibitory to Gram-positive bacteria, while lactic acid bacteria in beer exhibit resistance to iso-α-acids. Bittering procedures have evolved and have transferred the use of hops into almost all brewing stages, such as post-fermentation bittering, or dry hopping. Different hopping products are available as an aqueous extract or in pellet form. Such products have higher levels of cis-isomers relative to trans-isomers, and this results in a lower trans/cis ratio [115]. Chemically reduced derivatives of iso-α-acids (light stable tetrahydro-iso-humulones (tetra) and hexahydro-iso-humulones (hexa)) can ensure bitterness. The availability of the hop extends to different forms (cones, pellets, plugs) that can be added at different stages of the brewing process. Dry-hopping is a method of soaking hops in beer during fermentation or conditioning in order to add different aromas and flavors to the finished beer. This method grants oxidized α-acids, humulinones, to beer. Their concentrations in hop leaves and pellets range from 0.2–0.5% w/w [116,117,118]. Originating from hops, polyphenols contribute to bitterness as well [119]. Moreover, they are recognized as important acceptance factors in different beverages, including beer [120]. Flavan-3-ol monomers such as (+)-catechin and (−)-epicatechin can also impart bitterness to beer [7,121,122].

Grassy reminds one of freshly cut grass or must, and can also be a result of inappropriate hops storage. To avoid this, fresh hops should be used or stored properly.

Galena spent-hop (material remaining post-critical CO2 extraction) extract itself was characterized by intense fig and fruit-like aromas and when dosed into the base beer, provided remarkable hoppy aroma and flavor. Goldstein et al.46 report that these observations could be related to glycoside flavor precursors found in the spent hop material. According to the authors, water soluble glycosidic hop flavor precursors may undergo chemical or enzymatic hydrolysis to create a variety of flavor-active compounds that ultimately impact the overall hop aroma and flavor of the beer.

Hop CO2 extract contains hardly any polyphenols. Its usage causes lowered antioxidant activity, increased carbonyl formation, and ultimately increased staleness on sensory evaluation. However, the lower amount of phenolic compounds does lower harsh bitterness to some degree. Beers produced by the classical technology (i.e., use of malt and hop pellets ), were evaluated as superior even after six and nine months of storage.

Forster40 has conducted several brewing trials that corroborate these effects observed with pellets and spent hops on beer flavor stability. Forster brewed beer with pellets (PP rich) and CO2 extract (PP free). Results indicate that pellet hopped beers had a more pleasant aroma than beers hopped with CO2 extract and that pellet hopped beers aged slightly better than extract hopped beers. In a second experiment Forster brewed with PP rich spent hops (derived from processing of type-45 pellets) at the beginning and at the end of boil. Color and foam were not influenced, but the beer brewed with hop PPs did affect physical stability, especially when boiled for 90 min. The beers containing spent hops could be described as pleasant, hoppy, and slightly fruity in aroma and taste. Beers that were dosed at 600 g/hL at 90 min did possess a marked bitterness, however no harsh bitterness occurred with shorter boiling times. The majority of tasters judged PP rich beers positively, even after 4 weeks of storage at 27°C. Beer without the hop PP addition was deemed undrinkable and aged after 4 weeks of storage at 27°C.

Use of CO2 hop extract (which contains no phenolic content) is very common among in industrial beers.

In modern brewing, hop pellets and extracts are increasingly used to replace whole hops. Pure resin extract comprising hop acids and hop oil is added during wort boiling (conventional hopping) whether or not in combination with whole hops or hop pel- lets. Among the advantages of pure resin extract over whole hops and hop pellets are a higher bulk density, a more homogeneous product, reduced wort losses, lower levels of undesirable hop constituents in the final beer, and more consistent beer bitterness levels. Alternatively, non-reduced or reduced isomerized hop α-acids can be used together with hop essential oils, a combination often described as advanced hopping. This relatively new technology shows the benefit of yielding highly reproducible beer flavoring in terms of both hop-derived bitterness and aroma. However, since none of the above described commercially available hop extracts contain substantial amounts of hop polyphenols, both conventional hopping using pure resin extract and advanced hopping using isomerized hop extract results in beers with a very low concentration of hop polyphenols or no hop polyphenols at all.

Hops inhibit gram-positive bacteria by disrupting their cell membranes.

Hops may contribute sulfur-based aromas. Most notable is the familiar skunky, rubbery, light-struck smell common in import beers bottled in clear or green glass. These bottles offer no protection from the blue wavelength of light, which passes through the glass and reacts with alpha acids in hops to produce 3-Methyl-2-butene-1-thiol.

Hops can produce other sulfur aromatics as well. The old-time American hop, Cluster, as well as other U.S. varieties, can have a cat-pee or blackcurrant-leaf aroma that is obviously not to everyone’s taste—but all are sulfur compounds. Other hop varieties can smell strongly of toasted onion or garlic. A hop called Summit is famous for this, but much of the 2014 crop of Citra was pretty well loaded with it, resulting in a lot of IPAs out there last year that drank pretty much like a nice hoppy beer with an onion bagel perched on the rim.

Forster et al (21) have attempted to exploit some of the potential advantages of hop polyphenols in brewing by using a hop bracteole-enriched fraction containing hop polyphenols, which is obtained by mechanical separation of the vegetative hop cone bracteoles from the lupulin glands during the preparation of lupulin-enriched hop pellets (T45 pellets). It was found that beers to which this polyphenol-rich bracteole fraction was added during wort boiling had an increased polyphenol level, a higher reducing power, and a more pleasant taste compared to a reference beer prepared without the bracteole fraction. On the other hand, two drawbacks became apparent in the beers brewed with addition of the polyphenol-rich bracteole fraction: these beers had a significantly higher nitrate level than the reference beer, and in addition, the polyphenol-supplemented beers were more turbid and thus showed an inferior colloidal stability. The possible role of hop polyphenols in brewing was also examined by Mikyska et al (32) by comparing the stability of brews hopped with hop pellets versus brews hopped with polyphenol-free non-isomerized hop extract. These authors concluded that hop polyphenols, introduced via pellets, were beneficial to beer flavor stability. However, using conventional pellet hopping, it is clear that in addition to hop polyphenols, numerous other hop constituents with unknown pro- or antioxidant effects also enter the wort and beer matrix.

Beer contains from 20–60 mg/L of iso-humulones, and oxidized β-acids sum up to the rest of the bitterness sensation. Besides resins, hops contain 0.5–3.0% of essential oils, which provide certain beer flavors.

Valuable hop ingredients are grouped into hard and soft resins, polyphenols and hop essential oils. Hop resins are located in the lupulin glands. Total resins comprising 15% to 30% of the hop cone are obtained by extraction with ether or methanol. Soft resins containing the main bittering substances (α- and β-acids) are subsequently extracted with n-hexane. The part that is insoluble in n-hexane is termed hard resin. Hard resin phenolics comprise prenylflavonoids (total prenyflavonoids in commercial hop cultivars: 0.1 to 0.59 g/100 g hop cones) and multifidol glucosides (total concentration of co-, n-, ad-multifidol glucopyranosides in hop pellets type 90 (Herkules): 2.4 g/100 g). In hop hard resin, xanthohumol is accompanied by 13 other prenylated chalcone compounds, all of which are present at 10- to 100- fold lower concentrations than xanthohumol, and by a chalcone bearing a geranyl moiety. Prenylchalcones or prenylflavonoids are only found in a limited number of plant families, whereas xanthohumol is solely found in hops. The other phenolic compounds, which are part of the hard resin, co-, n-, and ad-multifidol glucopyranosides, are implicated in the bitter taste of hop hard resins. These compounds are (acyl-) phloroglucinol derivatives, just like hop α- and β-acids.

Hop resins are located in lupulin glands at the base of female cones and are classified as soft and hard resins. Soft resins are soluble in n- hexane and contain bittering substances ( α - and β-acids), while the insoluble part corresponds to the hard resins containing mainly pre­ nylflavonoids and flavanones. Prenylflavonoids have been identified in high levels and almost exclusively in hop plants (up to 10 g/kg hop cones) (Knez Hrnčič et al., 2019). Hop hard resins are an exclusive source of XN, in concentrations 10 to 100-fold higher comparing to other related compounds and representing 80–90% of the total prenylated flavonoids (up to approximately 1% w/w hop cone) (Knez Hrnčič et al., 2019; Sanz et al., 2019). The assessment of different hop products (pellets and spent hops) indicated that XN was at the highest concentration in all the considered products (164–592 mg/100 g), followed by catechin (106–365 mg/100 g) and proanthocyanidins (PC B3: 47–200 mg/100 g, PD B3: 3.1–20.3 mg/100 g). Rutin and ferulic acid were found in concentration between 50 and 133 mg/100 g and 4.3–14.1 mg/100 g, respectively (Goiris et al., 2014). A recent work intended to compare hop cone ethanolic extracts obtained from three cultivars (Magnum, Lubelski, and Marynka) reported high levels of chlorogenic acid (133.7–1077.0 μ g/g dw), epicatechin (343.5–2085.5 μ g/g dw), quercetin (222.9–395.1 μ g/g dw) and rutin (644.8–1764.7 μ g/g dw) among the studied hop varieties (Kobus- Cisowska et al., 2019). In another study, conducted in Magnum Halleraur and Saaz hop varieties, the most abundant phenolic compounds were p- hydroxyphenylacetic acid (19.32–55.86 mg/100 g), catechin (7.48–182.00 mg/100 g), epicatechin gallate (68.92–357.20 mg/100 g), myricetin (9.87–39.86 mg/100 g) and rutin (10.57–51.35 mg/100 g). Quercetin, caffeic acid, sinapinic acid, p-hydroxybenzoic acid, p-cou­ maric acid and kaempferol were present in concentrations from 1.2 to 7.7 mg/100 g, and vanillic acid in concentrations up to 0.28 mg/100 g (Šibalić et al., 2021). Hops are usually commercialized and used in the brewing process in the form of processed products such as hop pellets, extracts and iso­ merized products. Hop pellets are produced using finely ground dried raw hops which are then pelletized. They may thus contain lower phe­ nolics than hop cones due to drying and grinding steps. Given the fact that the polyphenol content of hop extracts depends on the extraction solvent, supercritical CO 2 is usually the first choice by the brewing in­ dustry as it is more suitable for the solubilisation of soft resins. These extracts contain a low content of prenylated chalcones and flavanones since supercritical CO 2 is relatively non-polar. For this reason, the spent hops resulted from CO 2 extraction are characterized by higher content of prenylated chalcones such as XN (above 30%) (Goiris et al., 2014; Knez Hrnčič et al., 2019).

Magnum is known as a bitter variety and contains relatively low levels of spicy compounds, whereas Saaz and Halleratuer Tradition varieties are characterised by higher levels of sesquiterpene oxidation products (Praet et al. 2016). Saaz is also known as the most stable variety regarding the changes in hop resins and polyphenols during long-term storage (Mikyška and Krofta 2012). In this research, it was shown that Saaz and Halleratuer Tradition difer also in the content of individual phenolics except in content of vanillic acids, myricetin, and kaempferol (Table 1). [...] The early addition of hops to the boiling kettle imparts highly desirable "spicy/herbal" top notes to lager beers (Praet et al. 2016). Sesquiterpene oxidation products, which are formed during aging of hops and wort boiling, are presumed to be responsible for this "spicy/herbal" favour characteristic.

Be wary of buying hop seeds.

Dry-hopping is a process consisting in the addition of hops to partially or fully fermented wort at low temperature to impart complex and intense hop aroma to beer. Dry-hopping may improve oxidative stability of beer due to its higher content in antioxidant compounds. Moreover, the contents of phenolic acids (gallic, protocatechuic, caffeic, syringic and sinapinic) and catechin, increased up to 2-fold and 10-fold after dry-hopping, respectively (Gerhards et al., 2021). In fact, higher transfer rates of catechin were achieved using dry-hopping (ca. 100% comparing to 66% during con­ventional boiling hopping) (Forster & Gahr, 2013). These compounds are significantly lost due to precipitation in wort, but not when added in beer during dry-hopping. For this reason, the increased levels of haze active substances catechin and proanthocyanidins after dry-hopping may result in lower beer colloidal stability and haze, requiring a more demanding stabilization and filtration steps. However, further studies regarding the individual phenolic compounds are required in order to fully understand the impact of dry-hopping on the phenolic profile of beer.

The addition of hop polyphenols at the onset of wort boiling has no negative impact on yeast performance.

In addition, during the growth of yeast, attention must be devoted to the effects on yeast of endogenous antioxidant components such as phenolic compounds in wort. For yeast, the presence of phenolic com­pounds has been found to result in a 50% reduction in cell growth, a 60–80% reduction in carbon dioxide production, and a 3-4-fold reduc­tion in amino nitrogen consumption, resulting in retardation of fermentation. The interference of phenolic compounds with the growth and metabolism of yeast may be mainly related to the interaction be­tween phenolics and yeast plasma membranes.

Hops contribute a negligible amount of most minerals in beer because of the small quantities used (200 g to produce 100 l beer). However hops make a notable contribution of nitrate to the beer wort. Approximately 5–30 mg of nitrate/l of wort comes from raw hops or type-90 pellets, depending on beer bitterness and hop variety. A reduction of some 60% can be achieved by using type-45 pellets, the reduction is about 90–95% when using pure ethanol resin extracts, and complete elimination is possible, by using a CO 2 extract (Forster, 1989). New hop products, like potassium salts of the dehydro iso-alpha-acids, are now available that can be added at the end of fermentation to produce a light resistant beer in clear glass bottles, which contribute to the potassium content of the beer. Similarly, the potassium salts of iso-alpha-acids are used to replace bittering hops for utilization or economic reasons and to adjust bitterness in beers that were underhopped in the kettle.

See Kunze section 1.2

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• Humulus lupulus – a story that begs to be told. A review - wow
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Plant Polyphenols 2 (downloaded)

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