Monthly Archives: July 2012

Hop Science III: Bitterness

Disclaimer: Please note that I wrote this in 2012, and the field has advanced considerably since then, so information might not be up-to-date!

This is part III of the mini-essay on hop science and factors influencing hop flavor and aroma in beer. The previous part (II – Hop Oil Composition) can be found here. The topic for this post will be bitterness and how the major components of hop resin contributes to the flavor and bitterness of beer.

One of the primary uses (if not the most important use) of hops in beer brewing, is to contribute bitterness to the flavor. Hops have been used for centuries as the prime bittering agent in beer, but hops have not been the sole or original bittering agent for beer, as a variety of different herbs were used far before the use of hops became popularized. Bitterness is usually thought of as an acquired taste, i.e. something you get used to and learn to like, and bitterness perception also varies from person to person, as some can be more sensitive to it compared to others. It is also usually agreed upon that humans have through evolution learned to become alert to bitter food, as it may be a sign of poison or a possible dietary danger, and hence very bitter beers can usually be very off-putting to a person that is not used to the flavor. Bitterness research is therefore complex, but thankfully a lot of knowledge has been gained in the area.

The general structure of an alpha-acid. For humulone, R=CH2CH(CH3)2, for cohumulone, R=CH(CH3)2, and for adhumulone, R=CH(CH3)CH2CH3.

The major contributors to hop bitterness in beer are isomerized alpha-acids, however it has been found that these are not the only compounds that contribute to the bitterness (others include products derived from beta acids and polyphenols). The iso-alpha-acids are derived from alpha-acids present in the lupulin glands of the hop flower cones, and these alpha-acids are then isomerized primarily during the wort boil into iso-alpha-acids. Dried hops typically contain around 2-20% alpha-acids by weight. The three major alpha-acids present in hop resin are humulone, cohumulone and adhumulone, while two minor alpha-acids, prehumulone and posthumulone, have also been identified. These alpha-acids are all weak acids (a pKa of around 5.5), structurally related (they differ slightly via their side-chain), and are poorly soluble in acidic aqueous solutions (approximately 50 ppm for humulone in 25° C water at pH 5 and 10 ppm for humulone in 25° C water at pH 4, while a limit of 14 ppm for alpha-acids in a lager beer has also been reported). Non-isomerized alpha-acids are usually regarded as having only a slight bitter flavor, which can be tested by tasting raw hop cones or pellets, but the bitter flavor of the non-isomerized alpha-acids is not nearly as intense as the flavor of their isomerized counterparts (which is nine times stronger). The flavor threshold is also close to the maximum solubility of the alpha-acids in beer (Intelmann et al. (2009) recorded flavor thresholds of 8 ppm, 6 ppm, and 8 ppm for humulone, cohumulone and adhumulone respectively in water by a trained sensory panel, while the half-maximum bitterness intensity concentration for humulone was over 180 ppm), meaning they don’t contribute to any of the bitterness in beer.

The isomerization reaction of humulone to iso-humulone.

Thermal isomerization of the alpha-acids to iso-alpha-acids occurs via an acyloin-type ring contraction, and the reaction rate is increased with increasing temperatures. Isomerization still occurs at pre-boil temperatures, but the reaction rate at 90° C is approximately half of the reaction rate at 100° C. Isomerization forms two epimers, a trans– and a cis-iso-alpha-acid (forming in a ratio of around 1:3 in wort), meaning 6 different iso-alpha-acids are formed from the 3 major alpha-acids present in hop resin. The trans-isomers are both less bitter in flavor and less stable (i.e. more prone to oxidation) than the cis-isomers, meaning the cis-isomers are usually more desired. Foam-stabilizing properties of the cis-isomer are though slightly weaker compared to the trans-isomer. The iso-alpha-acids have pKa values around 3.0 and have much higher solubility in wort than the non-isomerized alpha-acids (predicted solubility of trans-isohumulone in water 72.5 ppm). This, along with the fact that the flavor threshold of iso-alpha-acids in water is around 6 ppm, explains why the iso-alpha-acids contribute for the majority of the bitterness in beer. The total iso-alpha-acid concentration can reach over 100 ppm in more bitter beers. The limiting factors include the poor solubility of the alpha-acids, the reaction rate of the isomerization, and the reaction rate of the degradation of iso-alpha-acids. Prolonged boil times (over approximately two hours) do not increase the iso-alpha-acid content of the wort, since after this the rate of iso-alpha-acid degradation exceeds that of alpha-acid isomerization, and the iso-alpha-acid starts to decrease. Hence, the maximum amount of bitterness one can get from a 60-minute boil addition (independent of the size of the addition) is approximately 50 IBU (see this Basic Brewing Podcast and report). If one thrives for a higher bitterness, it must be achieved from a combination of hop additions.

Table 1 – Alpha-acid and Cohumulone content of various hop varieties
[table id=3 /]

The type of alpha-acid also presumably influences the quality of bitterness, as it is generally believed that iso-cohumulone contributes a harsher and more unpleasant bitterness compared to iso-humulone. However, recent studies seem to indicate that there isn’t any difference in bittering quality between the two alpha-acids. Because of a slightly higher polarity, cohumulone and iso-cohumulone have slightly higher solubility in wort compared to humulone and iso-humulone, meaning they are usually present in slightly higher quantities. Table 1 contains a table with the typical alpha-acid percentages and cohumulone contents of various hop varieties. Looking at the table, it becomes evident that hop varieties such as Simcoe, Horizon and Warrior are preferred, if one is after a hop variety that has a low-cohumulone, yet high alpha-acid, content.

A quartz cuvette used for IBU assays. (Source)

Beer bitterness is usually measured in International Bitterness Units (IBU), or the almost identical European Bitterness Units (EBU), and it is a measure of the absorbance of a solvent extracted sample of beer at 275 nm. The absorbance is of course dependent on the amount of isomerized alpha-acids present in the sample. 1 IBU approximately converts into to 0.7 ppm iso-alpha-acid, though this depends on the presence of other bittering compouds. The IBU assay is carried out by first acidifying a sample of beer with HCl (to make the bittering acids more hydrophobic), then adding twice the volume of 2,2,4-trimethylpentane (also known as iso-octane). The mixture is then shaken thoroughly for 15 minutes, to allow as much of the (now hydrophobic) acids to transfer to the organic phase. The absorbance of the trimethylpentane phase (organic) is then measured at 275 nm in an ultraviolet spectrophotometer (against a blank). The measured absorbance is then multiplied by 50 to convert into Bitterness Units. Since the absorbance is affected by oxidation products and beta acids as well, it doesn’t give a definite picture of the amount of isomerized alpha-acids in the beer, nor the actual perceived bitterness. HPLC can instead be used to measure the amount of, and types of, isomerized alpha-acids in beer, but it requires more labor and time, as well as more costly equipment.

The general structure of an beta-acid. For lupulone, R=CH2CH(CH3)2, for colupulone, R=CH(CH3)2, and for adlupulone, R=CH(CH3)CH2CH3.

As mentioned, there are also beta-acids present in the lupulin glands of hop cones, but it is generally regarded that these don’t have a large effect on the bitterness in beer. Like the alpha-acids, there are five types typically found in hop resin (lupulone, colupulone, adlupulone, prelupulone and postlupulone) and they are structurally related weak acids (pKa of around 6). The beta-acid content of hops is usually around 1-10% by weight, and the beta-acids usually precipitate out of the wort and beer. Some of the beta-acids can though undergo transformations into bitter-tasting products such as cohulupone, hulupinic acid, nortricyclocolupone, two tricyclocolupone epimers, two dehydrotricyclocolupone epimers, two hydroxytricyclocolupone epimers, and two hydroperoxytricyclocolupone epimers. These transformation products have flavour thresholds only slightly above those of iso-alpha-acids, meaning they most likely in some way contribute to the bitterness of beer. Another series of compounds most likely contributing to the bitterness of beer are polyphenols, which are naturally found in hops, as well as malt. Polyphenols can lend a harsh and astringent flavour to the beer in large concentrations. These polyphenols can transfer to the beer through hop debris or dry hopping (polyphenols are regarded as the cause of dry hop haze), and hence even though dry hopping doesn’t contribute significantly to the amount of isomerized alpha-acids, they do contribute slightly to the bitterness of the beer through the introduction of polyphenols.

As mentioned above, cis-iso-alpha-acids are usually more stable than their trans-counterparts, meaning the ratio between them in beer usually is a good indicator over bitterness stability. To improve the bitterness stability of the beer, various techniques can be applied. The use of pre-isomerized hop products allows for a higher cis:trans ratio, post-boil whirlpool hopping has shown to increase bitterness stability, and high mash temperatures can lead to an increased amount of coagulated protein before to wort boiling and consequently less humulone-loss in the trub, which then leads to higher hop utilisation and bitterness stability. It has also been proposed that the pH of the beer affects iso-alpha-acid oxidation, though the exact effect is unclear. Isomerized alpha-acids can also undergo reduction, to form dihydroiso-alpha-acids (RIAA), tetrahydroiso-alpha-acids (TIAA) and hexahydroiso-alpha acids (HIAA). These acids are also bitter-tasting, and have improved stability against light and also increases foam stability. In water, RIAA is about 0.67 times as bitter as iso-alpha acids, TIAA 2.03 times and HIAA 1.15 times. These reduced iso-alpha-acids have similar bittering qualities to their non-reduced counterparts, and can be added to beer e.g. post-fermentation. These reduced iso-alpha-acids are usually produced using sodium borohydride, and RIAA and TIAA are available on the market for brewers.

As can be seen, hop bitterness is also a complex subject, and the traditional views on hop boiling and bitterness are slightly flawed (e.g. very high IBU values can be achieved by adding hops only post-boil). The next part will also be the final part of the mini-essay, and it will be focusing on combining the theory from this and the previous part, and applying it to actual beer brewing. I should hopefully release it in about two weeks, together with information from a trial brew.


  • Briggs, D., Boulton, C., Brookes, P., Stevens, R., 2004. Brewing: Science and Practice. Cambridge: Woodhead Publishing.
  • Caballero, I., Blanco, C., Porras, M., Iso-alpha-acids, bitterness and loss of beer quality during storage. Trends in Food Science & Technology, in press
  • Intelmann, D., Batram, C., Kuhn, C., Heseleu, G., Meyerhof, W., Hofmann, T., Three TAS2R Bitter Taste Receptors Mediate the Psychophysical Responses to Bitter Compounds of Hops (Humulus lupulus L.) and Beer. Chemosensory Perception 2 (2009) 118-132
  • Kappler, S., Krahl, M., Geissinger, C., Becker, T., Krottenthaler, M., Degradation of Iso-Alpha-Acids During Wort Boiling. Journal of the Institute of Brewing 116 (2010) 332-338
  • Khatib, A., 2006, Studies of iso-alpha-acids: analysis, purification, and stability. Doctoral Dissertation, Leiden University.
  • Kishimoto, T., 2008. Hop-Derived Odorants Contributing to the Aroma Characteristics of Beer. Doctoral Dissertation, Kyoto University.
  • Malowicki, M., Hop Bitter Acid Isomerization and Degradation Kinetics in a Model Wort-Boiling System, M. Sc. Thesis, 2004, Oregon State University
  • Malowicki, M., Shellhammer, T., Isomerization and Degradation Kinetics of Hop (Humulus lupulus) Acids in a Model Wort-Boiling System. Journal of Agricultural and Food Chemistry 53 (2005) 4434-4439
  • Schönberger, C., Kostelecky, T., 125th Anniversary Review: The Role of hops in Brewing. Journal of the Institute of Brewing 117 (2011) 259-267
  • Techakriengkrai, I., Paterson, A., Taidi, B., Piggott, J., Relationships of Sensory Bitterness in Lager Beers to Iso-Alpha-Acid Contents. Journal of the Institute of Brewing 110 (2004) 51-56
  • USA Hops, 2011, Variety Manual – Hop Growers of America (link)

Reviews of my homebrews over at

I sent some homebrews over to Harri at Reittausblogi, hoping to get some objective comments on them, and he was kind enough to taste through them and write up some proper reviews (in Finnish) on them! Thanks!

It was surprising to note however that he had found a sulfur-like (burnt matches) off-aroma in three of the beers, which I at least have never noticed before, nor have I heard anyone else tasting my beers mention it. The beers with the off-aroma use oxygen-absorbing caps, and the bottles were shipped horizontally, so maybe that could explain it? To test this theory I will store a couple of bottles horizontally for a week, and then do a parallel tasting. The beers all use different yeasts, so it is probably not fermentation byproducts. The beers have used the same fermentation vessels, auto-siphon, and kegs, so one of them might be harboring some contaminations causing the off-aroma. I’m completely puzzled, especially since I haven’t noticed anything myself.

Thanks again for taking your time to taste and comment the beers! Much appreciated!

I have a small backlog of stuff to write about, including the third part of the Hop Science mini-essay, a small report on SOPP and a beer tasting we had yesterday, so hopefully these will be released during the week.

SOPP and a small update

Today will hopefully be a good day, as first we are going to finish up our automated brew kettle, then head over to Rautatientori to drink some beer at Suuret Oluet Pienet Panimot (SOPP). I highly recommend a visit (26-28.07, 12-01), as more than 100 microbrews will be available! The microbrewers at SOPP are:

Hollolan Hirvi, Hollola
Koskipanimo / Panimoravintola Plevna, Tampere
Kuninkaankartanon Panimo, Mustiala
Laitilan Wirvoitusjuomatehdas, Laitila
Lakeuden Panimo / Mallaskoski, Seinäjoki
Lammin Sahti, Hämeenlinna
Malmgårdin Panimo, Loviisa
Nokian Panimo, Nokia
Panimoravintola Beer Hunter´s, Pori
Rekolan Panimo, Mäntsälä
Saimaan Juomatehdas / Panimoravintola Bruuveri, Lappeenranta ja Helsinki
Stadin Panimo, Helsinki
Suomenlinnan Panimo, Helsinki
Teerenpeli Yhtiöt, Lahti ja Helsinki
Vakka-Suomen Panimo, Uusikaupunki
Taako / Pihtlan Olut, Saarenmaa, Viro

There hasn’t been much information about the beers available at SOPP, but Beer Hunter’s will be bringing:
Mufloni Falconer’s Flight 4,5%
Mufloni CCCCC IPA 7%
Mufloni Single Hop Simcoe 3,5%
Mufloni Single Hop Galena 3,5%
Mufloni Aamupala Kaurastout 4%
Mufloni Saison De Randonneur 4,5%
Mufloni Cornes à Pois 7%

And Rekolan Panimo (a new brewery which will be making its debut) will bring:

Part III of the Hop Science mini-essay will unfortunately be postponed until early next week, as I still have some text left to write and wont have time to finish it this week.

Homebrew: Imperial IPA (Catnipa) -Bottling and tasting

Today I bottled the keg of my latest Imperial IPA, aka Catnipa. Pia has again designed a nice label for the beer. The beer pours with a slight hazy amber color, and a slight off-white head is formed, that collapses quite quickly, leaving some lacing along the glass and an oily surface. The aroma is very hop-dominated, with dank tones of resin, citrus (grapefruit), and tropical fruits (pineapple and mango). There is some caramel behind the hops as well. The flavor begins with some caramel, but is quickly overtaken by a dank, resiny, piney, citrusy and earthy hoppiness, that clings on until the semi-dry and bitter finish. I’m quite happy with how this one turned out, as it is packed lots of hoppy goodness. Surprisingly drinkable for such a bitter and quite strong (with regards to ABV) beer. Hopefully this improves further with some time in the bottle, and hopefully the aroma stays for a while as well.

Hop Science II: Hop Oil Composition

Disclaimer: Please note that I wrote this in 2012, and the field has advanced considerably since then, so information might not be up-to-date!

This is part II of the mini-essay on hop science and factors influencing hop flavor and aroma in beer. The previous part (I – Introduction) can be found here. The topic for this post will be the composition of hop essential oil and how the individual major components contribute to the aroma and flavor of beer.

A gas chromatograph of the essential hop oil in Savinjski Goldings. Peaks represent different compounds present. (Source)
Hop oil research began in the 19th century, and the first fractions of hop oil were obtained in 1819. Alfred Chapman first identified six major compounds of hop oil in 1894-1895: myrcene, humulene, linalool, linalyl-isononoate, geraniol and diterpene. Of these, he mentioned that especially myrcene and linalool had a typical ‘hop scent’. With the use of various chromatographic techniques, researches have now been able to identify over 485 different compounds in hop essential oil (as of 2004), and suggest that up to 1000 different compounds exist, with several of the compounds being very potent aroma constituents. Since the hops and the wort undergo a complex process before it is turned into beer, not all of the aroma compounds from the hop oil make it into or have any influence in the finished beer. Myrcene and linalool have been found to still be major contributors to hoppy aroma in finished beer (especially when added late- or post-boil), and several others have been identified by analyzing hopped beers. Some of the more potent and relevant hop oil aroma compounds found in fermented beer are presented in Table 1 below, along with their typical sensory character (Schönberger & Kostelecky, 2011). The major aroma compounds in hop oil responsible for hop aroma and flavor in beer can be simplified as follows:

  • Grassy and green flavors are mainly caused by aldehydes, such as hexanal and hexenal.
  • Citrus flavors are mainly caused by esters (such as ethyl 4-methyl-pentanoate), nerol and linalool
  • Floral and fruity flavors are mainly caused by linalool, geraniol, β-ionone, citronellol, 4-mercapto-4-methylpentan-2-one (4MMP), and 3-mercaptohexan-1-ol (3MH), as well as other ketones, epoxides and esters.
  • Herbal and resinous flavors are mainly caused by myrcene, other monoterpenes, and oxidized sesquiterpenes (e.g. α-caryophyllene)

Table 1 – Major aroma compounds in hop oil (Kishimoto et al., 2006; Lermusieau & Collin, 2003; Nielsen, 2009; Schönberger & Kostelecky, 2011)
[table id=1 /]

Even though there has been contradicting results, research seems to indicate that the hop aroma compounds can have additive and synergistic effects, and thus are not individually responsible for the different aromas found in beer. The combined effect of different hop aroma compounds can lead to lower flavor thresholds (e.g. in the case of a caryophyllene (flavor threshold of 210 mg/L) and nerol (flavor threshold of 1200 μg/L) mixture (flavor threshold of 170 μg/L) or farnesene (flavor threshold of 2000 μg/L) in a linalool mixture (flavor threshold of 500 μg/L). The transfer rate of the different hop aroma compounds between the hops and the wort/beer have been shown to vary with hop variety, and experiments have even shown that the concentration of some hop aroma compounds (such as linalool and geraniol) increase during fermentation (more on this below). The amount of hop aroma compounds carrying over to the beer from the hops, depend largely on when they are added to the wort (e.g. during boil, after boil, and even after fermentation), the temperature of the wort, the contact time with the wort, the alcohol content of the beer, and even the composition of the wort (e.g. sugar content). Hence, the types of hop aroma compounds present in a beer where the hops have been added at flameout differ from the types of compounds present in a beer where the hops have been added as dry hops after fermentation, even though the same hop variety has been used. Next is some information on the major hop aroma compounds and how they behave in wort and affect the hop aroma of finished beer.

myrcene-invertedStructure of myrcene.
Myrcene is a hydrocarbon, more specifically classified as a monoterpene, found in hop and various other plants, and it is often responsible for the resinous, herbaceous, green, even metallic-like aroma and flavor in beer (especially in dry-hopped beer). Reports on the flavor threshold of myrcene in beer vary considerably: between 9.5 ppb (difference threshold value), to 30-200 ppb, to 30-1000 ppm (though this is a typo in Schönberger & Kostelecky (2011), and should be ppb (confirmed by author)), while the flavor threshold in water is thought to be lower, and reported values of 14 ppb can be found. Myrcene is usually not present in beers hopped only early in the boil. This is because myrcene has a quite low solubility (around 5 ppm in water) in aqueous solutions and a high volatility. Studies have shown rapid declines of myrcene concentration during wort boil, with the myrcene concentration almost halving every 5 minutes. Myrcene concentrations of around 0.4-1.1 ppb were measured in beers hopped with 67% of the hops added at 60 minutes and 33% added at 15 minutes left in boil. Similarly, myrcene concentrations of 5.5-90 ppb (depending on hop variety) were measured in beers when different hop varieties were added post-boil (during the cooling process). Hence, post-boil hops (either in the form of whirlpool hops or dry hops) seem to be necessary to achieve myrcene concentrations above the flavor threshold in beer (and consequently get the resinous, herbal, and green hop aroma and flavor associated with it). Hops varieties that have a high myrcene content are Citra, Simcoe, Newport, Amarillo and Columbus, while hop varieties with low myrcene content are Tettang, Vanguard, Saaz, Ultra and Palisade (see Table 2; USA Hops, 2011).

Table 2 – Myrcene content of various hop varieties
[table id=2 /]

linalool-invertedStructure of linalool.
Linalool is terpene alcohol, which is quite closely related to myrcene (you can compare their chemical structures above), and linalool is found naturally in numerous flower and spice plants. Linalool is frequently used in the perfume industry, because of its floral, spicy, lavender-like, sweet and even citrus-like aroma. Because of its structure, linalool is present in two stereoisomers, and these isomers have both a slightly different aroma and different flavor thresholds. (R)-Linalool has a spicier aroma and an odor threshold of about 0.14-0.8 ppb in water and 2.2 ppb in beer, while (S)-linalool has a sweeter aroma and a slightly higher odor threshold of 7.4 ppb in water and 180 ppb in beer. In fresh hops, around 95% of the linalool is in the (R)-enantiomer form. The reported flavor threshold of the linalool from hop essential oil in beer varies from 8 to 80 ppb, with most results around 20-30 ppb. As with myrcene, linalool is readily lost during wort boil. It has high volatility, and compared to myrcene, a higher solubility (around 1.6 g/L) as well. During wort boil, around one third of the linalool content of the wort is lost with the steam every 5 minutes. Hence, to get a sufficiently high linalool concentration in the final beer, it is required to add the hops late in the boil (preferably after the boil). Dry hopping does not necessarily give the highest linalool concentrations, as studies have shown that post-boil (whirlpool) hop additions give rise to higher linalool levels than equal dry hop additions. However, higher linalool concentrations can be achieved by using a combination of both late hopping and dry hopping. By adding a range of different hop varieties in equal amounts to different worts post-boil, Koshimoto (2007) was able to measure linalool concentrations ranging from 43 to 167 ppb, depending on variety. It has been shown in numerous studies, that linalool concentration has the tendency to increase slightly during fermentation (Daenen et al., 2007; Hanke et al., 2008; Kaltner, 2000; van Opstaele et al., 2010; van Opstaele et al., 2012), which might seem strange to brewers, who usually have the belief that hop aroma is lost during fermentation with the escaping carbon dioxide (this is of course partly true, since linalool is not by any chance solely responsible for the hoppy aroma of beer). The proposed reason for this increase, is that linalool glycosides (i.e. linalool molecules bound to a sugar molecule at its 1-position) are formed in the hop plants as they grow, and these carry over to the wort during the boil (they are water soluble and odorless). The glycosidic bonds are cleaved with enzymatic and acidic hydrolysis during fermentation, releasing the linalool to the beer. Various biotransformations between different aroma compounds (such as geraniol to linalool) can also be performed by the yeast during fermentation, causing changes in aroma compound concentrations. The linalool content of hops depend largely on variety, growing conditions and maturity at harvest, but some hop varieties lending to high linalool concentrations include Nugget, Citra, Simcoe and Summit.

1_3_5_undecatriene-invertedStructure of (e,z)-1,3,5-Undecatriene.
(E,Z)-1,3,5-Undecatriene is a quite recently discovered (2000) compound of hop oil, that has a very low odor threshold (0.02 ppb in water and 0.003 ng/L, i.e. 0.000003 ppb, in air), and has a pineapple- and citrus-like aroma. Its presence in hops hasn’t been studied much, but in a study by Steinhaus et al. (2007), it became evident that the compound had a large influence on the aroma of Cascade hops (compared to some European hops). (E,Z,E)-1,3,5,9-Undecatetraene has also been identified in hop oil, and it contributes with a similar aroma.

4mmp_wStructure of 4-Mercapto-4-methylpentan-2-one (4MMP).
4-Mercapto-4-methylpentan-2-one (4MMP), a volatile thiol, is another recently discovered (2006) compound of hop oil, which contributes with a black currant- and muscat-like hop aroma to beer. 4MMP has an extremely low flavor threshold in beer (1.5 ng/L, i.e. 0.0015 ppb), and it has been shown to largely contribute to the overall hop aroma intensity of beers hopped with some American hops. Kishimoto measured the 4MMP content of single hop beers brewed with 11 different hop varieties added during whirlpool, and found 4MMP in beers with Simcoe (0.183 ppb), Summit (0.116 ppb), Apollo (0.109 ppb) and Cascade (0.017 ppb). Hence, 4MMP content of beer influences the typical hop aroma contributed by at least these hops.

humulene_w Structure of humulene or α-caryophyllene.
α-humulene or α-caryophyllene is a monocyclic sesquiterpene, constituting a quite large volume of the essential oil found in hops (5-45% of the essential oil volume depending on the hop variety; noble hops usually have a higher humulene content). Humulene brings a herbal and spicy character to the aroma, and has a flavor threshold of 120 ppb. Because of the relatively high threshold, humulene quite rarely contributes to the aroma on its own, even though it is present in large amounts in the hop oil. Instead, humulene undergoes various reactions (e.g. during the boil), to form various epoxides and other reaction products (such as humulol and humulenol II). The exact impact of these, if there even is one, on the hop aroma of beer is still unclear though.

As you may notice, the essential oil in hops is complex, and there is yet no known certain composition that guarantees a certain aroma or flavor. We only covered a few of the major contributors, and missed important ones such as geraniol, citronellol and various esters. The next part will concentrate on the hop resins and bitterness in beer, and should hopefully be finished in a week is available here.


  • Ajisaka, N., Hara, K., Mikuni, K., Hara, K., Effects of Branced Cyclodextrins on the Solubility and Stability of Terpenes. Bioscience, biotechnology, and biochemistry 64 (2000) 731-734
  • Briggs, D., Boulton, C., Brookes, P., Stevens, R., 2004. Brewing: Science and Practice. Cambridge: Woodhead Publishing.
  • Hanke, S., Herrmann, M., Rückerl, J., Schönberger, C., Back, W., Hop Volatile Compounds (Part II): Transfer Rates of Hop Compounds from Hop Pellets to Wort and Beer. BrewingScience – Monatsschrift für Brauwissenschaft 61 (2008) 140-147
  • Kaltner, D. (2000). Untersuchungen zur Ausbildung des Hopfenaromas und technologische Maßnahmen zur Erzeugung hopfenaromatischer Biere. Doctoral dissertation. Freising, Technische Universität München, Fakultät Wissenschaftszentrum Weihenstephan für Ernährung, Landnutzung und Umwelt.
  • Kishimoto, T., Wanikawa, A., Kono, K., Shibata, K., Comparison of the odor-active compounds in unhopped beer and beers hopped with different hop varieties. Journal of Agricultural and Food Chemistry 54 (2006) 8855-8861
  • Kishimoto, T., 2008. Hop-Derived Odorants Contributing to the Aroma Characteristics of Beer. Doctoral Dissertation, Kyoto University.
  • Lermusieau, G., Collin, S., Volatile sulfur compounds in hops and residual concentrations in beer- a review. Journal of the American Society of Brewing Chemists 61 (2003) 109-113.
  • Nielsen, T., 2009. Character-impact hop aroma compounds in ale. In: T.H. Shellhammer (Ed.) Hop Flavor and Aroma – Proceedings of the First International Brewers Symposium 2007. St. Paul: Master Brewers Association of the Americas.
  • Roberts, M., Dufour, J., Lewis, A., Application of comprehensive multidimensional gas chromatography combined with time-of-flight mass spectrometry (GC x GC-TOFMS) for high resolution analysis of hop essential oil. Journal of Separation Science 27 (2004) 473-478
  • Schönberger, C., Kostelecky, T., 125th Anniversary Review: The Role of hops in Brewing. Journal of the Institute of Brewing 117 (2011) 259-267
  • Steinhaus, M., Fritsch, H., Schieberle, P., Quantitation of (R)- and (S)-Linalool in Beer Using Solid Phase Microextraction (SPME) on Combination with a Stable Isotope Dilution Assay (SIDA). Journal of Agricultural and Food Chemistry 51 (2003) 7100-7105
  • Steinhaus, M., Wilhelm, W., Schieberle, P., Comparison of the most odour-active volatiles in different hop varieties by application of a comparative aroma extract dilution analysis. European Food and Research Technology 226 (2007) 45-55
  • Takoi, K., Itoga, Y., Koie, K., et al., The contribution of Geraniol Metabolism to the Citrus Flavour of Beer: Synergy of Geraniol and β-Citronellol Under Coexistence with Excess Linalool. Journal of the Institute of Brewing 116 (2010) 251-260
  • Tokitomo, Y., Steinhaus, M., Büttner, A., Schieberle, P., Odor-Active Constituents in Fresh Pineapple (Ananas comosus [L.] Merr.) by Quantitative and Sensory Evaluation. Biosciences, Biotechnology, and Biochemistry 69 (2005) 1323-1330
  • USA Hops, 2011, Variety Manual – Hop Growers of America (link)
  • Van Opstaele, F., de Rouck, G., de Clippeleer, J., Aerts, G., de Cooman, L., Analytical and Sensory Assessment of Hoppy Aroma and Bitterness of Conventionally Hopped and Advanced Hopped Pilsner Beers. Journal of the Institute of Brewing 116 (2010) 445-458
  • Van Opstaele, F., Borremans, Y., van Holle, et al., 2012, Fingerprinting of hop oil constituents and sensory evaluation of the essential oil of hop pellets from pure hop varieties and single-hop beers derived thereof. 10th Trends in Brewing

Hop Science I: Introduction

I thought I’d write up some science behind hop aroma (and flavor), as I personally find it really interesting. This is part one (the introduction) of a multi-part mini-essay on hop science and factors influencing hop flavor and aroma in beer. I’m definitely no expert in the area, and have compiled most information from various scientific articles, books and web sources (list of sources will be found at the end of each post). Hope you enjoy it!

Hops are, along with malt, yeast and water, one of the main ingredients of beer, and they can, depending on beer style, have various influences on the aroma and flavor of beer. After tasting through various single hop beers (i.e. beers hopped solely with a single hop variety; see taste notes of Brewdog’s Bramling X, Citra, Nelson Sauvin and Sorachi Ace IPAs) and brewing some single hop beers myself, it has become evident (at least for me) that hops have an enormous influence on the flavor and aroma of beer. Through homebrewing I’ve also become aware that getting a strong hop aroma and flavor can be difficult, especially in lighter styles, since hops bring bitterness to the beer as well. Hop aroma also fades unfortunately fast. But how do hops contribute to the aroma and flavor of beer?

(Fresh hop cones. Source)

The hops used in beer are flower cones from the female Humulus lupulus L. plants. Over 100 different hop cultivars are grown globally today, and they all contribute various characteristics to the aroma and flavor of beer. Hop plants are usually harvested during the autumn/late summer, which means Northern Hemisphere harvest usually occurs between late August and early October, while Southern Hemisphere harvest usually occurs around March. Hops are rarely used fresh (though exceptions occur), and are dried after picking from around 80% to 10% moisture content to prevent molding. After drying, the whole hops are usually either vacuum-, CO2- or nitrogen-packed (to prevent oxidation), or processed into pellets (i.e. frozen, crushed with a hammer mill, homogenized, pelleted and packed under vacuum or inert gas), after which they are preferably stored cool before use (to maximize stability). Hops can also be processed into various non-isomerized and isomerized alpha acid extracts and hop oil extracts, and these are gaining popularity at breweries (because of less wort loss to hop absorption).

(Dried hop cones (above) and hop pellets (below). Source 1 and Source 2)

Hops produce and contain an aromatic essential oil (up to 3% by weight), and this oil is located in the yellow-colored lupulin glands of the hop cones together with the various acids (hop resins) that cause bitterness in beer. The essential oil and hop resin compositions are extremely complex and vary between different hop cultivars. Over 485 compounds have been identified in hop oil, while researchers predict over 1000 compounds are present. The hop resins and oil are transferred to the beer through the boil (or through post-boil contact with the wort). The essential oil is volatile, meaning most of it will be lost with steam when hops are boiled for a long time. Hence, hop aroma is added to beer by adding hops late to the boil (in the last 15 minutes of the boil), post-boil but pre-fermentation (e.g. whirlpool hops), and post-fermentation (e.g. by dry hopping or using a hop rocket). The hop compounds also undergo reactions during fermentation, meaning researching and analyzing the contribution to beer aroma from hops is complex.

(Lupulin glands. Source)

This is the end of the short introduction to the mini-essay. Next up will be a write-up on the major compounds found in hop oil and how they contribute to the flavor and aroma. The next part will hopefully be released later this week can be found here.


  • Briggs, D., Boulton, C., Brookes, P., Stevens, R., 2004. Brewing: Science and Practice. Cambridge: Woodhead Publishing.
  • Kishimoto, T., 2008. Hop-Derived Odorants Contributing to the Aroma Characteristics of Beer. Doctoral Dissertation, Kyoto University.
  • Schönberger, C., Kostelecky, T., 125th Anniversary Review: The Role of hops in Brewing. Journal of the Institute of Brewing 117 (2011) 259-267

Homegrown hops

Here are a couple of pictures of some homegrown Cascade (2 plants; upper picture) and Chinook (1 plant; lower picture) that were planted in the end of May this year. The Cascade plants are doing a lot better than the Chinook, but I am not expecting any harvest this year from either, as the rhizomes were planted quite late. Hopefully next year!

Homebrew: Plans for a 100% Brett B IPA

Bret you've got it going on

After reading some threads across the web on White Lab’s new WLP644 Brettanomyces bruxellensis Trois strain, I really want to try brewing a fruity IPA with it. White Labs describe the strain in the following way:

WLP644 Brettanomyces bruxellensis Trois
This Belgian strain, used traditionally for 100% Brettanomyces fermentations, produces a slightly tart beer with delicate characteristics of mango and pineapple. Can also be used to produce effervescence when bottle-conditioning.
Optimal Temp: 70-85F
Flocculation: Low
Attenuation: 85%+
Alcohol Tolerance: Medium-High

Reports on the web say that when used as the primary fermentation strain, the yeast gives off lots of tropical fruit aroma, drawing to tones of mango, guava, pineapple, and orange. This would probably compliment well with some fruity hops, such as Citra and Nelson Sauvin. There are commercially brewed Brett IPAs available (e.g. Ithaca Super Friends IPA and Anchorage Galaxy White IPA), but it would unfortunately be impossible for me to get my hands on one here in Finland. I have never brewed with Brettanomyces before, and have only tasted a couple of commercial Brett beers, so this will probably be an interesting and exciting experiment. I plan on brewing a 10 liter IPA batch and a 5 liter Brett Blonde batch (to get a better impression of what the yeast brings to beers) from the same mash, and splitting a 1.5 liter starter of the WLP644 between them. The grain bill will consist of 10% munich for some maltiness and 10% flaked oats for improved body, and I will be mashing quite high to prevent the beers from fermenting too dry. I haven’t planned the Blonde recipe yet, but the IPA recipe is presented below. I will be naming the beers after Bret McKenzie from Flight of the Conchords (because of the Brett yeast and the New Zealand hop), and the IPA will probably be called ‘Bret you’ve got it going on’.

[codebox 1]

Homebrew: First sampling of the Imperial IPA

My latest Imperial IPA has been in the keg for a week at 0.8 bar CO2 and 1 degree C, so today I poured up the first taste sample. As I was expecting, the beer was only partially carbonated (it usually takes me at least two weeks with the ‘set-and-forget’ method to reach proper carbonation levels), and it poured really murky with almost no head (I poured with low pressure from the keg, so that also explains the non-existing head). I’m positive the beer will clear up after I’ve poured the first few pints, as these usually contain the yeast and other sedimented particles from the bottom of the keg, and also form a much bigger and retentive head, as the beer gets more carbonation and a more vigorous pour. This beer is heavily hopped (both in the kettle and dry-hopped), so that naturally makes it murkier, and right now the appearance reminded me a bit of that of Alchemist’s Heady Topper. Well enough about the appearance, lets go to how it smelled and tasted.

Even though the carbonation level was low, the beer had a great and strong aroma, featuring dank, resiny, grapefruity, citrusy and earthy hop tones. These will hopefully still be amplified with more carbonation and by resting on the keg hops for another week. The flavor was also nice, though not perfect at the moment, as the beer felt a little ‘green’ still (muddled flavors, bland mouthfeel from low carbonation, slight yeast tones). This is not surprising though, as it has only been a week in the keg. The flavor featured some initial caramelly maltiness, which was overtook by resiny, dank, citrusy and earthy hop tones, that lingered long with the dry and bitter finish. I have high expectations for this, and I think that in a couple of weeks this will be fantastic.

A trio of Juhani’s homebrews

Today I will be tasting and reviewing a trio of my homebrewing neighbor Juhani’s homebrews. The beers include his #24 Dubbel (with Orval dregs?), #28 ‘Ryssänpelko’ Imperial Stout and #42 Blonde Ale (brewed with yeast harvested from a bottle of Lente Blond). I’ll be starting with the latest, #42 Blonde Ale, moving on to #24 Dubbel, and finishing off with #28 Imperial Stout. This should be interesting!

  • Brewery: Juhani’s homebrewery
  • Country: Finland
  • Style: Belgian Ale
  • ABV: 4.7%
  • Size: 330 ml
  • Bought from: Gift
  • Not on Beer Advocate
  • Not on RateBeer


This should be a Belgian-style blonde ale, and it has been brewed with a tin of Coopers Lager extract (1,7 kg), 700 g Brewferm light DME and 300 g sucrose (20 l batch, OG 1,044, FG 1,008, ABV 4.7%). It has been hopped with Styrian Goldings, and the yeast (as I already mentioned) has been harvested from a bottle of Lente Blond. Let’s see what it tastes like!

[easyreview title=”Juhani’s #42 Blonde Ale” cat1title=”Appearance” cat1detail=”The beer pours with a brilliantly clear golden color, and a large fluffy white head, that collapses quite slowly. Nice appearance! The second pour was hazier, as I poured in some yeast as well.” cat1rating=”4.5″ cat2title=”Smell” cat2detail=”The aroma is spicy, phenolic, yeasty and has slight fruit notes as well. The aroma reminded me quite a bit of the Tripel I brewed last December. No off-aromas, and it would be hard to tell this is a homebrew from the aroma alone. Typical Belgian-like aroma, difficult to give any tips for improvement. As the beer warms up you can pick up some alcohol notes as well despite the low ABV.” cat2rating=”3.5″ cat3title=”Taste” cat3detail=”The flavor begins with a slightly honey-like, almost sweet, maltiness, and also features many of the tones present in the aroma. There are spicy and phenolic yeast tones (reminding me of coriander), together with a slightly citrusy and grassy hoppiness. The finish is quite dry and has a subtle bitterness. The flavors are quite well balanced, but the spicy yeastiness is definitely in the center. With the second pour they become even more evident.” cat3rating=”3.5″ cat4title=”Mouthfeel” cat4detail=”The beer has a medium-light body with a medium-high carbonation level. Quite refreshing and definitely a nice beer for the summer!” cat4rating=”4″ summary=”Overall a really nice Belgian-style blonde ale, that is both refreshing and tasty. Can’t find many faults in this, just some slight alcohol notes in the aroma, which are acceptable for the style. This is gonna be a good beer for the summer! Good job!”]

  • Brewery: Juhani’s homebrewery
  • Country: Finland
  • Style: Abbey Dubbel
  • ABV: ?
  • Size: 330 ml
  • Bought from: Gift
  • Not on Beer Advocate
  • Not on RateBeer


Next up is the Dubbel. I unfortunately have no information about this brew, but if I remember correctly Juhani added some Orval dregs to this batch, so it should contain some Brettanomyces bruxellensis. Should be interesting, since I haven’t had many Brett beers before!

[easyreview title=”Juhani’s #24 Dubbel” cat1title=”Appearance” cat1detail=”The beer pours with a slightly hazy dark amber color, and a cream-colored head that quickly collapses.” cat2title=”Smell” cat2detail=”The aroma is a lot subtler than the last beer, with much less phenolic yeast, and more tones of dark fruits and red berries. There is a strange off-aroma as well that I have noticed in some of Juhani’s beers before, reminding me a bit of plastic.” cat3title=”Taste” cat3detail=”The flavor begins with a sweet maltiness, drawing to tones of dark fruits, red berries and raisins, but it is quickly taken over by a plastic-like (maybe even burnt) flavor that could be detected in the aroma as well. Not sure what the origin of this off-flavor is, but have tasted it in some of his earlier beers before, but beers from his latest batches have been free from it.” cat4title=”Mouthfeel” cat4detail=”A medium body and carbonation level, with a slightly acidic/astringent bite.” summary=”Unfortunately it seems as if there was something wrong with this beer, as the plastic-like aroma and flavor made it impossible for me to finish the bottle. If it is from an infection via contaminated gear, Juhani has hopefully been able to get rid of it, as I haven’t tasted it in his recent batches.”]

  • Brewery: Juhani’s homebrewery
  • Country: Finland
  • Style: Imperial Stout
  • ABV: 9.5%
  • Size: 330 ml
  • Bought from: Gift
  • Not on Beer Advocate
  • Not on RateBeer


The last beer of the night is the Imperial Stout. I have limited information on this brew as well, but it has been fermented with Westmalle yeast (harvested from a bottle?), aroma-hopped with Amarillo, and it should have an IBU around 90 and an EBC of around 115. I think it contains malt extracts, but am not fully certain.

[easyreview title=”Juhani’s #28 Ryssänpelko Imperial Stout” cat1title=”Appearance” cat1detail=”The beer pours pitch-black with a compact tan-colored head that collapses quite slowly.” cat1rating=”4″ cat2title=”Smell” cat2detail=”The aroma is dominated by tones of roasted malt, coffee, vanilla, coffee, and caramel. I couldn’t really detect anything from the Belgian yeast in the aroma. There is some alcohol in the aroma as well.” cat2rating=”4″ cat3title=”Taste” cat3detail=”The flavor begins with roasted flavors drawing from roasted malt, coffee, dark chocolate and ash. The flavors are then coupled with some caramel and vanilla. The finish is sweet and quite bitter. The bitterness is smooth and I wish it was a bit more pronounced, even though it apparently should be around 90. The flavors are quite balanced, but the roastiness is definitely in the center. Really nice! Only complaints are that the roastiness is just slightly astringent, and it seems just slightly one-dimensional.” cat3rating=”4″ cat4title=”Mouthfeel” cat4detail=”The beer has a full body and medium carbonation level. This is definitely a slow-sipper. The astringency draws away some points.” cat4rating=”3.5″ summary=”Overall a really nice Imperial Stout, and the flavors are definitely down my alley with the roasty coffee-like tones. Liked this a lot more than the last Imperial Stout I tasted from Juhani, and this was my favorite of the trio. The beer was missing some complexity, and was a little astringent, but otherwise not much I would change. Maybe raise the bitterness just a bit? Otherwise, a great beer!”]

Thanks Juhani for the beer!