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)
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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.

Structure 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
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Structure 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.

Structure 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.

Structure 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.

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.

Sources:

• 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.
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• 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