Linalool is a terpene alcohol, and is closely related to myrcene, being its hydration product.  It is found in dozens (if not hundreds) of plants, flowers, and spices, but in freshly dried hop cones it is generally found at levels of about 25-150ppm (mg/kg).  Due to the ability to have two configurations at the #3 carbon, linalool is found as two stereoisomers (S, R), each having different thresholds and aroma qualities:  S-linalool has a sweeter, more floral aroma and an odor threshold of about 7ppb, while R-linalool has a wood, spicy, and lavendar-like character and a much lower threshold at less than 1ppb.   Regardless of the total level of linalool in a hop variety, the ratio of these stereoisomers in fresh hops seems to be fairly consistent:  about 93% of it is in the R form.  Generally, linalool’s threshold in beer is much higher – upwards of 100ppb  (2.2 and 180ppb, for R and S respectively).   The pure linalool that I use in my sensory department (which I have to assume is a mixture of isomers) has a very pleasant sweet, tropical, fresh floral character, but is also not unlike the aroma of Froot Loops® cereal.   To be honest, it’s one of the most pleasant aromas I’ve ever smelled, which is probably why it’s a very popular addition to many fragrant commercial products, from perfumes to laundry detergents.   Apparently, it’s so pleasant smelling that it has been shown to reduce stress levels in laboratory rats and inhibit the activity of genes associated with stress hormones.

Despite having an aroma that is only remotely reminiscent of hops, linalool is widely accepted as one of the few compounds that directly contributes to hop aroma in beer.  Being a volatile flavor-active compound, however, the presence of linalool in beer hinges greatly on a number of factors.  Not surprisingly, the variety of hop plays a crucial role in the amounts of linalool available for the beer, but also the growing conditions and the maturity of the plant at harvest time.  Some research breeders have considered using linalool as a way to gauge the readiness of a crop for harvest.  Apart from variety, growing conditions, and maturity, production processes in the brewery have the biggest impact on linalool levels in the beer.  The sweet and floral aroma characteristics of linalool are quite distinct from the hop aromas generated by the noble hop varieties favored by many European brewers, but this is not necessarily due to the varieties themselves (although they do tend to be lower in levels relative to the newer American aromatic hops).   More influential is how these nobel hops are often used in such beers, and that is often via “kettle hopping”, where noble aroma hops are added near the beginning of the boil.  Despite most of the hop essential oils being lost via steam distillation throughout the boil some hop aromas must remain, as this kettle hop aroma is generally described as spicy, woody, and herbal – terms which are obviously not malt-related.   But these aromas are also dissimilar to that of linalool, which must be lost with the rest of the essential oils.  As one might expect, European kettle hopped beers have very low levels of linalool and “sweet, fresh, and floral” hop aroma terms are not associated with these beers.

So, just like hop aroma in general, in order to get more linalool in the beer, one would need to add hops later in the boil so that not so much is lost via distillation.  ”Late hop aroma” is imparted by adding hops at kettle knockout or in the whirlpool.  This is where you really begin to notice the impact of some of the essential hop oils, leading to various aromas like citrus, piney, floral, perfume, etc.   The later the addition (ie, the closer to wort chilling) the more linalool will remain in the wort.  Some research has demonstrated that when adding hops at 10 minutes prior to the end of boil, linalool levels can rise from about 8ppb to 60ppb before tailing off near 30ppb by the end of boil.   By contrast, adding the hops just 2 minutes before the end of boil, linalool levels can rise to 85ppb before stabilizing at about 80ppb.   Even without any indication as to what hop variety was used or other parameters, this is a dramatic demonstration of the importance of timing with regards to hop additions.

As you probably already know, dry hopping is the best way to get the elusive hop aroma into beer in significant amounts.  But, interestingly, research has shown that this doesn’t necessarily apply to linalool.  While some linalool may be imparted by dry hopping, it seems that it is rather negligible compared to late kettle hopping.  In addition, many of the other aromatic compounds contributed to the beer during dry hopping (such as methyl esters and ketones) will likely mask the presence of linalool to the point that it would be difficult to detect.

The final way that linalool is introduced to beer is via glycosides (which I’ve discussed before).  Glycosides are interesting because they can carry aromatic compounds into the beer rather surreptitiously and release them under various circumstances.   A glycoside is essentially a sugar molecule which is bound to another molecule at its #1 carbon position, and in such a configuration they are not flavor-active.  However, during fermentation, yeast enzymes break this bond and release the aromatic molecule.  Some of these glycosides contain linalool as the aromatic compound.  It’s not always enzymes that are needed to cleave these molecules in twain;  sometimes an acidic environment is all that is needed.  In fact, glycosides also play an important role in wine flavor, as wine is quite acidic.   But while most beer not as acidic as wine, acid hydrolyzed glycosidic cleavage reactions still take place in beer.   It’s been shown that if glycosides survive into the finished beer, they can continue to release linalool as the beer ages, and depending on what kind of hop product is used and other production parameters, this source of linalool may actually be more influential to beer flavor than the linalool which comes from the hops directly.  I’d really like to delve a little deeper into glycosides and how they are introduced and modified throughout the brewing process, but besides the little bit of research I’ve found regarding them, all the references I can find now are in German…  and I don’t speak German.  If anyone knows of any papers about glycosides and beer flavor, please let me know.

And, for now, that’s all I have to say about that.  I think the next posts will be about some beer we’ve drank recently.

See you next time!

—

Sources:

Peacock, V., “The Value of Linalool in Modeling Hop Aroma in Beer”, Master Brewers Association of the Americas Technical Quarterly, 47:4, 2010, p.29-32.

Kaltner, D., Mitter, W., “Changes in Hop Derived Compounds During Beer Production and Aging”, Hop Flavor and Aroma:  Proceedings of the 1st International Brewers Symposium, MBAA 2009, p.37-47.

I’ve read only a handful so far (picking out the contributors whom I routinely enjoy reading), and many of them are pretty interesting.

One that stood out for me, and is somewhat on-topic, is Richard Dawkins’ discussion of “Redundancy Reduction and Pattern Recognition”, and it briefly explains how our brain chooses which sensory signals to pay attention to and which to filter out.

You may also enjoy Vilayanur Ramachandran’s discussion of the question of consciousness and where it may arise. I’ve always found Dr. Ramachandran’s work fascinating, particularly his work on phantom limb pain and synesthesia (mixing of sensory signals in the brain; ie something tastes “brown”). You can find a bunch of his material on the internet, and he has done a couple TED Talks, too. Google it up!

More beer topics soon!

What’s not despicable: a couple of the beers I had this weekend.

Friends visited the house this weekend and we had a wonderful time reveling in the bacchanalia. We had some lovely grilled London Broil, some garlic mashed potatoes, some salad, and beer. One of the beers we had was brought as a gift by my good friend. It was Firestone Walker’s 15th Anniversary Ale. We shared it between the two of us. Good thing, too, because this beer is enormous: 12.5% v/v. But I didn’t realize that when I poured it and began to experience it; I hadn’t yet looked at the label for any details so I didn’t really know what to expect. Turns out this beast is a blend of about 8 different barrel-aged beers.

Orthonasal aroma (front of nose, before tasting): fruity, figs, raisins, molasses, and a moderate alcohol aroma (certainly did not belie the true alcohol content in the aroma).

Retronasal aroma (back of nose, after tasting): the barrel aged character comes out, whiskey/bourbon-like, vanilla, slight burnt aftertaste.

Taste, etc: thick mouthfeel (syrupy), chocolate, significant alcohol burn, sweet. Not overly bitter or astringent.

This is not a beer that I could drink 22oz of, so if you have one it might be a good idea to share with 1 or 2 friends. Personally, I think an 8-10oz glass of this would be just about right. It’s a really big beer that you just shouldn’t overdo. A good beer if you like a lot of flavor, but the drinkability on this beer is very very low.

BONUS! Had an organic IPA from Hopworks Urban Brewery (HUB) as well this weekend – excellent beer! Great citrus hop aroma (orange and grapefruit) without being overly catty. I’m pretty sure I had this when I visted the brewery in November, but this bottled version stood out as a very fine beer. Clean, fairly drinkable (if you don’t mind bitterness), and a great hop aroma. I will seek this one out again.

DOUBLE-PLUS BONUS! A co-worker gave me a bottle of Anchor Steam’s 37th Christmas Ale. Didn’t really pay attention to some of the flavor characteristics, but this is a surprisingly smooth and drinkable beer, considering its fairly dark color. Very low bitterness, very clean flavor, no lingering issues. The balance of this beer is tipped in the malt direction, which is usually not where I prefer it, but there’s really nothing wrong with this beer. Pretty good!

Now, sour beers are really not my preferred style of beer so my goal was just to find some beers and food that I enjoyed. I would have no problem trying the sour beers, but I certainly wasn’t going to be ordering any for myself. I also didn’t go into this trip with the idea that I’d analyze, or even remember, each beer I tried, but I did want to form some general opinions on the bars that we visited, and some of the food and beverages that they served. Below is a general overview of the places we visited, and what we thought about the products they offered.

The first place we visited was the Cascade Brewing’s Barrelhouse. I ordered a Test Flight IPA for myself and found it to be quite nice: a clean and drinkable IPA with a nice and floral hop aroma. We also ordered a series of beers from their sour beer lineup: the Kriek was a tooth-enamel destroying sour beer with a fairly subtle cherry flavor – it was alright; Nightfall is their blackberry sour which was decent; and whichever of their cranberry offerings we had was not that exciting. Our group’s favorite of the sours was probably the Sang Rouge, a moderately dark beer with a milder and mellow sourness and a pretty rich and complex flavor profile. I believe it was this beer that Sarah bought in 3 large bottles.

Other Highlights: the pulled pork sandwich, and noticing that their 750ml bottles of beer have stated alcohol contents on the labels which go to 3 decimal places. Service was on the slow side as it was difficult to get the attention of the Shel Silverstein look-alike behind the bar at times, despite not being very busy. Overall grade (beer, food, ambiance): B+

Next up, we headed over the Hopworks Urban Brewery. We happened to show up during Happy Hour, so we ordered a bit of food on the cheap: some awesome warm and doughy pretzels (with not enough delicious cheese sauce) and some bruschetta which was a bit uninspiring. For beer, we ordered a sampler platter, which serves 3-4oz glasses of each of their 10 main beers. Pilsner: sulfury and corny, but still clean and drinkable – pretty decent. Pale ale: my favorite here, balanced with some citrus hop aroma, maybe slightly catty. ESB: not terrible, but nothing to write home about. IPA: big brother to the pale ale it seems, more catty, more Simcoe-like citrus hop aroma. 7-grain stout: too much coffee flavor for me personally, but not bad. Organic ale: similar to the ESB but a bit on the grainy side, uninspired. Porter: a smooth and quaffable chocolatey/roasted offering. Helles: more sulfur, very basic, not bad I guess. The alt and Abominable Ale I do not remember. Sorry.

Highlights: Cheap Happy Hour prices. Overall grade: A-

Our last stop on the east side of the river was Upright Brewing. Just a bare-bones tasting room here, right in the middle of their basement brewery. We had a tasting rack of 5 of their offerings. Four: this wheat beer seemed too clear for the style, and had a harsh character to the bitterness – not awful but not great. Five and Six: the Farmhouse ale and the dark rye were actually fairly similar in that they both were quite tutti-fruitti (Juicy Fruit bubble gum, estery, ethyl butyrate) and had some medicinal phenol characters. Seven and “other”: I think these were a Saison and an Engleberg Pilsner, both of which did not stand out to us as being particularly good.

Highlights: realizing there were still plenty of places to visit. Overall grade: C+.

Back to the west side of the Willammette River we stopped at Bailey’s Taproom, which was a scant 2 blocks from our lodging. A nice selection of beers on tap here, with a low-key and relaxing environment. We ordered a Pelican IPA which was as good as I remembered, a Logsdon Farmhouse Kili-Wit (as in “Dave Logsdon”, yeast master of Wyeast) which wasn’t bad as phenolic Belgian-style beers go, and the Lagunitas Fusion 7 [censored] Red ale, which was big and robust and was thankfully much fresher than when I’ve had it from the bottle.

Highlights: the huge flatscreen display behind the bar with a price list for their available offerings, even showing how much remains in the keg. Overall grade: A

We noticed that Tugboat Brewing Company was right across the street, so we helped each other up and made our way over there next. A quaint, dark, and quiet pub, this place doesn’t look like much. We ordered the 13.5% abv Chernobyl stout (didn’t taste like 13%, but big in flavor nonetheless), the Hopped Red ale which was reminiscent of Deschutes Cinder Cone Red (pretty good), and I think third was the Czech Bitter, or perhaps some German-style lager. Joel, who digs European lagers and pilsners, loved this beer and we found it pretty true-to-style. We also ordered some nachos which, while not the greatest nachos in the world, were readily eaten.

Highlights: chilled ambiance, good beer, reading random passages from random books off the shelves. Overall grade: A

Our final stop for beer for the evening was the Deschutes pub. Nice ambiance here as well, like a large and spacious hand-carved lodge. The evening was getting late, so Sarah called off any further beer drinking for herself, but Joel and I both ordered the Hop Trip, mine on draught, his on cask. Both were fine beers, but we speculated on the differences in perceived bitterness. It seemed like the cask beer was more bitter than its draught counterpart. We wondered if the slight pain from the carbonation in the draught version distracted from the bitterness, or whether the bubbles tended to lower the surface area of the tongue that the beer had contact with, while the flatter cask version sat right on your tongue longer. At any rate, these beers were tasty, as was the food: sweet potato french fries, some kind of german sausage, and some spicy Thai chicken wings which will make your face melt off.

Highlights: I’m amazed I can still see straight by this time. Overall grade: A-

Finally, we drop Sarah off at the hotel room because she’s passing out and Joel and I decide to go for a walk to Voodoo Doughnut. We get there at about midnight and the place is packed, with a line just about out the door. A few minutes later, we’ve made it to the counter and we find out that many of our first choices are out of stock, including the infamous “Cock and Balls”. I order a Neapolitan cake doughnut and a Maple Bacon bar. Joel ordered a chocolate/peanutbutter/rice krispie conglomeration, among others. All the doughnuts were good, although I recommend eating any Maple Bacon bars soon after ordering them, as the fat on the bacon will eventually get rather gooey and gross if you let it sit, say overnight.

Highlights: informing some Cock-and-Balls-seeking customers outside that they were out of them, and being met with wails of despair.

So, that’s the essence of our tour. We discussed a a great many things regarding sensory science, and the brewing industry, from shelf-life studies, to training panels for various tasks, to the coming World Brewing Congress (in Portland, next summer, see link to the lower left), and more. It was a good trip with good food and good beer, and I’m glad I was able to join my friends and colleagues.

Hope to keep it up well into the future!

Anyway, this weekend some sensory colleagues of mine and I will be assembling in Portland to discuss various matters, as well as to tour local facilities. Part of the focus for some of us is to be tasting various sour beers. Here’s where you come in: know of any pubs and breweries in Portland, Oregon with any sour beers on tap? It’s been awhile since I’ve spent any appreciable time in Portland, and there are likely a number of breweries which didn’t exist the last time I was there.

So if you have any tips or suggestions for establishments that we can darken with our presence, please let me know by the end of the week!

(Maybe I’ll turn it into a post as well)

Anyway, let’s get on with it. I’ve already discussed the primary source of bitterness in beer, so now I’ll revisit the topic of bitterness from a different view: the physiology of bitterness.

The sensation of bitterness is not well understood at all. Nearly every aspect of bitterness is shrouded in complexity, and although new research is continually expanding our level of understanding there is still a great deal to be learned. The reasons bitterness is a tricky subject to elucidate are numerous and varied. There are a wide variety of chemical compounds which are bitter, such as polyphenols, organic acids, peptides, salts, sulfimides, and acyl sugars. This variety in molecular size and shape in turn implies a variety of mechanisms of operation. There is also a huge variation in how bitterness is perceived by individuals, and these variations are largely genetic in origin. Further confounding our understanding of these mechanisms are the difficulties that arise when attempting to communicate the qualities of bitter sensations. There are no agreed upon vocabularies for describing bitterness and its qualities, so while one person may describe caffeine as having a harsh and unpleasant bitterness another person may call it medicinal and lingering. Are they perceiving the same sensation, and is there even a way to tell for sure? Yet another factor in the complexity of bitter taste is the interactions that bitter sensations have with other sensations, most notably sweetness. Certain mixtures of bitter and sweet compounds can have interesting and unexpected effects on each other, with some sensations being suppressed by the presence of other compounds. In some cases there can be a synergistic effect where the total sensation is greater than what would be expected from a merely additive effect. In this article I will explain some of the mechanisms and characteristics of bitterness as we understand it so far.

Structures

Caffeine

Epicatechin

Urea

Quinine

Sucrose octaacetate

Mechanisms

Many bitter compounds are detectable at very low thresholds (eg. 4.5ppb for denatonium benzoate), and because of this it’s apparent that bitterness is a receptor-based sensation, meaning that there are specific structures on the tongue which respond to specific molecules. With the wide variety of compounds which are bitter, it seems likely that there are a number of different kinds of receptors as well. The fact that genes have been discovered which control sensitivity to specific (but not all) bitter compounds, and also that compound mixtures can have synergistic effects (beyond merely additive) also support the idea that there are numerous types of taste receptors, both for bitterness and sweetness. The specifics of these synergistic effects (or lack of) are rather enlightening: in the case of saccharin and acesulfame, artificial sweeteners with similar molecular structures, there is no synergy as they are similar enough that they are probably competing for the same receptor sites. As yet, however, there have been no studies which examine the synergistic effects of bitter compounds that I am aware of, but given the various parallels between bitterness and sweetness, I wouldn’t be a bit surprised to find that they exist.

So, on a general level, how do these receptors work? How does a molecule tell your brain that it is bitter? Each cell in your body has an electrochemical gradient across its membrane which means that the inside of the cell has a different ionic charge than the outside. Cells have a number of different methods for maintaining this difference, including various channels and pumps for transporting different charged particles across the barrier, usually the positively-charged hydrogen ions. This means that in a normal resting situation, taste cells have a slight negative charge internally. Whenever something happens to the cell which alters that gradient to a particular level, the cell’s membrane-bound channels open briefly and the cell depolarizes which stimulates the nearby nerve cell and a signal is sent to the brain.

More specifically, this process involves a whole series of proteins and other molecules that probably operate in a variety of different ways, each leading to the same end result: bitterness. The receptor, or ligand-binding site, is a large membrane-bound protein made up of a number of different sections both on the inside and the outside of the cell. When a ligand (or, in this case, a bitter molecule) attaches to its specific binding site, it causes a change in the shape of this protein which allows its internal portions to interact with another protein called a GTP-binding protein (or G-protein). This G-protein in turn activates a second messenger inside the cell which is a type of enzyme, either adenylyl cyclase, phoaphodiesterase, or phospholipase C. Each of these enzymes does different things, either producing cyclic adenosine monophosphate (cAMP), hydrolyzing cAMP, or releasing internal stores of calcium ions, respectively. These resulting molecules then go on to affect the activity of the membrane-bound ion channels which causes the cell’s electrochemical gradient to change and the nerve cell to be stimulated. Below is a graphic I found on the intertubes. It does a reasonable job expressing the basic idea without being overly complicated.

G-Protein Signaling, from: www.mdc-berlin.de

This system can be stimulated at different levels as well, and the various sections can also overlap simultaneously. A single receptor can activate a few different G-proteins, or multiple receptors can activate a single G-protein. Multiple second messengers can also be involved, as well as multiple ion channels. Once you begin to see all the ways that these different combinations can lead to a nerve impulse, it does not become implausible that this is probably a big reason why there are vast differences in how different compounds can be bitter in different ways, whether more intense, longer lasting, more or less pleasant, etc.

Other Factors

So we now have a somewhat better understanding about how the tongue and the brain perceive bitterness, but what about all the other factors that affect our perception of bitterness? Well, one factor is age: bitter and salty perception declines with age, although it’s been shown that sweet and sour perception is relatively unaffected. Interestingly, the decline of bitterness sensitivity is really only applicable when analyzing specific locations on the tongue. It is believed that whole-mouth perception may compensate for these localized defects to the point where the elderly may not even realize that they are losing sensitivity. We’ve already mentioned how some sensations can be altered by other compounds present in the food/beverage, but the extent of this is quite striking. Caffeine has been studied extensively in this regard. Caffeine’s bitterness can be supressed by sugar and salt, with natural sweeteners working better than artificial sweeteners. Acids can either suppress or enhance caffeine bitterness, depending on concentration. Some research has shown that these suppression effects might be taking place in the nervous system rather than by receptor competition on the tongue. There is a fair bit of speculation that a person’s diet has an effect on how they perceive bitterness as well, but there is not agreement among researchers on this. As mentioned, genetics play a role in sensitivity. Genetic research has shown that the ability to taste PROP (6-propyl-2-thiouracil, a very bitter compound used to test bitterness sensitivity) is linked to the chromosome locus at 5p15, with some influence from a modifier locus on chromosome 7. Considered a dominant trait, 70% of Caucasians are sensitive to PROP while at least 90% of Asians and African Americans are PROP tasters. There is also a subgroup of PROP tasters who are extremely sensitive to it; they are known as “supertasters”, and they are mostly women. These supertasters have more fungiform papillae on their tongues and more taste buds on the papilla. It is unclear whether these supertasters are also more sensitive to other tastes. Some research has shown that PROP tasting is linked to heightend sensitivity to caffeine, saccharin, and quinine, but not urea. Again, more evidence that bitterness perception is controlled by a variety of different types of receptors and other factors.

There’s lots of stuff we’re learning about taste and bitterness, but there is still a long way to go. It’s pretty fascinating, since this topic is a convergence between genetics, food science, physiology, and nutrition. All of these confounding aspects also tend to make it a very complicated topic.

Sources and Further Reading:

Drewnowski, Adam., The Science and Complexity of Bitter Taste. Nutrition Reviews, Vol. 59, No. 6, June 2001, p163-169.
Walters, Eric., Roy, Glenn., Taste Interactions of Sweet and Bitter Compounds. Flavor-Food Interactions, Chapter 12, 1996, p130-142.
Guinard, JX., Zoumas-Morse, Christine., Dietz, Jason., Goldberg, Shelley., Holz, Mylan., Heck, Elizabeth., Amoros, Abel., Does Consumption of Beer, Alcohol, and Bitter Substances Affect Bitterness Perception?, Physiology and Behavior, Vol. 59, No. 4/5, 1996, p625-631.

Chocolate, maybe not surprisingly, is one of those flavors that is made up of a strange hodge-podge of flavor compounds which, taken on their own, have no relation or similarity to the flavor of chocolate. Research from the Technical University of Munich is starting to show just how complex chocolate flavors are. They’ve found that there are up to 600 different aromatic compounds in cocoa beans, but you really only need about 25 of them to make a decent chocolate flavor. Twenty-five is still a big number for a single flavor and the ones on that list come from a wide-range of flavor categories, many having no obvious connection to chocolate: potato chips, cooked meat, peaches, raw beef fat, cooked cabbage, human sweat, earth, cucumber, honey… etc etc. Certainly not the types of flavors you contemplate as that decadent Swiss chocolate melts in your mouth, are they?

While not part of the research mentioned in this latest press release (for an ACS meeting), here is a table from a book about ‘chocolate science’ which includes data from the same researcher (Schieberle) which shows a large list of compounds found in the aroma of chocolate (milk chocolate, pg 67; dark chocolate, pg. 69). Since chocolate also undergoes Maillard reactions and is fermented as well (like beer in both regards), a number of these flavors are also found in beer: maltol, phenylacetaldehyde, diacetyl, dimethyl trisulphide (ew!), gamma-nonalactone, butanoic acid, various furans and pyrazines, just to name a few. Fascinating stuff!

I stumbled upon this gem of an article written by a “very knowledgeable” winery tour guide from the Napa valley area. In it, he discusses how the aroma of wine depends on which way you swirl the glass, clockwise or counter-clockwise. The reasons he posits for this are… interesting. You’ll just have to read it for yourself.

Enjoy.

Please also note the link at the top leading to an equally entertaining follow-up article where he further attempts to explain his wine prowess and reasoning.

Facepalm, headscratch, mouth agape, etc.

Myrcene is an aromatic hydrocarbon which is an important part of the essential oils of a number of different plants, most notably hops. In perfumery, it is used as an intermediate in the production of various aromatic compounds like geraniol, nerol, and linalool. In brewing, it is considered the headlining feature of the “green hop aroma” and is often found in many dry-hopped beers. It has an odor which is described as “herbaceous, resinous, green, balsamic, fresh hops, and slightly metallic” and can be quite pungent at higher levels sometimes smelling a bit like floor-cleaner. In water its odor threshold is about 14ppb, but it is a good deal higher in beer. While it is found at very low levels in kettle-hopped beers, its high volatility and low solubility in aqueous solutions means that it doesn’t tend to stick around very long during the kettle boil. In fact, some studies have shown that myrcene levels in beers which were hopped at the beginning of the boil are around 0.13ppm, while beers which were hopped after wort cooling had about 66ppm – a 508x difference! Myrcene is also readily oxidized and there are some ideas that if it doesn’t volatilize up and out the kettle stack, then it probably degrades and leads to a handful of other aromatic compounds.

Cascade hops tend to be regarded as the classic “myrcene hops”, and in fact it makes up roughly 50-60% of the total hop oil fraction of Cascades. Some hop varieties do have higher levels of myrcene than Cascades, however. Amarillos (~70%), Citra (65%), Crystal (40-60%), Horizon (55-65%), Simcoe (60-65%) and others can have higher levels than Cascades. Conversely, most of the European Noble hops have some of the lowest levels of myrcene: Saaz (5-13%), Hallertau Mittlefrueh (20-28%), and UK Fuggle (24-28%) are among the lowest. Keep in mind, however, that geography, growing conditions, and storage conditions all play a part in dictating myrcene levels. The same study mentioned above showed that a post-wort-cooling hop addition with hops aged at 40C for 30 days yielded myrcene levels of 0.82ppm (as opposed to the 66ppm with cold-stored hops). As with most other aspects of hop quality, there is a difference between whole hops and pellets as well. Whole hops can have as much as 70% more myrcene than pellets of the same variety, but that difference is flipped when the wort is hopped as only 5% of myrcene is extracted from whole hops compared to 17% from pellets.

(note: there are some discrepancies in the literature regarding myrcene levels in Hallertau Mittlefrueh, with some levels reported to be around 10-30% of the total oil fraction, while another study has found higher levels of myrcene in Hallertau MF than in Cascade hops. Since more sources are reporting that H.MF has very low levels compared to most other hop varieties, that is the idea I would stick under most circumstances).

References:
IndieHops, In Hop Pursuit Blog, Hop Oil: Is Bigger Better? A Preview of Ongoing Research at OSU

Kishimoto, T., Investigations of Hop-Derived Odor-Active Compounds in Beer, Hop Flavor and Aroma, Proceedings of the 1st International Brewers Symposium, 2009, pg 49-58