Category Archives: Hop Flavor

It’s alive!

I’ve just finished relocating to a new city so it seems like a good time to dust off the ol’ blog and create some content!  Let’s pretend that I haven’t left you all hanging for more than a year without any new beer sensory science content and get down to it with a short literature review:

Many brewers and beer aficionados already know that one of the first ways that beer degrades as it ages is by the loss of the hop aromas which are often considered to be marquee flavors in many products and styles.  As such, if one wants to know how to extend the shelf life of beer and maintain a fresh-tasting product for as long as possible some investigation into how these aromas are lost is warranted.

This paper explores the various ways that hop oils (a major source of hop aroma) are lost throughout the shelf life of beer and focuses mostly on the loss of the aromas into packaging materials like the rubbery plastic liners under the bottle caps or crowns.  It was published in the Journal of the American Society of Brewing Chemists in 1988 and written by Val Peacock and Max Deinzer – a former AB chemist and hop guru, and an experienced analytical chemist from the Oregon State University chemistry department, respectively.   Both of these men have been extensively involved in beer research for years, and hop research in particular, so they know their hop chemistry; I can’t think of many too many more researchers more capable of attacking this question.  Let’s see what they have to say about this.

First, the researchers present data from some analyses they performed on commercially-available products:  a “super premium American brand” (Beer 1), a “Central European Import” (Beer 2), and an “American product from a mini-brewery” (Beer 3).  Flavors were extracted from these beers via continuous liquid-liquid extraction with dicholormethane and prepared with 2-octanol as an internal chromatographic standard.  In addition to analyzing the beer itself, they removed the foamed-PVC crown liners and extracted them in hexane prior to being made up for gas chromatography/mass spectrometry analysis.  Relative concentrations of analytes were calculated by finding the ratio of the amount found in the beer vs. the crown liner.  Analytical results for roughly 36 flavor-active compounds (15 from hops) are presented, with concentration values for both the beer and the crown liner indicated.   Overall, they found that the more polar, or less oily, the compound, the less it migrated into crown liners.  Therefore, alcohols and the water-soluble esters (like isoamyl alcohol and isoamyl acetate) were not found in liners in any appreciable levels (0% and 2% found in crown liners, respectively), while the non-polar compounds, like the hop terpenes and sesquiterpenes myrcene and humulene as well as the long-chain fatty acid esters, were found only in the crown liners.  Other hop aromas, like terpene alcohols, linalool, and geraniol, were only found in the beer.

In order to understand the rate of uptake of some of these compounds into the crown liners the researchers created model systems of non-carbonated 3.5% and 3.0% (v/v) ethanol/water solutions and spiked known amounts of several hop-derived compounds, then re-crowned the bottles and stored them for 18 and 28 days, respectively.   In the 18-day 3.5% ABV model, 79-87% of the hop-derived hydrocarbons (myrcene, caryophyllene, and humulene) were lost to the crown liners.  As was seen in the commercial beer analysis very little, if any, of the water-soluble compounds were detected in the crown liners.  In the 3.0% ABV model system after 28 days of storage, the researchers found that only small amounts of the oxygenated hop compounds (alcohols, epoxides, and diepoxides) were captured by the crown liners.  Some of the results ran counter to what was seen in the previous analysis, and it was speculated that either some of the compounds degraded by oxidation after they were captured by the liners, or that the 3.0% uncarbonated model system was different enough from the other beers analyzed and that this unpredictably affected the results.

Finally, the researchers looked at the rate of decomposition of four hop aroma compounds which they had spiked into a “premium American beer” (implied later to not be a pilsner):  linalool, geraniol, humulenol II, and humulene diepoxide A.  Beers  were stored at room temperature for about 60 days to simulate warehouse and market storage.  11% of linalool was lost after 57 days, and the steep-then-level nature of the decomposition curve indicates that the degradation of linalool is not a first-order reaction and implies that there are other factors at play in the decomposition of linalool – either that there is an equilibrium that is reached or that linalool is reacting with beer components that also get depleted over time, such as oxygen.  Breakdown products of linalool were analyzed in the final (57-day) sample and the amounts found only account for 10% of the lost linalool, which is somewhat puzzling – perhaps there are other breakdown products which were not realized in this study.  Geraniol behaved similarly to linalool:  12% lost in 56 days, with the majority lost in the first couple weeks and few anticipated breakdown products detected.  Humulenol II degraded much more rapidly than linalool and geraniol, with 66% being lost after 61 days.  While the decay curve isn’t as “curvy” as the previous compounds, it still leveled off somewhat.  They also found some additional compounds in the final sample which they guessed were humulenol II breakdown products, as there was none of these detected in the fresher samples, nor in the linalool/geraniol samples.  GC-MS results implied that both oxidation and acid-hydrolysis were at play.  Lastly, humulene diepoxide A decayed the fastest of the four compounds, where 84% of it was lost at 56 days in a nearly-linear rate.  Numerous supposed degradation compounds were detected, but the reactions are so complex that identification was not feasible.

Overall, this paper provided an interesting look into a couple of the main reasons that hop aroma is lost in aging beer:   adsorption/absorption into crown liners (and likely aluminum can liner material as well!) and oxidation/acid hydrolysis reactions leading to their conversion to other compounds, both flavor-active and not.  When one considers both the importance of hop aroma to so many craft beers and the fragile nature of hop aroma, it seems like some attention should be paid to maintaining sufficient hop aroma over time.


Fate of Hop Oil Components in Beer. Val E. Peacock and Max L. Deinzer, Department of Agricultural Chemistry, Oregon State University, Corvallis 97331. J. Am. Soc. Brew. Chem. 46:0104, 1988.

Hop Candies: woah.

If you are a fellow hop appreciator, then you may be interested in these.   A reader of this blog contacted me with some questions a little while back, and also offered some of his unique products to me to try:  hop candies.  These are Jolly Rancher-like candies made from the aromatic oils of a single hop variety, and they are very interesting.

Despite tasting like you’re chewing on a fresh hop cone, these candies are not really bitter at all (there may be a hint of bitterness if you stretch your imagination), but the aroma is very authentic.  Lots of myrcene and caryophyllene, the types of flavors that you’d pick up from a dry-hopped beer (and that are lost when you hop in the kettle).  He sent me a sampling of 3 varieties (Cascade, Fuggle, and East Kent Goldings) and while they are all similar they do have their distinctions.  For one, the Fuggle seems slightly more bitter than the Cascades, but it is a minor difference.  The bitterness comes across more like a slight tickle from time to time, so it’s not that disagreeable if you aren’t normally a fan of bitterness.

The main concern that I had, and was shared by the various panelists who tried them, is that they are a little too big.  These candies are about the size of the small Tootsie Rolls but slightly longer, and I think they could be about 1/2 – 2/3 of that size, as they seem to drag on a little too long.  Some of my panelists thought that the aftertaste that lingered for quite some time was unpleasant in the way it stuck around, but personally that was my favorite part.  With the candy in your mouth you had the continuous resinous raw/fresh hop flavor explosions, but after it was gone it became more subtle, with floral and citrus notes. A unique take on breath fresheners…

They are available from a number of online sources, listed below.  Check them out!  They’re probably unlike any other candy you may have had.
Grape and Grainery

2 more articles on deck, maybe today, maybe some other day soon!

Linalool – Fresh and Floral Hop Aroma

Linalool has a rather prominent, but at times contested, place in hop aroma. Over the past several years, many brewers and research groups have attempted to use it as a marker in the assessment of the qualities and quantities of hop aromas, both in the hop field and in the beer bottle. Other investigators have been hesitant to distill the representation of such a complex phenomenon as hop aroma into a single compound, and have downplayed its usefulness as a chemical marker. Nevertheless, linalool is an influential part of hop aroma in many varieties and, depending on hopping regimes, in finished beers as well. Here, we’ll discuss the importance of this aromatic compound in hops and brewing.

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!


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.

Bitterness, Pt. II

Phew! Where’s the time gone?

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.

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Myrcene: the Green Giant of hop aroma

And we’re back!

Myrcene chemical structure

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 0.13ppmppb, while beers which were hopped after wort cooling had about 66ppmppb – 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).

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

10/24/12: An EDIT that took too long to initiate: fixed myrcene levels from ppm to ppb.

7 IPA’s reviewed

In lieu of the normal training session activities that I normally give my panel on Thursday mornings, I decided to switch it up a bit today and present them a number of IPAs to judge. Each one was purchased from a local grocery store or specialty beer/wine store, and was tasted blind by the panel of 11 tasters. I asked them to throw out any term or descriptor they could think of, and when we finished with that I went around the room and asked them express their preference for each sample on a 1-10 scale (10 being the best beer they’ve ever had). Below are the terms that were used for each sample, as well as the average rating and the range of ratings given for that sample. I’m assembling them here in order of “worst” to “best”.

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The IBU Assay

The spectrophotometer is among a small set of tools and equipment that are essential for a quality control lab to be adequately productive and accurate in a brewery setting. The reason for this is the BU assay which, apart from the HPLC, offers the best and most direct way to analytically measure the bitterness of beer that is available to brewers today. Relative to HPLC, the BU assay lacks precision, accuracy and sensitivity. But what makes it appealing are a number of things, foremost being the price: a couple thousand dollars can net you a new UV/Vis spectrophotometer and some supplies, whereas an HPLC can be an order of magnitude more expensive. Convenience, simplicity, and to some extent reliability are also among the benefits of this method, since HPLC, indeed chromatography in general, can be fickle and prone to error if rigorous procedures are not followed. Here, we’ll explore various aspects of the BU assay, including its origins and the fundamentals behind it.

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