Category Archives: Oxidation and Staling

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.


It’s been recognized in beer since the 1870’s, and it may just be one of the most well-known flavor defects in beer across the world. Just because it’s recognizable, however, doesn’t mean people are necessarily bothered by it since Corona and Heieneken sell plenty of bottled beer. It’s becoming common knowledge among some of the beer-drinking public that putting beer in clear or green bottles will allow it to become skunky or “lightstruck”. Following right behind is the growing awareness that, with the use of specialized hop extracts, brewers can successfully put beer in such bottles without resulting in skunked beer (Miller products being the best known of these). We’ll discuss these topics and more as we explore this phenomenon. A warning: there will be chemistry.

First, the molecule: 3-methylbut-2-ene-1-thiol, but you can call it 3-MBT for short. With a threshold of around 4 parts-per-trillion in beer, 3-MBT is among the most potent flavor compounds that can be found in beer; as such, it does not take much to ruin your beer. If you drink your beer from a pint glass on a sunny patio you may notice this flavor by the time you reach the bottom of the glass – that’s how quick this problem can arise.

Here’s the little bugger now. Rather innocuous looking, isn’t he?

3-methyl-2-butene-1-thiol, or 3-MBT: Source of Lighstruck / Skunk flavors in beer

We’ll take a small break here for a minor organic chemistry lesson involving molecular nomenclature. The great thing about O-chem is that there are rules by which molecules are named, and if you know the rules you can figure out the molecule’s shape and features. Let’s take 3-methylbut-2-ene-1-thiol: the “but” part of the name tells you that there we are dealing with a carbon-chain that is 4 carbons long (1 carbon: meth-; 2 carbons: eth-; 3 carbons: prop-; 4: but-; 5: pent-; etc etc). Each carbon is numbered sequentially, and I’ve included the numbers in the image. As the name implies, there is a “thiol” group on the number 1 carbon. A thiol is like an alcohol group but with a sulfur atom in place of an oxygen, -SH instead of -OH. Continuing to look at the name we see that on the number 2 carbon there’s an “ene” group, which means that there is a double bond between that carbon and the next. Finally, on the number 3 carbon, there is a methyl group (essentially a branch made up of a carbon “chain” made of only one carbon – remember “meth-” meaning 1 carbon?). So, if one of the carbons off of #3 is a methyl group, which one is the #4 carbon? For the purposes of this molecule, it can be either one, and whichever is #4 then the other is the methyl group. This concludes the nomenclature lesson. Now, back to the… well, more chemistry I guess.

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Does Guinness travel well?

Well, beer in general doesn’t travel well, so I’m going to guess “no”.

I ran across an article about a study by the Institute of Food Technologists (link in the sidebar) which sought to find out if there was any truth to the idea that Guinness served in Ireland tastes better than that which is served elsewhere. Their preliminary results indicate that, yes, Guinness served in Ireland tends to score higher in flavor preference to those outside the country, even after accounting for various variables.

I’m not surprised. In fact, I bet this is the case for most any beer in the world: it’s very likely going to taste better the closer you are to the brewery. This is because there is less transportation and time required to get the beer to various parts of Ireland compared to the rest of the world, meaning less time for the flavor to deteriorate from oxidation and aging. Also, Guinness probably has more influence over local bars and how they maintain their tap lines. This is also related to proximity; there are probably more Guinness reps combing the pubs of Dublin than there are in Boston. They also mention the affect on popularity and the effect on freshness: Irish pubs will probably be going through more Guinness than pubs elsewhere, meaning there is fresher beer on tap since the turnover rate is higher. All of these factors should be no-brainers.

Other thoughts I had when I read this was about the methodology. I haven’t read the paper, but I’m going to assume that all 4 researchers tasted Guinness in a number of different countries, and overlapped their territory, because otherwise there would be no way to control for the “assessor” variable during the analysis. I also note that in the abstract, they say their researchers were “non-expert”. I would have hoped that they would have had some level of beer flavor training, however basic, before undertaking this project. Another thing I wondered about is how they controlled for the “ambiance” variable. While they did mention the possibility that ambiance could affect the assessment of the beers, in the abstract they say the statistical significance remained even after controlling for ambiance. Now, I’m not sure how you can control for ambiance without tasting all the beers in a single location; it doesn’t make sense to me.

I was also a bit irked at the tone from the following passage, which seems to assume that the Journal of Food Science, or beer research in general, might be considered a non-scientific discipline:

That the Journal of Food Science is a serious publication can be inferred from some of the other material in the March issue. One feature is headed: “Technological Optimization of Manufacture of Probiotic Whey Cheese Matrices”. A second reports: “Improved Sauerkraut Production with Probiotic Strain Lactobacillus plantarum L4 and Leuconostoc mesenteroides LMG 7954”.

It’s almost like they needed to convince themselves that Food Science is actually science…

PS: the pint of Guinness I had at the Panorama Sky Bar at the end of the Guinness tour was the most expensive “free” beer I’ve ever had: 17€. But they poured a little shamrock in the head of the beer, so that’s got to be worth something, right? Of course the 17€ was for the tour of the “brewery” (read: “museum”), but really the only worthwhile part of the tour was the view at the top and the pint in your hand.

Looks like someone beat me to it: how to find fresh beer.

I wrote a post last week asking for requests for production information on beer labels, in an effort to accumulate a database that you can reference in your quest to buy fresh beer.

Well, one commenter has enlightened me to the fact that this has pretty much already been done.  What a load off my back!  This could have been a huge and on-going project, and I’m a bit relieved that I don’t have to assemble and maintain such a list.

I’ve had a look over it and it’s huge, and from the entries I’ve seen, pretty accurate too.  Of course, breweries change their labels and equipment all the time, so there may be some inaccuracies hiding in there somewhere, but it’s a great start.

Fresh Beer Only.

So, find your favorite breweries in this list, and make a note of where and how they put their information on the label (hopefully they put something on there; there’s a disturbingly high number of packages that have no information whatsoever on them). Then when you’re standing in front of the beer aisle at the store, don’t be afraid to shuffle the bottles around in order to find the freshest. You deserve it.

Does this make sense?

So I was watching Brewmasters on The Discovery Channel the other night (about Sam Calagione and the Dogfish Head Brewery) and the episode was “Grain to Glass”. The “B-plot” of the episode dealt with a batch of their 120min IPA in a stuck fermentation back in June where the batch started at about 32 Plato and never got below 9 Plato. They were trying to figure out what the problem was, so they went back and pulled some bottled 120min IPA from storage and tasted them. I don’t know how many they tasted, but there was one from 2008 and one from 2006, at least. They decided the one from 2006 had the characteristics they were aiming for, and then implied that they used the notes from that brew to influence the recipe for the brews going forward.

The problem is, beer is not a static and shelf-stable product. Unless the beer is kept cryogenically frozen (generally -80C or less), it will degrade. Even the “bigger” beers which use many dark malts and have high alcohol will be altered during storage, for better or for worse. While some beers may be altered less than others, it seems a bit foolish to base recipe modifications on a sample of beer which is no longer representative of what that beer tasted like when it first hit the market.

Of course I could be missing something. Television editors are known for spinning the story in whatever way they want, and their lack of familiarity with the subject matter would make it easier for them to accidentally mislead.

On a side note, I also noted that Dogfish Head “tested” their sensory panelists with the FlavorActiv standard for “Musty” (trichloroanisole – “cork taint” for you wine enthusiasts). They must have intentionally picked that particular one for filming day, since Musty is literally one of the two easiest FlavorActiv standards to identify at the usual 3X threshold level, and it would have looked better for the camera if all their panelists were seen picking the right one. Now if they did this with FA’s “Kettle Hop”, “Grainy”, or “H2S” standards, I’d be impressed.

And while I applaud the presence of the craft brewing industry on television, I can’t help but think that this show is just a giant ad for Dogfish Head…

Say “Cheese”! Isovaleric acid in beer.

Cracking cheese, Gromit!

Have you ever smelled cheese in your beer? How about dirty sweatsocks? It’s more common than you may think. If you’re a homebrewer and you don’t use your hop supply as fast as you should, or if you store them improperly, you may be familiar with this aroma. This is isovaleric acid, and it’s a short-chain fatty acid commonly found in cheese, the valerian herb, foot odor, and sometimes beer. Now that’s an interesting selection of sources!

The commonly accepted threshold for isovaleric acid is about 1ppm, but like most other aromatic compounds, this can vary greatly depending on your genetics. This brief article gives some information about the genetic component of isovaleric acid receptors, exploring some of the sources of variability in how subjects perceive this compound. One of the more interesting things mentioned is that its detection threshold can apparently differ between individuals by up to 10,000 times. Personally, I think my nose has what I call an “acquired anosmia” to this compound. To be anosmic to a particular compound means you can not detect it at any concentration. While my case isn’t that dramatic, I think my sensitivity has dropped due to being frequently exposed to the purified compound when I spike it into my samples (despite using a fume hood and taking protective measures, it’s still possible to get it on you). If you get this stuff on your hands, you’ll stink for the rest of the day, if not longer. For this reason, I often have a hard time being able to tell if my spiked samples are at an appropriate level for the panel. Many times, I have to trust my math more than my nose.

So, how does isovaleric acid get into beer? Most of the time, it’s formed when hops get old, particularly when the alpha acids degrade. I’ve discussed hop acids already in the bitterness article, so if you need a quick overview, head over there and it might clarify some things. This image (from the above-linked article) shows the basic structure of the alpha acids (on the left) and the iso-alpha acids (right) that they isomerize into during boiling in the brewing kettle (at which time they become the source of bitterness in beer). Basically, there are 3 main types of alpha acid (and the 3 corresponding iso-alpha acids) and while they have the same basic structure as each other, there are differences at the “R-group” (top right of the molecule in the images). The differences are minor, but these minor differences can be interesting and influential nonetheless. One of these 3 alpha acids (humulone) has an R-group which is called an isovaleryl group. When this alpha acid oxidizes (due to age and/or improper storage), this R-group can be removed from the molecule and becomes flavor-active, leading to the cheesy/sweatsock flavor I’m on about.

Another way isovaleric acid can get into beer is through a Brettanomyces infection. It’s not the most common source in beer, but infection by this yeast genus can produce cheesy aromas, as well as a host of other undesirable flavor-active compounds like acetic acid (vinegar), 4-ethylphenol (bandages), and 4-ethylguaiacol (smoky). Some breweries intentionally “pitch” Brett into their fermentors as they try to achieve a certain flavor profile or match a particular Belgian style, but more often than not a Brett infection is a bad thing. Brett is also used in winemaking to achieve certain flavors, but it can also be a spoilage organism here depending on the intent of the oenologist.

So limiting undesirable isovaleric acid levels in your beer comes down to using fresh and high-quality raw materials (store hops in a cool, dark environment and, if possible, oxygen-free), and maintaining sanitary brewing conditions and using plentiful and healthy yeast to limit the potential for beer spoilage.

Jackpot! The Beer Fishbone Diagram

This PDF is a bonanza of information, enumerating the multitude of factors involved in all sorts of beer phenomena. It’s called a Fishbone Diagram, and the reason is obvious once you see it. I can’t even begin to explain everything that’s in here, I mean it would takes hours (days?) to pick it apart.

It’s pretty easy to interpret, although it is a bit of an information overload. Each page explains the various factors that influence a particular quality issue in beer. For example, below is a screenshot for the one of the pages [!] about how packaging and brewing issues interact to promote or limit beer oxidation. Other issues covered are controlling beer pH, fusel alcohols, H2S levels, foam quality, beer stability, yeast flocculation/vitality/viability, etc etc etc.

Brewing/Packaging Parameters and Beer Oxidation

You can find it here:
[see below]

Please excuse the rotated table of contents; I rotated the PDF so that the first page was the only one (of 42) that you needed to crane your neck to read. Better yet, print it out and enjoy it with a pint or two of your favorite beer. I’m going to go get a blonde ale out of the fridge right now.


Edit, 1/6/11:  Looks like these fishbone diagrams were developed by Greg Casey, recently (currently?) of Coors Brewing.  I hope it’s OK that they’re posted here…

Edit, 1/2/13: I’ve recently been informed that the file on the host site disappeared, so I’ve rehosted it at another site. If it disappears again, shoot me an email and I’ll try to get it back up.

Edit, 1/23/13:  At the moment, the free file-hosting websites I’ve been using don’t seem to have much of a shelf-life.   Either that or Greg Casey has a Google Alert on “beer fishbone diagram” and every time he sees the file posted he submits a takedown request to the hosting site.

Anyway,  I’m going to do this on an on-demand basis.   If you’d like a copy of the Beer Fishbone Diagrams, email me (found on “About” page) and I’ll get you a copy within a couple days.