Coming soon to a brew near you

Have you ever taken that first sip of a beer and had to put the bottle down because it was skunked? You liar. You drank it anyway, didn’t you. In between some of the poop jokes on the very exclusive Aleheads email list someone mentioned skunked beer, and it got me thinking as it’s something we’ve all experienced.  Since I’m a science nerd, I thought it might be interesting to write a little bit more about the chemistry of beer and why beer gets skunked in the first place.

There are a lot of ways to initiate a chemical reaction, and  UV light is one of them: some organic molecules have the ability to absorb high energy UV radiation.  When they absorb that energy, they can respond by undergoing rearrangements, fragmentations, or other reactions. UV light is famous in chemistry for enabling some really unusual transformations – [2+2] cycloadditions generate a 4-membered ring (this reaction causes crosslinking of DNA in your skin upon prolonged sun exposure and can be a cause of melanoma.  Sunscreen works by absorbing the UV radiation so your DNA doesn’t. Lather up.), or the Norrish fragmentation are two examples for the chemists in the crowd.  Heh.

Like any Alehead, you are probably asking yourself “Yeah, but how does this relate to beer?” Well, it turns out that many of the thousands of components of beer also have the ability to absorb UV light, and can undergo some of these same (mostly undesirable) chemical reactions.  When beer is exposed to sunlight (and less so to incandescent/fluorescent lighting) the UV rays cause a few different reactions to occur.

For skunked beer, the molecules that are primarily affected are humulones.  Humulones are a class of molecules that represent some of the bitter principles that come from hops and are perhaps better known as alpha-acids. When you boil wort, one of the things that happens to your bittering hops is that humulone is converted to cis- or trans-isohumulone* (these two molecules are collectively known as isohumulones, or iso-alpha acids) Notably, that the 6-membered ring in humulone is changed to a 5-membered ring in the isohumulones. The isohumulones are more soluble and desirable as they provide a different flavor than unisomerized humulone. You can control the amount of isohumulones in your beer by the amount of time you boil hops.

It turns out though, that those isohumulones are somewhat fragile.  When beer is exposed to UV light, the isohumulones that were so desirable in the brewing process can break down and through a process initiated by a Norrish reaction generate a compound called 3-methylbut-2-ene-1-thiol. You can see where the atoms in this offensive compound come from in the color coded picture.  The sulfur here arises from the decomposition of methionine, a harmless, odorless amino acid.  While this is a very specific process, there are a number of other reactions that can occur upon exposure of beer to UV light that produce other sulfur containing compounds, and sulfur compounds in general are pretty smelly (think rotten eggs).

So why is it called “skunked” beer anyway? It smells like a skunk, idiot.  While that little gem might not surprise you, you might be interested to know that some of the components of skunk spray are actually not that different than the skunk smell in beer.  As you can see in the picture, the major components of the skunk spray are really similar to the 3-methylbut-2-ene-1-thiol that is the main smelly component of skunked beer.  In fact, 3-methylbut-2-ene-1-thiol itself is probably found in skunk spray as well.  I can’t be sure, because I don’t have access to an exhaustive compendium of the components of skunk spray at the moment.

Fortunately, skunked beer can be avoided to the most part by the use of brown bottles (or even more desirably, cans). As if you needed another reason to stay away from Corona.  Interestingly, a high oxygen content appears to protect against the damages of UV light as well.  Oxygen works presumably by oxidizing the radical-generating species before they get the chance to form radicals (or at least before they react with the humulones), most probably the sulfur containing precursor molecules.

So the short of it is that the “skunk” flavor in beer is actually not too far off from real skunk spray. Just at lower concentrations. Think about that next time you take a sip of a skunked beer, and happy drinking.

*Wikipedia calls this process a degradation.  It’s really rearrangement.

22 thoughts on “SKUNKED

  1. See, this is the kind of stuff you don’t get anywhere else. Professor Lager, breaking down one of the scourges of the beer world…skunkiness. Although I was never much for Chemistry, I have been watching Breaking Bad lately, so I feel like I understand this pretty much perfectly. The key to preventing skunkiness is a little Chili P…

  2. I can’t help wonder, given this information, why Brewdog has yet to cram a beer inside a skunk. What the hell are they waiting for?

  3. WOW! That hurt my brain.. reminded me of my organic chem class..ouch. Got a load of Molson Golden ale that brought the skunk down on me… now I know how. Thanks for the intel and yet another reason to grab a craft brew… quality control is a serious mission for this alehead.

  4. Awesome article, simple enough that a lay person could understand the principles and enough juicy tidbits that will send a chemist like myself scurrying off for a google search. Are there any papers published on skunking in beer that you know of?

    Anyone know what the entry on skunking is like in the new oxford compendium?

  5. Thanks for the comments! I’m sure that there is a decent amount of work that’s been done on skunking, although that sort of thing isn’t usually published in academic journals as it’s too industry-focused. You can bet that the big brewhouses have teams of scientists who have figured all this stuff out already. We’ve talked about it before, but even though Bud is swill, it is kind of a triumph of modern engineering that no matter where you are in the world it tastes exactly the same. Any of us who’ve tried to make beer at home know how tough it can be to control those variables!

    The actual skunk spray analysis was done by William Wood at Humboldt State University — most of the stuff i’ve seen references his work. He published it in the journal of chemical ecology or something like that. It’s not exactly a hot topic, presumably since… no one wants to work with bottles of skunk spray in the lab.

  6. Heat can be used to generate radicals as well. However, I think that heating beer generally gives rise to other detrimental reactions that negatively affect flavor, not specifically the rise of thiols.

  7. What about Kegs? Doesn’t seem like too much UV light gets in there, but I’ve had keg beer that tasted like a sunburnt corona.

  8. Piels, you’re right…kegs can’t “skunk” like green or clear-bottled beer can because light can’t get to it.

    BUT, other things can happen to a keg (extreme temperature changes, oxidation, bacterial growth in an improperly stored, unpasteurized keg, etc.) that can create off flavors similar to the skunkiness the Professor describes above.

    Actually, one of the most common sources of off flavors in kegged beer comes from unclean lines at a taphouse. If you go to a bar that doesn’t do a good job sterilizing its lines, it’s remarkable how close the beer will taste to a UV-bombarded bottle of Beck’s or Heineken.

    So while it might not be undergoing the same chemical changes that the Professor describes, keg beer can still taste pretty goddamn terrible.

  9. As Barley mentioned, bacteria can also be problematic (i.e. from tap lines). Dimethyl sulfide is another sulfur containing compound that smells roughly like rotten eggs, and is produced by bacteria — in fact, the bacteria in your mouth can produce dimethyl sulfide and give you bad breath. Bacteria in tap lines can have the same effect.

  10. >*Wikipedia calls this process a degradation. It’s really rearrangement.

    That’s not a rearrangement, since the carbon skeleton remains the same. You could call it a degradation, since “degrade” is a relative term, depending on what your desired compound is.

    If you want to call it something else, “isomerization” would do.

  11. Barley McHops: “See, this is the kind of stuff you don’t get anywhere else. ”

    Except on my blog,

    Steve: “Are there any papers published on skunking in beer that you know of?”

    Beer Lighstruck Flavor: The Full Story., DeKeukeleire, Denis., Heyerick, Arne., Huvaere, Kevin., Skibsted, Leif., Andersen, Mogens., Hop Flavor and Aroma, Proceedings of the 1st International Brewers Symposium, 2009, Master Brewers Association of the Americas and American Society of Brewing Chemists, pp 1-16.

    Slouch: “So, is it a myth that heat causes skunking as well?”


  12. Thanks, BeerSensor! Great resource! I was mostly just touting our brilliant Professor, but I do apologize for not mentioning your blog as another great site for learning about the Chemistry of beer. You’ve got some cool stuff on there!

  13. Thanks for reading, Erik! Anything else you want to hear about let me know – I often struggle with topics to write about.

  14. What a smart group of aleheads we have here… The “buffalo theory” applies to us very nicely. Here it is. Back in the wild west the American Indians hunted the buffalo. They naturally caught up to the slowest weakest buffalo in the herd for the kill thus making the herd stronger and faster as time went on. Well as you know, when we drink, the alcohol kills brain cells. Naturally the slower ones get caught first thus making us smarter and faster but culling out those weak ones. So, drink and be smarter my friends! Logic just caught up to chemistry.

  15. beer-meister: the buffalo theory is the only way I can explain how I got to where I am today. There’s certainly no reason I would have gotten a job had I not spent all those years carefully weeding out all my weak brain cells.

  16. There is a way to keep beer in Clear/Green bottles from becoming skunked. There is a glass coating called SPF 2000 that is applied to the bottles during manufacturing. It blocks 99% of UV rays. The only problem is trying to find a bottle that has this stuff on it! is where I found it.

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