Your beer is skunked you say? Damn those thiols!

Mr. Science recently spent a week in Panama with Mrs. Science. Many, many drinks were consumed at the swim-up bar in the pool. Mostly, I was drinking Seco, a sugar cane liquor that was pretty tasty with pineapple and lime; gallons of Abuelo Cuba Libre; and quite a bit of beer. The standard light lagers of Panama available were Atlas, Panama, and Balboa. Balboa had more flavor, so I ordered it more than the others. They were coming in plastic cups, of course, and I found it interesting that if you were not slamming them down like a sailor with a one day pass, you could progressively taste the beer getting skunky as it sat in the equatorial sun. An interesting, albeit nasty, phenomenon.

Sit back, open a Heineken (green bottle, not can!), and learn something.

Skunky or light-struck beer has to do with those wonderful oily, resiny plants that make beer so rad, Humulus lupulus itself, the hop! Whoopee! Fanfare and such. Hops are used in the brewing process to add flavor, aroma, and varying levels of bitterness to the finished product. Hops, as we know them, are the female flowers of the plant that have not been allowed contact with male pollen. This prevents the formation of seeds and the flower just continues to produce resin glands. I suppose that the Spanish term for “without seeds” shared by one cousin of hops might just apply here. Anyway… the more resin, the better.

The resin glands or lupulin glands are tucked down in the crevices of the bracts and bracteoles. The glands are filled with an oily fluid that is essentially what we are after when it comes to flavoring a brew. Evolutionarily, one can assume that these sticky glands serve the plant in various ways. Hops are generally considered to be a wind-pollinated plant. Sticky oils are going to give the benefit of trapping male pollen. Fragrant aromas may also be used to attract some pollinator but this is just an assumption. One thing that you can be sure of is that some of the alkaloids that are the focus of our discussion are used as a defense mechanism by the plant. Less damage is going to be done by pests and the opportunistic herbivore if there are deterrent molecules in their food.


All Hail Humulone!

All Hail Humulone!


So there are a lot of chemicals in the lupulin. Probably the most discussed are the family of alpha-acids, including humulone and its related compounds. As we know, part of the beer making process is boiling. During the boiling process, hops are added at varying times–hops added during the early part of the boil are the bittering hops. At this point, more of the flavor and aroma molecules are going to volatilize, leaving just the bitter compounds. The heat from the boil isomerizes (changes the structure of) the alpha acids and forms isohumulones, molecules that are more bitter than their predecessor molecule. Control of this process allows the brewer to produce beers of varying intensity with regard to bitterness.

The finished beer is then packaged and we plan to enjoy a nice beer in the sun. How can you go wrong? Well, a few critical ways:

  • If your beer is in clear bottles or any bottles for that matter, sunlight can hit the beer.
  • If you put it in a clear glass or the aforementioned plastic cup, you get that sunlight business again.


I mention it because the sun, specifically ultraviolet light (and less so just plain old visible light shining on the beer), results in an interesting series of molecular changes. Riboflavin (Vitamin B2) is a nutrient in malt that ends up in the final beer. Riboflavin is activated by UV light and when activated, it catalyzes reactions that modify other molecules. In the case of beer (heheh case of beer), oxygen in its natural state of two atoms hanging out together (O2) is split apart and forms two reactive oxygen atoms. These atoms bounce around looking for a friend it reacts with– an arm of the isohumulone molecule forming a free-radical molecule. Frankly, this is decidedly not radical. Then carbon monoxide is pushed off forming 1,1-dimethylallyl. This old boy reacts with the sulphur containing amino acids, cysteine and methionine, shearing off a sulfhydryl or thiol group and making 3-methylbut-2-ene-1-thiol. This thing smells and tastes very similar to the compounds expressed from the anal glands of a skunk. Not that I know what a skunk’s ass tastes like, but you know what I am saying.


Aww, cute


Anyway, this is the stuff I think about when I am supposed to be relaxing in the sun. I guess the idea here is keep your beer out of the sun. That or just chug the hell out of it. By the way, if anyone tells you that repeated cooling of a beer skunks it, you can proudly and with great assurance, tell them that they are a stupid buffoon.


Blatantly stolen from

Not rad radicals...


About Chris Washenberger

What is my favorite drink? Huh... That is a tough one. What do you have? That is probably it.

  • ericmsteen

    You make science a bit more interesting, thanks! I’m not a fan of skunked beers, but I have repeatedly had this Saison from France that comes in a green bottle and has a light skunkiness to it. I actually really dig it that way, it’s surprisingly good. I imagine the brewery intended that, I mean they put it in a green bottle and it’s definitely a full-flavored beer. I like to think it’s intentional. Just curious if there are any examples where beer is supposed to be a bit skunky?

  • Chris Washenberger

    There are a lot of beers out there where part of the brand characteristic is a slight thiol flavor. I can’t conjecture as to the original intent or the history in these cases but I would bet that there are a lot of green bottle beer fans out there that would riot if they changed the flavor. I think this should be opened up to folks who might have a better idea on European brands than I do.

    What saison was it? I remember a green bottle saison from way back in my early beer days that was a bit skunky as well. I want to say it was Saison Dupont but I am pretty sure that it is in a brown bottle.

  • Justin Lloyd

    Another great article. Mr. Science rules!!!!

    This light triggered thiol is the reason you rarely if ever see Heineken on draft anywhere. Heineken fans don’t think it’s actually Heineken. Same goes for St. Pauli Girl, and Moosehead – take off you hosers.

    Another reason why beer in cans are WAY better than bottles. After all the can is just a mini, single serve, keg.

    Mr. Science – What makes sour beers sour?

    • Chris Washenberger

      Their terrible attitudes.

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