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.