Essential Oils and… Skunks?

All Hail Humulene

Alright folks, time for a little chemistry lesson of sorts. If you don’t like chemistry and are more of a “left brain” person that’s ok you can tune out like you did for all your other science and math classes and go read some of my super high quality reviews! However, if you’ve ever marveled at the aroma and flavor of beer and wondered where that comes from, I have an answer for you! Well… kind of. The short answer, and the one that most people know, is the hops. But there’s a long answer to that question as well.

A long time ago, someone decided they were gonna boil some green cone looking plant they found on a vine in the woods into hot water that had barley soaking in it. Some yeast fell in and a month later beer was born! Just kidding. I have no clue what possessed early brewers to use hop plants, otherwise known as Humulus lupulus, which is the scientific name for it. But regardless of what made them do it, we are forever in their debt. Now, more about that Humulus lupus plant…

The hop plant has one thing that humans have found very useful in nature for a very long time: essential oils. This is kind of a buzz word these days, along with “chemicals, GMO’s, Paleo diet, handlebar mustaches, etc.”, but an essential oil is basically just a smelly oil derived from a plant. The smell comes from a property known as volatility, which describes a compound’s (compound basically meaning molecule) ability to aerosolize. The more volatile a compound, the easier it becomes a gas.

Basically, anything you can smell has some degree of volatility. A good household example of a very volatile compound is acetone. In fact, if you left a cup of acetone uncovered for long enough it would eventually all just evaporate. An example of a compound with zero volatility is table salt! Have you ever smelled table salt? No, because it cannot aerosolize and thus is not volatile.

Ok, so by now you’re probably saying, “shut up and tell me more about beer!” Right you are. Hops, just like many other plants have extractable oils, and brewers use these oils to flavor beer, as well as to get its characteristic aroma. There are 4 main essential oils that brewers take advantage of; caryophyllene, humulene, myrcene, and farnecene, but beyond these there are hundreds of other compounds that are present in beer in small amounts that contribute to both flavor and aroma. The resin within the hops themselves that contains all these oils is known as lupulin, hence the name Humulus lupulus.

In their plant form essential oils are insoluble, meaning they can’t dissolve in water. As most people know, oil and water don’t mix, and if you didn’t, well now you do! Heat is applied to the hops in the form of boiling, and the hop oils, humulene being one of them, change structure. This changing of structure is called “isomerizing” and is just chemistry speak for maintaining the same number and type of atoms in the molecule, but changing its shape slightly. Once the oils isomerize, they are soluble in water, and are free to float around among the water molecules.


Humulene (Image courtesy of Wikipedia Commons)

Humulene can be responsible for up to 40% of the essential oils by weight (Source: Wikipedia), so naturally the hop plant was named after it, or vice versa, I don’t really know. So there you have it, fellow beer enthusiasts. This compound and a few (150 or so) other compounds are responsible for the hoppy goodness of beer. Different hops have different ratios of these oils, which can be used to get the myriad different flavor and aroma profiles that exist in beer today. But what happens when good beer goes bad?

Pepe le Beer

It’s happened to all of us at least once in our beer drinking careers. You crack open a beer, usually in a green or clear bottle, and it tastes and smells like a skunk. That’s weird, you probably thought at first, took a second sip and yup… definitely skunk. Now, if you’re secure in your man or womanhood, you do the logical thing and throw the beer out, but if you’re bro-ing out hard, throwing the skunked beer out isn’t an option and you just gotta power through lest you look like a wimp in front of all your bros!

For your sake I hope the second scenario has never happened to you because drinking a skunked beer is terribly unpleasant, depending on how far gone it is. But you’re not wrong about the skunky-ness that you smelled and tasted. The compound responsible for that terrible flavor and aroma is actually one of the compounds that skunks spray when they feel threatened! So just how exactly does that end up in your beer you might ask? Well, let me tell you.

There is a compound mostly responsible for bitterness in beer known as isohumulone. This is different than humulene, though, so don’t mix those up. When this compound gets hit by light, a radical reaction occurs. And no, a radical reaction is not a reaction wearing his hat backwards and kick-flipping over little kids at the skate park. It is actually a reaction that produces what’s known as a “free radical”, which is a molecule that is extremely reactive due it’s instability.

When isohumulone get hit with light, especially blue wavelength lights, it undergoes a radical reaction and produces a nasty sounding molecule; 3-methylbut-2-ene-1-thiol, but for my sake and yours, I’ll just call it 3-MBT. Below is a picture of the innocuous looking little guy that has caused many a beer to go skunky.

3-MBTImage courtesy of Wikipedia Commons

Once this guy is in beer, it’s over. With a taste threshold of 4 ppt (parts per trillion), just a tiny amount of this will completely skunk a beer. To put that into perspective, if you had an olympic sized pool of beer (660,000 gallons), it would take just one milliliter (about 0.034 fluid ounces) of this compound to skunk the entire pool. Check out Beer Sensory Science for more info on this little terror and really any other beer aroma or flavor questions (warning: it is a fairly science-y blog).

Luckily for us, isohumulone isn’t terribly common in beer, though it does contribute significantly to its bitterness. Even more lucky is that not every isohumulone molecule will undergo this kind of radical reaction when struck with light. And even better news is that there are specialized hop extracts that brewers that refuse to use cans or brown glass for whatever reason can use that have minuscule amounts of isohumulone. This limits the skunkiness potential of the beer but it still may be possible.

Whew, you made it. The science is over. Hopefully you found it interesting and informative. If not, that’s ok too! I’ll get you on the next one. But now you can drop the words humulene and isohumulone when drinking with you bros and explain why it would be better for your manhood to just pour that skunky Heineken out.



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