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.

So, how does it all happen? It’s a bit of a complicated process consisting of a small handful of reactions, but let’s take a look at them. Since about the 1960′s it’s been well established that the blue part of the visible light spectrum (~350-500nm) is the most efficient at generating lightstruck flavor, although ultraviolet light (below about 380nm) is capable of initiating this process as well. Brown glass bottles block light that is below about 500nm, while green glass bottles begin to block light below about 400nm and clear glass blocks no visible light. So visible light between 400 and 500nm poses a problem for beer in green bottles, and brown bottles are roughly 4 times as protected at these wavelengths. Owens-Illinois is rolling out some black glass bottles soon and as you can imagine, and from what they’ve told me, it has similar light protection capabilities to the standard brown glass.

There are essentially two potential mechanisms for lighstruck flavor to occur. The first involves direct irradiation, by UV-B light, of the isohumulones. In this mechanism, UV-B light excites the isohumulones into what is called the “singlet state”. Basically, part of the molecule has a higher energy level than it did previously. Through a few “shuffling” mechanisms, this energy is transferred to the acyloin group (which is just a hydroxyl group adjacent to a ketone; can be seen at the lower left of the 5-membered ring) which then undergoes a “Norrish Type 1″ cleavage which separates the side chain at that acyloin group. This newly cleaved side chain, now also a radical, then loses its carbonyl group and goes on to trap a sulfhydryl radical to form 3-MBT. The sulfhydryl radical has been shown to be produced by a reaction between triplet-excited riboflavin and the sulfur-containing amino acid cysteine. The mechanism is shown below. If you open the image in another window, it’s slightly clearer.

Mechanism for production of 3-MBT by direct irradiation (UV-B), source cited below

The second possible mechanism for the development of lightstruck flavors is the “sensitized irradiation” mechanism, which is caused by visible, particularly blue, light. This happens when riboflavin is excited to its triplet state where it then removes an electron from an isohumulone molecule. This leads to a cleavage of the radical side chain and decarbonylation of said-side chain, similar to the direct irradiation mechanism. Finally, the triplet-excited riboflavin reacts with a cysteine amino acid which produces a sulfhydryl radical that can recombine with the side chain to form 3-MBT. Does your head hurt yet? The mechanism is shown below.

Mechanism for visible light-induced formation of 3-MBT, source cited below

Reduced iso-alpha acids: Inhibiting lightstruck flavor formation with hydrogenated hop products

So some of you may already know that there are a few different specialized hop products that are available which can be used to prevent lightstruck formation, but I bet that you probably do not realize that these hop extracts do not actually prevent the photodegredation of iso-alpha acids as much as they prevent the formation of 3-MBT. So, what’s the difference?

First, let’s take a step back and look at what these hop extracts are. When isomerized alpha acids (known collectively as isohumulones) are hydrogenated at various locations, they can form what are referred to as “reduced iso-alpha acids” – the “reduced” referring to the gain of electrons from the hydrogenation process. There’s two different ways to do this and three different hop products can be made from these procedures. The graphic below shows how the isohumulones relate to these three reduced forms.

Formation of Reduced Iso-alpha acids

If you look at the four “chemical reaction” arrows, you’ll notice that the two methods involve the use of sodium borohydride (NaBH4), which reduces the carbonyl group (=O) on the side chain to an alcohol group (-OH), as well as the catalytic hydrogenation, which uses hydrogen gas and some palladium catalyst to hydrogenate the double bonds in the side chains. When using the first method, dihydroisohumulones are created (also called “rho-isohumulones”) and these are most commonly used to add light-stable bitterness to beer. When the second method is used, tetrahydroisohumulones are created (also called “tetra”) and these are most often used to increase bitterness (they are more bitter than native isohumulones) and head retention. When both of the above reactions are used, then hexahydroisohumulones are created (“hexa” for short), and these apparently aren’t used for much at the moment.

These reduced extracts are usually considered “stable” from a photodegradation standpoint, since when they are used in place of traditional hop products the beers seem protected from producing 3-MBT. But these compounds are not, in fact, resistant to photodegradation. They degrade and take part in the same basic reactions as regular isohumulones, but they do not result in the same radicals or, ultimately, sulfur compounds that are as flavor-active as 3-MBT with its ridiculously low threshold. So effectively they do protect the beer from skunking, even if they still degrade and produce some potential flavors.

In conclusion, how can we inhibit the development of lightstruck flavors in beer, today?

  • use brown glass, cans, kegs, or packages otherwise protected from light
  • limit exposure to visible and ultraviolet light as much as possible during production
  • use reduced iso-alpha acids, either rho-, tetra-, or hexaisohumulones
  • Minimize the use of sulfur in the production of hop products

Solutions for the future?:

  • there is some ongoing research into decomposed riboflavin, but it hasn’t been applied to beverages yet
  • use of free-radical “quenchers” with oxidation potential between 1.4 and 1.7 (currently in preliminary research phase; identity of quencher candidates is proprietary, although some research shows potential for phenols and purines to be used as quenchers)
  • Decrease riboflavin levels in beer to delay onset (tricky at best, since flavins come from both malt and yeast).

So, we now know that this is quite a complex set of reactions that take place, so much so that it has only been fully understood in the last 5-10 years or so. I hope all the chemistry-speak didn’t make your face melt and your children weep over your exploded body, but there really isn’t a way to discuss this without all the big scary terms.


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.

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10 responses to “Lightstruck.

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  3. First let me say how much I love your blog – even though at times the detailed technical aspects are beyond my desire of what I want to know (I am more into empirical observation and experience than theory and hardcore science when it comes to beer evaluation and homebrewing) – its nice to read something beyond the usual beer geekery and try to absorb a little science when it comes to sensory evaluation of beer.

    There is currently a discussion on BeerAdvocate about lightstruck beer – and the crux at most of the argument is how fast a beer can exhibit skunky aromas. My experience tells me that it can happen almost immediately when exposed to strong sunlight, but many say that it highly unlikely that it will happen within 10 minutes.

    Also, what makes one beer succeptible to skunking so quickly and another seems to not skunk before finishing it? My theory is that it is not just the amount of hops in a beer, but the variety of hops (or maybe malt?) used in the beer as well. I had one experience while vacationing in Victoria BC where a hefeweizen (pretty low hopping rates) was served to my wife and she thought it was skunky – we (and the beer) were sitting outside in the shade, but the waiter had to walk about 10-15 yards in direct sunlight to reach our table. I did not believe that skunking could happen so fast, so I went into the bar to see if maybe the beer sat in sunlight after being poured, but the bar area was well shielded from sunlight and there were no flourescent lights nearby that I could see. I asked for a sample of the same beer and it exhibited no skunky aromas – then I proceded to take the same route with the sample to our table – by the time I got to the table the beer showed some skunky aromas. Now I was curious to how fast a beer could skunk, so I took the IPA I was drinking and set it in the sun, after about a minute is started to skunk, yet another IPA they were offering that day did not skunk in the 20 minutes I had it in the sun. Of course some are taking me to task for suggesting a beer can skunk by the time a waiter delivers a beer to the table. What is your experience/knowledge?

    • Thanks for the kind words. Glad you like the site. I may not update as much as we’d like, but with my other duties there are times when it’s tough to get something posted.

      About your skunking scenario:

      I have definitely heard of beer in a glass in the sun being skunked by the time you finish the glass, but I don’t know what to think about one being skunked just by being walked out to your table. Of course there are a variety of things that can influence the perception of this flavor, although its threshold is so ridiculously low that it doesn’t take much. It seems likely that a beer that has a lot of iso’s and a lot of riboflavin (high-hopped, all-malt beers) will reach a higher level of 3-MBT than a beer with less hops and more adjunct (not considering malted wheat as an adjunct here). However, there will also be more other flavors present in that type of beer which can mask the skunk a bit, so there’s that trade-off to consider. Cloudier beers and darker beers will protect the beer a little bit longer from the light as well. Also, the temperature of the beer should affect it: if it’s a lager served at chilled temperatures, it will take longer just because of the thermodynamics, whereas an ale that might be served warmer will allow these reactions to proceed faster.

      It all depends on your personal threshold for the compound as well. If yours is lower than other people’s threshold, then you’ll obviously detect it before they do.

      So I’d say it’s theoretically possible for the beer to be skunked that fast for you, but it’s a bit surprising.

  4. Thomas Barnes

    Empirically, I’d say that extremely bitter, pale beers can get skunked with heartbreaking quickness. Several years ago when I was drinking on a beer-bar’s patio on a hot, sunny day, I had a glass of German Pils (Jever – poured on draught, so no chance of “green bottle” faults) go skunky on me within 10 minutes of being brought outside.

    Also, don’t assume that brown/amber/flint glass will protect you. If there’s enough light energy, brown glass just slows down the skunking process. The moral: Beer hates sunlight as much as Dracula. Keep it covered!

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  6. I’d just like to add a point of perspective here, something that was drilled into me when I did my Brewing Science degree, and it is especially relevant to 3-MBT.

    It’s easy for us to throw around ppm, ppb and ppt without fully appreciating what it means.

    The taste threshold for 3-MBT is 4 parts per trillion. That’s tiny, no, that’s really, really tiny.

    It equates to 4ml in 1 gigalitre.

    That’s saying we could detect 3-MBT even if we disolved 4ml of the stuff in the amount of water it takes to fill 500 olympic size swimming pools.

    As humans, we’re not too bad at this sensory lark.

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  8. A long time ago I took a hazardous materials class. One of the biggest catch phrases I can remember is, “SH smells like Sh*t.” It was amusing to me to put that together here. The smell is not quite the same but it makes a lot more sense to me now to tie the smell of mercaptan (added to nat gas and propane) to skunky beer. This article made my eyes cross a little but I still found it very interesting and actually a little entertaining. Thank you!

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