Thursday 23 June 2011

When beer goes bad: Light-struck or Skunked

Beer contains hops (it also contains sulphur). Hops contain alpha acids. When hops are boiled the alpha acids get isomerized and become isohumulone. When light hits the isohumulone compounds it breaks them down and causes a reaction with the sulphur in the beer and produces some of the same stinky chemicals that skunks spray. Hence the term ‘skunked’ beer.

Brown bottles offer the best protection from those beer-harming UV rays, green bottles don’t help much and clear bottles offer no protection against light. Open a beer in a clear bottle and it will have a similar dusty, funky character to any other beer in a clear bottle; that's not some unique British hop you're smelling, that's skunked beer. All bottles let through some light and therefore all are susceptible to being light-struck. Casks, kegs and cans are safe from the light.

Beer can be light-struck almost immediately in direct sunlight; it takes a little longer in non-direct daylight and a little longer still in fluorescent light (the sort of light that brightens the supermarkets which have aisles filled with beer...), but it can happen quickly. And it’s the hops which make it go bad; Coke or wine in clear glass bottles aren’t affected the same way.

I’ve never smelt a skunk but I have smelt and tasted skunked beer and it’s bloody horrible. I now won’t buy beer in clear bottles because I hate the aroma and flavour of light-struck beer. Sometimes it’s just a little moldy and funky, like over-stewed vegetables, dust or damp cardboard, other times it’s like sewage or really bad breath. Sometimes it’s like all of the above. Never is it nice.

I don’t understand why any brewery would make beer and then put it in clear bottles when they know it will be affected by light. I’m guessing it’s a marketing choice but if that decision is done on aesthetics while knowingly spoiling decent beer then what’s the point? Some well known beers have an essential quality of being light-struck (Newcastle Brown Ale, Shepherd Neame and Becks to name just a few) so the brewers are making decent beer and then having to let the light do its damage to create the flavour. Why?

I also think beer in clear bottles looks cheap and horrible.

It is possible to use clear bottles without it getting light-struck but it involves using tetra-hop extract, a lab produced hop replacement that is pre-isomerised and doesn’t get affected by light. The trouble is that it doesn’t taste much like real hops (the beers it gets used in don’t taste much like real beer though...).

Beer in clear bottles is bad. If you want to taste it for yourself then buy two beers in clear bottles (although they will already be affected by the time you get them), leave one in the light for a week or two and the other in a dark cupboard and then try them side-by-side (for more extreme results leave it in direct sunlight but watch out because it’s horrific).

Do you buy beers in clear bottles or not?

If I’ve missed any interesting science stuff then let me know below. If you know why a brewery uses clear bottles then also please tell me. Of all the off-flavours I think this is the stupidest because it can largely be avoided simply by using a different coloured bottled. And sure, most people are used to that flavour in the beer, but that doesn't make it right, does it?

I used this and this to help write the post.


  1. Totally with you. It's nearly as bad as the idea that air getting at beer is somehow a good thing.

  2. I'm still on a learning curve with regards to off flavours in beers (couldn't make the beer bloggers conference this year and would have loved that talk).

    Did you read Ghost writer's piece on light strike in beer though? He didn't find any results really?

    Interesting set of posts this Mark, I think a lot of drinkers (myself included) know little about the off flavours that can happen in beers.

  3. The key to making progress on this lies in your line 'I also think beer in clear bottles looks cheap and horrible'. If we can get this message across to the marketing departments then we have a chance. Brewers are banging their heads on their mash tuns in frustration at the stupidity of their marketing departments, which only understand imagery and not flavour.

  4. As Jeff says, the trend for clear glass in recent years is purely marketing driven eg the gimmickry that was the ICE beer aberration from Fosters and Budweiser with its contoured shoulders. Thankfully they seem to be seem to be restricted to novelty "beer" like Desperadoes and Bud Light Lime. continue to Whether this was the case in olden times or there was a more practical reason why Becks used green or Newcastle Brown Ale used clear etc., I'd like to know. Why some brewers like Greene King, Shep etc continue to do so, I'd also like to know.

  5. Another fine piece Mark and I think Jeff has a really good point too -- though there are some apologist brewers around bizarrely claiming that "sunkissed" is part of the flavour characteristic of their beers!

    I ought to dig out my piece on good beer going bad from Beers of the World and repost it on my blog, given the growing interest in this subject.

  6. "Clear bottles: do you buy beer filled with them or not?"

    No, but I have been known to buy bottle filled with beer.

    Innis & Gunn beers are all sold in clear glass, and it may explain why I have such a love-hate relationship with the stuff.

  7. At the BBC11 Kristy from Molson Coors and I had a conversation about this one.

    I was arguing on the side of taste and she was saying that clear glass bottles sell more and appeal more to the women. I suggested that if the beer was lightstruck then the person trying it would not come back for more. She countered with not everyone can detect the skunky flavour and if she can get some people trying real ale then they were doing a good job..... but you are selling a beer with a known has become part of the taste can you control the process?...etc etc... until we agreed to differ and get on with dinner.

    From this you can see two things:
    Sales are King.
    It is hard to change the mind of a single person towing the company line.

    I too avoid beer in clear glass bottles even though I don't hate the skunky just tastes like cheap bottled lager to me so I don't really like it either.

  8. Neil - The problem there was that they were probably already light struck anyway or that it takes longer for it to happen. Try it for yourself and you'll see but give it a day or two to make the difference really stand out. Or just drink a few bottles of Sheps beer and the familiar characteristic (that dusty, cardboardy, funky flavour) is lightstrike.

    Jeff - There's a whole other post in that comment! I can't understand it and I don't see how the marketing department have more sway over the brewery department on issues like this.

    Bumpby - I'd also like to know why they use them knowingly! I'm also interested in a side issue - when these beers go in cans do they have a lightstruck taste? If consistency is key then do these breweries add a lightstruck flavour to canned beer?!

    Des - The people saying that were in marketing!! If lightstruck has become the known characteristic of the beer then things are not good! I'd also like to read your piece so get it posted!

    Pete - Thanks for that, I've updated it! It made sense in my head but reading it back I see that it doesn't so I've edited slightly! I&G is an interesting one. There's so little hops in them and they are very strongly flavoured from the wood that I don't know if I've tasted/smelt lightstrike in them? I think there's some diacetyl but not sure about skunking - I'll have to try another bottle.

    Dredpenguin - I'm on the taste and quality side. It's also important that people understand what is good and what is not. If someone buys a bottle of skunked beer as their first experience of real ale then will they order it in the pub out of the cask? Will it taste the same? Sales are king sadly but even so, it's making money out of bad beer! But also not everyone is sensitive to it... Odd one!

  9. I heard a story when I was at Heriot-Watt about Newcastle Brown and clear glass. Apparently the man in charge of the brand managed to get them to change to brown bottles...and sales plummeted as the beer tasted different and people didn't like the change!

  10. This is an interesting one, why people think clear bottles are appealing and think cans look cheap (though cheap beer in cans sell so well). In sweden i often see both the can and the bottle version of Bishops Finger on the shelf right next to eachother and I wonder about how people tend to choose. But one thing that really confuse me is why such a highly ranked brewery as De Molen have some of their beers in clear bottles.

  11. Really interesting.

    I wonder how feasible it would be to cover the clear bottle in a UV film? The label would then just be printed onto the UV film and this might solve the damaged beer issue.

  12. Mark, Jeff, there exists the eternal struggle between production and sales. Sales will win out because money is king. Brewers will try to negate the effects by not only using isomerised hop extract, but by washing the yeast several times prior to pitching to minimise any hop carryover from the yeast and supernatant. Canned beer by its nature will not be lightstruck and will taste different but the canning brings its own taste complications which I'm sure you'll address. Another point is that brown bottles do not eliminate the problem only slow down the reaction. Try leaving a Budweiser or a Punk IPA in direct sunlight for a couple of days ...

  13. I can' t believe nobody mentioned that Mark posted a pic of a skunk's anus!

    Seriously though, having most definitely smelled skunk spray (a nightly occurrence in Albany), skunky, light-struck beer does smell a lot like skunk musk—just not as intense. Skunk musk contains low molecular weight thiol compounds which are made up from carbon-bonded sulfhydryl. The word thiol is a blend of the word "alcohol" and "thio," a variant of "Thion," meaning sulphur, from Greek root.

    How's that for super-geeky science stuff? And I'm not even a chemist—I'm a graphic designer!

  14. Ed - I can believe it, sadly. I heard a story that when Newcy Brown was first kegged people didn't believe it was the same beer because it tasted different (i.e. not skunked!)

    Fredrik - I didn't know De Molen used some clear bottles?! Given a choice with beers like Bishops Fingers I would go with the can!

    Matthias - Good question and I don't know the answer... I guess some smart scientists are working on an answer but I also guess that it adds extra cost to the bottles.

    Bumpby - Interesting! And I've had some lightstruck beer in dark bottles. Light is not good on beer!

    Craig - I could've used any picture of a skunk but I thought the one of the anus was most appropriate :) And good knowledge!

  15. Oh yes. I have a Hel & Verdoemenis Misto B.A. in a clear 25cl. bottle. Might have something to do with the size though.

  16. Really enjoyed reading this, Mark. Saw this on the Botanix website the other day.

    "Conventionally brewed beers can easily become “lightstruck”. The action of light on such beers results in the production of the “sunstruck compound” 3-methyl-2-butene-1-thiol (3MBT), which has a very unpleasant “skunky” taste as well as a very low taste threshold of 1 to 2 parts per trillion. To avoid this Reduced Isomerised Extracts (Redihop and Tetrahop Gold) have been developed. These products can be used in combination or separately to produce a light stable beer."

    All of which begs the question: Given that a "well known Kentish brewery" not far from me uses this product, how come their bottled beer still tastes so crap?

  17. "Clear bottles: do you buy beer filled with them or not?"

    Yes, quite frequently (not just Shep's but a lot of the mid-sized brewers supermarkets stock use clear glass), and I've never once noticed this. Maybe I've just been lucky.

  18. Sean - Do they really use the extract in the beers?! I didn't know that...

    Phil - Have you enjoyed the beers? If they are in clear glass then they are almost certainly affected without you realising.

  19. Wow, this post is bit of an eye-opener here! Yeah sure beer can go bad but it is absolutely another story to know that certain color or the clarity of beer bottles can help the beer from going bad!Anyways, very well researched and informative post. Good work on that!

  20. I have in the past noticed the kind of flavour you describe in Spitfire, but I have bought plenty of other beers from Shep's's, Greene King and Badger in clear bottles and not been aware of anything unpleasant. The current Shep's seasonal, Goldings, is to my palate very nice.

    I notice that Marston's recently seem to have shifted all their beers to brown bottles, though.

  21. Ditto to Curmudgeon's comments. In fact I'd go further - I've got no memory of ever pouring beer out of a clear or green bottle and thinking it tasted mouldy, funky, stewed-cabbagey, cardboardy or whatever. (I had a dark Budvar last night (courtesy of the tombola at the Stockport Beer Festival!) which was very nice, despite the greenness of the bottle.) I think you must be overstating this one.

  22. Mudgie - Interesting that Marston's have made a change. See the comments above about Sheps and using hop extract which may explain something. One of the things about lightstruck beer is that it isn't always VERY unpleasant and can easily be seen as the characteristics of the beer. When it gets worse it gets more apparent and more horrible.

    Phil - I disagree about overstating it. Budvar Dark is a dark beer and so able to disguise the flavour more. For an experiment, try a beer in clear glass next to a canned equivalent (Tanglefoot, perhaps - I think that's in clear glass). Also, put one on a windowsill for a few days to increase the flavour and then do the three side-by-side and you'll see the difference.

  23. it isn't always VERY unpleasant

    I'll agree with that at least - one of the nicest bottled British beers I've ever tasted was Bateman's Dark Lord, poured from a clear bottle. I really don't think you can say that clear glass always makes beer taste foul, particularly not if you then cover your retreat by saying maybe it did taste foul and you didn't notice - that way extreme beer connoisseurism lies. But I will try the Tanglefoot can experiment some time.

  24. Phil - What I should've added yesterday is that not everyone can actually detect skunked beer, not that that makes it more or less ok. Some people, like diacetyl and phenols, just can't taste them. It's like asparagus - some people don't get stinky piss, of those that do, some can smell it and some can't.

    I also think there's enough research to say that beer in clear glass is always affected.

  25. Does beer get light-struck "almost immediately" when you drink it out of a glass, or is this something that only happens in bottles?

  26. If you are drinking from a clear glass then it will be affected. You'd have to either be very unlucky or a very slow drinker to taste it that fast in a glass though.

  27. I've had a pint in front of me for an hour before now, when I'm drinking in the daytime and want to pace myself. I don't think I'm the only person that's ever done this. It's really hard to reconcile with what you said in the OP -

    Beer can be light-struck almost immediately in direct sunlight; it takes a little longer in non-direct daylight and a little longer still in fluorescent light ... but it can happen quickly

    There's no question that prolonged exposure to light is a bad thing - that's why I keep all my beer under the stairs. But if the passage I've just quoted were accurate beer gardens would have to carry health warnings, and CAMRA would be lobbying for lidded steins.

  28. Phil - As soon as light hits the beer the reactions will start to take place. If they start then that's a bad thing, right, whether we can taste them or not. Science tells us that it happens almost immediately but like many things it takes time and intensity to make it worse and more noticeable. When your skin is subjected to sunlight it is immediately at risk from sunburn but it's only if you stay there longer that it gets worse.

    What is it you disagree with in this post? I'm not the first person to write about this topic and am only writing about the science which is already there.

  29. Mark - there appears to be more weight to your writing than just science behind light struck beer. Are you planning to start a campaign? Are you really boycotting beer in clear glass bottles? Steve.

  30. Hi Steve, no campaign just personal preference and taste. I don't understand why a brewery would deliberately put their beer into something which will make it taste bad! And I don't buy beer in clear bottles for that reason. It's not very difficult to stop buying these beers because not many of them are beers I enjoy (lightstruck or not!).

  31. What is it you disagree with in this post?

    Most of it!

    Open a beer in a clear bottle and it will have a similar dusty, funky character to any other beer in a clear bottle

    Never experienced this and don't believe it's true. I've drunk - or refused to drink - beers that tasted foul & had clearly gone bad, but never tasted anything resembling these flavours. I'm certainly not getting them every time I open a clear bottle (or drink from a clear glass).

    Beer can be light-struck almost immediately in direct sunlight

    I don't believe this can possibly be true, as it would happen every time anyone drinks a glass of beer in natural light.

    Sometimes it’s just a little moldy and funky, like over-stewed vegetables, dust or damp cardboard, other times it’s like sewage or really bad breath. Sometimes it’s like all of the above. Never is it nice.

    Again, this is far too categorical. And again, if this was always true of light-struck beer in bottles, and if beer was light-struck 'almost immediately' in direct sunlight, it would be impossible to drink beer from a glass without these various horrible flavours being present.

    Some well known beers have an essential quality of being light-struck (Newcastle Brown Ale, Shepherd Neame and Becks to name just a few)

    ??? Everything that Shepherd Neame puts in a bottle has an "essential quality" of being light-struck, and tasting of mould, damp cardboard and sewage? Are you sure you're not overstating this a bit?

    Beer in clear bottles is bad. If you want to taste it for yourself then buy two beers in clear bottles (although they will already be affected by the time you get them), leave one in the light for a week or two and the other in a dark cupboard

    I don't doubt for a moment that a beer that's stood in the light for a couple of weeks will taste foul because of it. What you've been saying throughout this post is that the other beer will also taste foul - after all, it will already have been affected.

    I'm not arguing with the chemistry, but I do think you're overstating the effects.

  32. From reading your blog, it is quite clear that you have been courted in the past by breweries with respect to PR. We all know how the ol' machine question is, if you refuse to buy beer in clear bottles, how can you be impartial? And will you still be accepting the free hospitality from the breweries using/moving to clear glass products? Steve

  33. Phil - I'm saying it as I believe it. I know what lightstruck beer tastes like and I've noticed all beer in clear bottles tastes this way (I'm not saying like sewage, just that it's lightstruck - when it's severely skunked it's like sewage and yes I think all Sheps beers taste like dusty cardboard). I'm not going to convince you and that's fine but try it for yourself - open 3-4 beers in clear bottles side by side and tell me that they don't share a similar funky, dusty, rubbery aroma. I think you are currently just drinking these beers and tasting it skunked without realising it.

    Steve - I will still drink the beer if someone gives it to me. If I like it then I will write honestly about it, if not then I won't. I don't write about most of the beers I get sent. And it's just a personal choice with what I spend my money on. Other people are quite happy with clear glass but I'm not. And accepting free hospitality is pretty rare, to be honest! If it happens and the beer is in clear bottles then I will treat it like any beer. My beef is that the marketing departments are choosing to put these beers in containers which are not the best for the beer and that's a shame. It also means that people get used to the beer tasting lightstruck.

  34. OK, I think we've more or less run this one into the ground. I'll just do a Columbo and ask one final question - if beer can be light-struck "almost immediately" in direct sunlight, doesn't this mean that every glass of beer anyone's ever drunk in the sun is light-struck, unless they neck it in one?

  35. It will inevitably be affected if you are sitting outside and it's bright but it's come straight from a dark container and, unless you leave it a few hours, then I very much doubt you'd notice it. The bottles of beer on the supermarket shelves have been exposed to light for weeks and so the effects are worse. But, as soon as light hits the hops the reaction starts to take place.

  36. Aside from some people not being able to detect the thiol (everyone has different detection abilities, hence such a variety of tasting notes for any given beer if blind tasted) perhaps there are other factors which affect the amount of lightstrike detectability. Either other compounds from the brewing process react to reduce the levels present or they act as a mask by being a similar shape and block the tastebuds. It would be interesting to do quantitative studies (mass spectrometry for example) of similar recip light vs dark beer in clear bottles left in the light for equivalent time periods to see how they are affected. Sounds like a future phD project in fact!

  37. Actually, the beers that I buy in clear bottles (recently Tanglefoot, Abbot Ale and Sheps' Goldings) I would say turn over on the shelves within a week, often less, and for most of that time a particular bottle won't be at the front of the shelf. Once I get them home I put them in a dark cupboard. So any lightstrike effect will be minimised anyway.

    And I think the idea that anyone can detect lightstrike in a pint poured from a clear glass and left outside for an hour is absurd. In any case, the principal problem will be that it's far too warm ;-)

  38. The reason certain brewers have switched to bottling in clear glass is a recent development in bio-chemistry. Scientists have come up with a specific modification on certain molecules in hops, specifically the ones sensitive to light. As a result, they've produced a hop extract that will remain stable even in contact with light, the so-called 'tetra-hops'. My assumption is this is a fairly costly project, as you only tend to see the larger commercial breweries using these clear bottles. Also, I have no idea if and how this alteration would change the taste and smell of these hop extracts.

  39. Anon - I don't trust these tetra hops! I don't know how good they are in terms of flavour though so that's pure uneducated opinion. I do prefer the idea of pellets or flowers being used though!

  40. I created a poll on my blog asking which of the popular premium bottled ales people had drunk recently.

    Interestingly, at an early stage, the three leaders are all beers sold in clear bottles - Bishop's Finger, Old Speckled Hen and Tanglefoot.

  41. I think what you guys fail to understand is that many people like the taste of skunked beer. Duh.
    Skunked beer taste stronger and more complex, and the skunky flavor tends to fade anyway on the second or third beer. There is a reason Heineken is so popular. It's popular because for many people it tastes good.