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What makes old beer taste bad? Why, it’s the trans-iso-alpha acids, of course

The views expressed are those of the author and are not necessarily those of Scientific American.

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Man with a bitter beer faceBeer, for the most part, is not like wine—it does not improve with age. Quite the contrary, in fact. Old beer is a comparatively unpalatable shadow of its former self—skunky in odor, bitter in aftertaste.

So what happens between the brewery and the bottle opener to make long-in-the-tooth beer taste bad? A team of researchers from—where else?—Germany is on the case. A group from the Technical University of Munich and the Bitburger brewing company last month reported a comprehensive analysis of how beer becomes bitter over time in the Journal of Agricultural and Food Chemistry.

Beer is supposed to have a pleasant bitterness, thanks to the contribution of hops. But over time some of those bitter compounds degrade into less appealing substances that lend the aged beer a harsh, bitter aftertaste.

With mass spectrometry, the researchers tracked how those hop-derived compounds changed during storage at a variety of temperatures. They found that a family of compounds known as trans-iso-alpha acids underwent significant degradation into nasty, bitter by-products, including a compound called tricyclocohumol. The concentration of tricyclocohumol in pilsner beer, for instance, increased by nearly a factor of four after eight months of accelerated aging in a bottle at 28 degrees Celsius. Pilsner kept in a bottle at roughly room temperature (20 degrees C) for four years had even more tricyclocohumol—nearly six times the initial concentration.

The breakdown of trans-iso-alpha acids is temperature-dependent, meaning that beer ages faster in warm storage conditions, but it also appears to depend on the initial acidity of the beer. The researchers collected samples of 10 different pilsner brands to compare how the beer’s pH value affected the aging process. Even though the beers were all comparable in acidity, ranging from pH 4.3 to 4.55, the slight differences had a measurable impact on how much the hop-derived compounds degraded into unwanted bitter compounds. The more acidic beers accumulated more tricyclocohumol during storage.

The key to producing a fresh-tasting beer, then, is to control its pH during the brewing process and to store it in a cool place once it has been bottled. As a consumer, the easiest thing to do is probably just to avoid old beer altogether. Who knows what kind of trans-iso-alpha acid degradation might be going on in those dusty bottles on your grocer’s shelf?

Photo credit: © iStockphoto/Alex Gumerov


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  1. 1. ultimobo 8:10 pm 04/15/2011

    I believe this applies to most commercial or ‘dead’ beer from which the yeast has been removed, so there is no room for further improvement or maturation.

    My home brew where I leave the yeast in the bottle, ‘mit hefe’ (yeast) like hefeweizen, definitely improves with age, and the few I have managed to keep for a year have been fantastic, the closest thing to champagne with fine beaded yeasty bubbles – delicious !

    More generally I consume it within 1-2 months of bottling, it needs 2 weeks to settle and taste OK, after which I notice a definite improvement with age.

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  2. 2. Cramer 2:31 am 04/16/2011

    John Matson said, "Beer, for the most part, is not like wine–it does not improve with age." And then concluded, "As a consumer, the easiest thing to do is probably just to avoid old beer altogether."

    He is definitely not a beer connoisseur, otherwise he would have known that beers that are sweeter, maltier, higher in alcohol, and bottled conditioned age very well. The optimal age varies depending on the type of beer, but five years is not uncommon. Many beers have been aged up to 25 years. Beers have remained good up to 140 years.

    In a good beer bar it is possible to find vintage bottled beer. The Blind Tiger in NYC even ages beer in the keg. Craft brewers do this, too.

    Some beers do age very poorly. A North German Pilsner does not age well due to it’s high bitterness in a clean crisp dry lager style. Hoppy, dry, low alcohol beers taste worse with age. Alpha acids from the hops are what creates the bitterness in beers. There are some high alpha acid hops like Cascade and Centennial that give some beers a distinct grapefruit bitter taste. These beers age terrible, but since high alpha acid hops are used in many west coast American style ales, the off flavors can be masked well. Noble hops from Europe are lower in alpha acids and age better.

    Ironically, more hops were historically used in beer to protect the beer from bacterial spoilage. Alcohol was also increased for the same reason. Higher alcohol was only relative to time period when much of the beer was only 3% or less in alcohol because it was drank in place of contaminated water. These types of beers were brewed in the spring for keeping through the summer when beer was not brewed (called March or October beers) or for shipping for export or the military (such as IPAs).

    This is why the German researchers studied Pilsners, because freshness is important with that style — not so much for a sweet stout.

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  3. 3. jmatson 12:01 pm 04/16/2011

    Cramer: Whether or not I meet your criteria for a "beer connoisseur" is beside the point. You’ll notice that I included the words "for the most part" in the very first sentence here. You then cite a very specific counterexample ("beers that are sweeter, maltier, higher in alcohol, and bottled conditioned") that doesn’t apply to most beers. Sure, some beers can be bottle aged and in fact improve that way, but I would argue that, for the most part, that is not the case.

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  4. 4. Cramer 1:06 am 04/17/2011


    Thank you for your reply. I would like to apologized for my first sentence in my second paragraph about you not being a beer connoisseur (or maybe you don’t care). That did sound a little snobbish. It was not my objective to belittle you.

    I also thank you for pointing out my error. In the sentence you quoted, I should have said, "…and/or bottled conditioned beer." The four factors I listed all help a beer age well, but they all don’t need to be present. I can see how it did sound like I was referring to "a very specific counterexample."

    But this is not the case in reality. MOST styles of beer either age well or do not have any significant problems with aging. This is especially true, if concerned with periods of less than three months in which most beers are drank; and which you implied by your statement of "between brewery and the bottle opener." The analysis by Daniel Intelmann, mainly focused on pilsners and the study was co-sponsored (or fully sponsored) by Bitburger brewery. Their biggest selling product, Bitburger Premium Pils, is exactly the type a beer I said does not age well. Their beer does have a problem with skunkiness (not just my opinion).

    They were also only looking for a scientific basis for remedies that have been used for over a century in extending shelf life (such as controlling pH and temperature).

    Also, your use of the escape clause "for the most part" does not provide a reasonable loophole in your defense considering the context of your entire article. I also tend to use qualifiers in my writing, too. But when I use them, it’s an attempt to defend against insignificant anecdotal exceptions to the rule. I also try to eliminate most of them upon proofreading (if I proofread at all when commenting on SciAm).

    For the "most" part, "most" beers do not have a problem with aging. So, your advise that "the easiest thing to do is PROBABLY just to avoid old beer altogether" is terrible advise, especially since most beer is not labeled with a brewing date. The research article you referenced mentioned nothing about "old" beers turning into "a comparatively unpalatable shadow of its former self." That was only your opinion. In my opinion, I have rarely had a palatability problem with respect to the age of a beer.

    My advise: If you like to write opinion pieces, do not use so many qualifiers to hedge your opinions. And, since you live in NYC, please visit the Blind Tiger or Brooklyn Brewery and interview some experts before trying to give more opinions about beer.

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  5. 5. markodarko1 12:29 am 04/18/2011

    I just wanted to point out one other thing. 28C = Roughly 83F. I will all but guarantee that very few, if anty, wines will get better in taste when age at this temperature. Hell, even Room Temp a bottle of Wine isn’t going to age well. What a completely ridiculous conclusion it is to say that since beer doesn’t hold up well for 8 months at 82+ degrees F, it doesn’t age like wine. Or saying that beer held at room temp(roughly 68F) for 4 years doesn’t age well. Neither does wine, to quote the article "for the most part". I’d love to go to a wine person’s house and have him show me his room temperature cellar of fine wine!!! LMFAO. Ridiculous.

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  6. 6. chinabeergeek 1:47 am 04/18/2011

    This article is preposterous. First off, "for the most part", MOST WINES ARE ALSO MEANT TO BE CONSUMED YOUNG. Ask any sommelier and they will tell you that only a small percentage (5-10% or less) of wines will actually benefit, even with optimal cellaring conditions. The percentages are even smaller if you’re taking into account the VOLUME of wine, where the disproportionate bulk of production worldwide is of brands which will NOT
    improve with age, and in fact will DEGRADE. (Even at proper cellar temperatures, let alone your bizzare figure of 28C/83F.)

    Even the article about "skunky beer" you cite is flawed. It takes less than 10 minutes of direct sunlight to "skunk" a beer in green, blue or clear glass. Ambient indoor lighting, depending on intensity, will also "skunk" beer in such containers at varying times, but usually in less than a week. None of these timeframes are even in the category of being "aged". Furthermore, green glass, contrary to the claim in the article you cite, is about as ineffective as clear glass.

    Something far more scientific than your attempts at explaining lightstruck/"skunked" beer can be read here

    Beer that has truly spoiled by age (and not just "skunked"/lightstruck, which can occur in a very short timeframe with vulnerable packaging such as green or clear glass) is usually oxidized or infected. This is totally different from "skunked".

    Even if you equate "shelf life" with "capacity to age" (which you shouldn’t, but whatever), many beers are perfectly "shelf stable" for a year or more.

    And indeed as Cramer noted, please do some more research about the aging potential of beers. Brooklyn Brewery’s brewmaster, Garrett Oliver, has a wonderful book which you should take a look at.

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  7. 7. chinabeergeek 1:59 am 04/18/2011

    As an addendum, tricyclocohumol is not the cause of "skunk" in beer. Instead it is the mercaptan, 3-methylbut-2-ene-1-thiol, which is formed from hop isohumulones by wavelengths of light (both UV-light AND near-UV wavelengths above 400nm) in the presence of riboflavin.

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  8. 8. Cramer 3:47 pm 04/18/2011


    Thank you for clarifying skunking. I didn’t know if the author understood this. I decided not to bring it up considering the amount I wrote and the apparent fact that the SciAm author did not read my full comment. This was obvious from his erroneous reply.

    He doesn’t appear to have any knowledge of how many different ways a beer can go bad, as well as, improve with age. [Thank you for also pointing out oxidation and infection.] I have found this arrogance common among many SciAm editors. Many of them seem to voice opinions with very little scientific knowledge of whatever scientific research they are discussing. Then they attack their readers in attempting to defend their faulty positions. I hope SciAm is not paying them much — maybe they are volunteers.

    I only registered here a month ago. I doubt I will stay much longer, but it is one of the few websites to find reader comments on scientific news. Most websites allow it, but nobody ever comments. The NY Times Science section gets a lot of comments, but there is not that much dialogue and the comments are not immediately posted.

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  9. 9. Beer And Whiskey Bros 1:19 pm 04/19/2011

    Well done Cramer. I actually wrote a post about your comments here over on my blog. It’s great to see a beer geek set the record straight.

    You can see the post here:

    Feel free to stop by and comment any time. We say many stupid things, so no doubt you’ll have plenty of material.

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  10. 10. Cramer 2:23 am 04/20/2011

    Thank you. I bookmarked your blog. I also say stupid things and I am sure I could learn quite a bit from you and your brother. I believe I could have done a better job commenting if I would have avoided putting down the author and talked more about flavor stability.

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  11. 11. Cramer 2:38 am 04/20/2011

    An additional note about flavor stability:

    Beer flavor does change with age, but that is also the case with wine. Unless the brewer has made a serious flaw, most of the time the change in flavor is not of much negative significance. Beers do loose some of their bitterness with age. This is usually described as mellowing. Even hoppy beers like Dogfish Head’s 120 Minute IPA or Stone’s Arrogant Bastard can age well. They are not the same beer, but they still taste good.

    Major brewers are very concerned with flavor stability because brand identity is important to them and the pale lagers that they typically produce do not mask the slight off-flavors that might develop with natural aging. Jever does not want their beer turning into a Bitburger Pils and Bitburger does not want their beer turning into a St Pauli Girl. [e.g. the coffee bitterness from dark roasted malt in a stout should mask any harshness from the degradation of alpha acids.]

    And just like beer, only the top 5% to 10% of wines can significantly improve with age. I wouldn’t expect Sutter Home White Zinfandel to age any better than PBR. But that doesn’t equate with avoiding it or throwing it out due to its age.

    Both wine and beer can go bad in the same way (not the same as natural aging). That’s why the waiter has you sample it first. Has a waiter ever done that for beer? Maybe it just has to do with tradition, but you should be checking for infection, oxidation, sulfur and other problems that can also happen with beer.

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