Category Archives: Defects and Taints

A pretty corny post.

One of the most ubuiquitous flavors in beer, present to some degree in pretty much every beer, is dimethyl sulfide, or DMS. It’s a normal part of beer flavor but, as usual, its acceptability is dependent on the intentions and desires of the brewer. It can be a large portion of the flavor profile of certain beers, while in other beers it is expected to be at much lower levels. For example, Rolling Rock is considered to be a prominent example of a beer which is high in DMS (although in the past it may have been swamped by skunky/lightstruck flavors as I believe Rolling Rock has not always been brewed with light-stable hop extracts).

DMS has the aroma of canned vegetables, particularly corn or creamed corn. It’s a small and simple molecule; as the name conveniently implies, it has two methyl-groups flanking a sulfur atom:

Typical flavor threshold for DMS in beer is about 35ppb, and beers from around the world can contain anywhere from 10-200ppb. Typically lagers tend to have a bit more DMS than ales do, but what dictates the DMS levels in your beer more than the yeast is the production parameters in your brewery, particularly the kettle boil and wort-chilling.

DMS is considered to originate from malt, although it is actually formed in the brew kettle. All malt contains a variant of the amino acid methionine called S-methyl methionine (SMM), and it is an intermediate in a number of biosynthetic pathways which plants use to make other compounds. SMM is pulled into the wort during mashing and lautering and as the wort is heated the SMM degrades and is converted into DMS. While the wort is boiled, however, the volatility of DMS allows it to be removed from the wort and out the ventilation stack to never return. If there is no way for steam condensate to escape from your kettle (like if your homebrew pot is covered during the boil) then that condensate will drip back into the wort returning that DMS back into your beer. In such a case, the SMM is still creating DMS because the wort is hot, but the DMS can’t go anywhere so it just continues to build up into higher and higher levels. For this reason, the whirlpool and/or wort-chilling stage after the boil is a critical time in DMS control: if the wort sits too hot for too long, DMS will continue to develop since there is no boil to drive off the compound. I recall some power outages here at the brewery which knocked out the brewhouse for a bit. The brew that was in the whirlpool at the time wound up staying in there for much longer than it should have, which lead to a beer with astronomical DMS levels. At first glance, you may think that controlling DMS might be as easy as making your boils longer to convert all the SMM to DMS and drive it out the stack, but there is enough SMM in most malts that this is not a practical solution: the boil times required to convert it all and volatilize it could be hours long and would be quite detrimental to other wort parameters and the quality of the beer. Some kettles are more efficient at stripping DMS than others, as well. A simple direct fire or steam-jacketed kettle would be less efficient at removing DMS than a kettle with a calandria, which would be less efficient than a Merlin kettle. In fact, I recall brewery which, after installing a Merlin-style kettle, wound up being so efficient at stripping volatiles from the wort that they had to “de-tune” the kettle to make it less efficient since it was throwing off the flavor profile of their beers.

Even after the wort is chilled to the point where SMM is no longer being converted into DMS, the whole corny story isn’t over yet. During fermentation, the carbon dioxide that is produced by the yeast has a scrubbing effect on the DMS, carrying some of it out of the beer. This happens more efficiently at higher temperatures since the fermentations are more vigorous, and for this reason ale fermentations are better at this than lagers. This is why many lagers tend to have somewhat higher levels of DMS than ales do.

Here is a pretty handy chart I found on the internets showing some data about the DMS levels in wort/beer over the course of production. You can see how the levels drop significantly during boiling, but how they can potentially rise again before the wort is chilled. Then they fall again during fermentation as the yeast help blow off some more of the remaining DMS.

Finally, another potential source of DMS can actually come from bacterial infection. Some species of Enterobacter can produce DMS, along with diacetyl. This is quite uncommon in normal production scenarios, but it could conceivably happen more frequently in homebrewing situations. However, the vast majority of DMS in beer comes from the malt and the boil, so if you have an issue with corny beer, check the brewhouse parameters first.

But the story of corny beer doesn’t stop there, no! A challenger appears!

As I collected various flavor compounds that I understood were in beer, I came across a reference to another malt-based compound which was described as “biscuity/malty”. I thought this might have something to do with the biscuity aroma which is a dominant malt flavor in beers which use Victory or “biscuit malt”. Well, I was wrong. As I opened the package of 2-acetylpyridine which Sigma-Aldrich had shipped to me, I realized “This isn’t biscuity at all. This smells like freshly cooked corn tortillas!” It was like opening up that container of steaming tortillas at a Mexican restaurant, or a bag of high quality corn chips. So that’s what my panel calls 2-acetylpyridine now, “corn chips” rather than “biscuity”. Wikipedia says that 2-ap has an odor threshold of about 60 parts-per-trillion, but other literature values I’ve seen indicate that in beer it is closer to 40 ppb (and my experience with it shows this to be pretty close).

2-acetylpyridine, as seen below, is found in malt (and corn chips) and is created by the Maillard browning reactions. These reactions take place when certain types of sugars are heated in the presence of amino acids. It’s a highly complex series of reactions that take place which lead to a whole slew of compounds, including flavor compounds and color compounds. It’s not caramelization, but it can be confused with it if you are unfamiliar with the differences. The browning of the bread as it toasts, the malting of barley, the browning of beef as it cooks – these are examples of Maillard browning reactions.

I’m not going to go much into 2-ap, but just brought it up to show that not all corn-type flavors in beer come from DMS. In fact, I’m starting to think that some of the flavors in our beer that I have previously associated with low levels of DMS might actually wind up being 2-ap and that’s pretty interesting.

Hope to post again soon! But probably not.



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.

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That’s not what I meant by “Lawnmower Beer”.

Today, we’ll take a quick look at the source of grassy flavors in beer. This off-flavor is caused by the “leaf alcohol” known as cis-3-hexenol. This compound arises in various vegetative systems (flowers, leaves, stems, etc) when unsaturated fatty acids such as linolenic acid are degraded. As you can see by the picture in that last link, linolenic acid is a fatty acid with a long 18-carbon chain (tail) with a few points of unsaturation (meaning double-bonds along the chain). These double bonds are highly reactive and the fatty acid chain can be broken here. When this happens, cis-3-hexenol can be formed as the tail-end piece and grassy flavors will result. In beer, this happens most frequently when old hops are used particularly if they haven’t been dried thoroughly or stored properly. So, if you’re growing hops at home and intend to use them in some homebrew, take note:  the picked hops need to be dried down to about 30% of their original weight; roughly 8-10% moisture.  So with improper hop production and storage influencing grassy flavor production, it stands to reason then, that these conditions could also lead to isovaleric acid production. All things being equal, however, the cheesiness of isovaleric acid will probably be noticed before the grassy flavors, since not only does isovaleric acid have a lower threshold than c-3-h (1ppm vs. 15ppm) but the source material for isovaleric acid (humulone; one of 3 alpha acids) is likely at a higher initial concentration than the various poly-unsaturated fatty acids (total fat content averages around 3% of the weight of hops).

There aren’t too many beers that I can think of that are heavy in grassy flavors, but I would hazard a guess that you are more likely to find them in European pilsners and lagers as they tend to be lighter in flavor (meaning they can’t hide defects as well) and they tend to use the traditional nobel hops which are used as aroma hops rather than bittering hops. This means, among other things, less source material for isovaleric acid production, as well as poorer storability and higher tendency to oxidize.

If anybody knows of any commercial beers which seem to be grassy in character, I’d love to hear from you. Shoot me an email through the “Contact” link above and I’ll see if I can track any down. For now, all my grassy beer is made by me, my stock solution of cis-3-hexenol, and my pipette.


[edit:  3/3/11, 12:53 EST, added language about hop drying.]

Does this make sense?

So I was watching Brewmasters on The Discovery Channel the other night (about Sam Calagione and the Dogfish Head Brewery) and the episode was “Grain to Glass”. The “B-plot” of the episode dealt with a batch of their 120min IPA in a stuck fermentation back in June where the batch started at about 32 Plato and never got below 9 Plato. They were trying to figure out what the problem was, so they went back and pulled some bottled 120min IPA from storage and tasted them. I don’t know how many they tasted, but there was one from 2008 and one from 2006, at least. They decided the one from 2006 had the characteristics they were aiming for, and then implied that they used the notes from that brew to influence the recipe for the brews going forward.

The problem is, beer is not a static and shelf-stable product. Unless the beer is kept cryogenically frozen (generally -80C or less), it will degrade. Even the “bigger” beers which use many dark malts and have high alcohol will be altered during storage, for better or for worse. While some beers may be altered less than others, it seems a bit foolish to base recipe modifications on a sample of beer which is no longer representative of what that beer tasted like when it first hit the market.

Of course I could be missing something. Television editors are known for spinning the story in whatever way they want, and their lack of familiarity with the subject matter would make it easier for them to accidentally mislead.

On a side note, I also noted that Dogfish Head “tested” their sensory panelists with the FlavorActiv standard for “Musty” (trichloroanisole – “cork taint” for you wine enthusiasts). They must have intentionally picked that particular one for filming day, since Musty is literally one of the two easiest FlavorActiv standards to identify at the usual 3X threshold level, and it would have looked better for the camera if all their panelists were seen picking the right one. Now if they did this with FA’s “Kettle Hop”, “Grainy”, or “H2S” standards, I’d be impressed.

And while I applaud the presence of the craft brewing industry on television, I can’t help but think that this show is just a giant ad for Dogfish Head…

Say “Cheese”! Isovaleric acid in beer.

Cracking cheese, Gromit!

Have you ever smelled cheese in your beer? How about dirty sweatsocks? It’s more common than you may think. If you’re a homebrewer and you don’t use your hop supply as fast as you should, or if you store them improperly, you may be familiar with this aroma. This is isovaleric acid, and it’s a short-chain fatty acid commonly found in cheese, the valerian herb, foot odor, and sometimes beer. Now that’s an interesting selection of sources!

The commonly accepted threshold for isovaleric acid is about 1ppm, but like most other aromatic compounds, this can vary greatly depending on your genetics. This brief article gives some information about the genetic component of isovaleric acid receptors, exploring some of the sources of variability in how subjects perceive this compound. One of the more interesting things mentioned is that its detection threshold can apparently differ between individuals by up to 10,000 times. Personally, I think my nose has what I call an “acquired anosmia” to this compound. To be anosmic to a particular compound means you can not detect it at any concentration. While my case isn’t that dramatic, I think my sensitivity has dropped due to being frequently exposed to the purified compound when I spike it into my samples (despite using a fume hood and taking protective measures, it’s still possible to get it on you). If you get this stuff on your hands, you’ll stink for the rest of the day, if not longer. For this reason, I often have a hard time being able to tell if my spiked samples are at an appropriate level for the panel. Many times, I have to trust my math more than my nose.

So, how does isovaleric acid get into beer? Most of the time, it’s formed when hops get old, particularly when the alpha acids degrade. I’ve discussed hop acids already in the bitterness article, so if you need a quick overview, head over there and it might clarify some things. This image (from the above-linked article) shows the basic structure of the alpha acids (on the left) and the iso-alpha acids (right) that they isomerize into during boiling in the brewing kettle (at which time they become the source of bitterness in beer). Basically, there are 3 main types of alpha acid (and the 3 corresponding iso-alpha acids) and while they have the same basic structure as each other, there are differences at the “R-group” (top right of the molecule in the images). The differences are minor, but these minor differences can be interesting and influential nonetheless. One of these 3 alpha acids (humulone) has an R-group which is called an isovaleryl group. When this alpha acid oxidizes (due to age and/or improper storage), this R-group can be removed from the molecule and becomes flavor-active, leading to the cheesy/sweatsock flavor I’m on about.

Another way isovaleric acid can get into beer is through a Brettanomyces infection. It’s not the most common source in beer, but infection by this yeast genus can produce cheesy aromas, as well as a host of other undesirable flavor-active compounds like acetic acid (vinegar), 4-ethylphenol (bandages), and 4-ethylguaiacol (smoky). Some breweries intentionally “pitch” Brett into their fermentors as they try to achieve a certain flavor profile or match a particular Belgian style, but more often than not a Brett infection is a bad thing. Brett is also used in winemaking to achieve certain flavors, but it can also be a spoilage organism here depending on the intent of the oenologist.

So limiting undesirable isovaleric acid levels in your beer comes down to using fresh and high-quality raw materials (store hops in a cool, dark environment and, if possible, oxygen-free), and maintaining sanitary brewing conditions and using plentiful and healthy yeast to limit the potential for beer spoilage.

Jackpot! The Beer Fishbone Diagram

This PDF is a bonanza of information, enumerating the multitude of factors involved in all sorts of beer phenomena. It’s called a Fishbone Diagram, and the reason is obvious once you see it. I can’t even begin to explain everything that’s in here, I mean it would takes hours (days?) to pick it apart.

It’s pretty easy to interpret, although it is a bit of an information overload. Each page explains the various factors that influence a particular quality issue in beer. For example, below is a screenshot for the one of the pages [!] about how packaging and brewing issues interact to promote or limit beer oxidation. Other issues covered are controlling beer pH, fusel alcohols, H2S levels, foam quality, beer stability, yeast flocculation/vitality/viability, etc etc etc.

Brewing/Packaging Parameters and Beer Oxidation

You can find it here:
[see below]

Please excuse the rotated table of contents; I rotated the PDF so that the first page was the only one (of 42) that you needed to crane your neck to read. Better yet, print it out and enjoy it with a pint or two of your favorite beer. I’m going to go get a blonde ale out of the fridge right now.


Edit, 1/6/11:  Looks like these fishbone diagrams were developed by Greg Casey, recently (currently?) of Coors Brewing.  I hope it’s OK that they’re posted here…

Edit, 1/2/13: I’ve recently been informed that the file on the host site disappeared, so I’ve rehosted it at another site. If it disappears again, shoot me an email and I’ll try to get it back up.

Edit, 1/23/13:  At the moment, the free file-hosting websites I’ve been using don’t seem to have much of a shelf-life.   Either that or Greg Casey has a Google Alert on “beer fishbone diagram” and every time he sees the file posted he submits a takedown request to the hosting site.

Anyway,  I’m going to do this on an on-demand basis.   If you’d like a copy of the Beer Fishbone Diagrams, email me (found on “About” page) and I’ll get you a copy within a couple days.  

Diacetyl – “Who put butter in my beer?”

Diacetyl (dye-assa-TEEL, or dye-ASS-itle) is probably one of the most well-known flavors related to brewing.  It’s buttery aroma is easily recognized at levels above threshold but, as much as is known and recognized about this compound, I’m constantly amazed and disappointed by how much “butter-beer” is still being produced. This post will briefly explore the various ways that diacetyl arises in beer.


Diacetyl (2,3-butanedione) is one of a class of compounds called “vicinal diketones” (VDK). In chemistry-speak, “vicinal” essentially means “adjacent”, and “diketone” means that there are two ketone functional groups (a ketone is an oxygen double-bonded to a carbon in the middle of a carbon chain). If you look at the molecular structure in the link above, you’ll see that it is a 4-carbon chain (hence the “butane” root in the name), and the two ketones are on the vicinal positions of the #2 and #3 carbons (ergo, 2,3-butanedione: two ketones on #2/3 carbons of a 4-carbon chain). The other main VDK in beer is 2,3-pentanedione (pentane – can you guess what this one looks like?), but it is usually found in beer at levels below that of diacetyl.

Diacetyl, as mentioned already, has a buttery or butterscotch-like aroma. Open a bag of microwave buttered popcorn and you’re hit in the face with diacetyl. At very high levels it can even start to affect the mouthfeel of the beer, causing a slick or oily mouthfeel.  The detection threshold of diacetyl in beer is typically between 10 and 40ppb, although I have determined the personal thresholds for 11 of my panelists and they range from 7-190ppb, with an average of about 60ppb. Personally, my threshold is about 20ppb and beers with higher levels are so offensive to me that any beer with detectable diacetyl usually goes down the drain. Many times you may be served a beer which appears to have no hint of diacetyl, but as you work your way down to the bottom of the glass you begin to detect it. This is because the beer is warming up, which allows more diacetyl to volatilize into the headspace of the glass (and your nose). Too many times have I started in on a good and cold beer only to dump the second half because the “Big D” had begun to show its stinky face.

Common levels for diacetyl in beer range from 30ppb to over 1ppm [1]. Until recently, I would have had a hard time believing that there was packaged beer out there with 1ppm diacetyl. That is, until my panel tasted a beer from a local micro-brewery which was so high in diacetyl that we just had to run it through our gas chromatograph to find out how much was actually in there: almost 900ppb. Nearly all of our beers that we make here at The Company are below 30ppb or so, with most falling easily below 20ppb. I would hazard a guess that the big American Lagers/Pilsners are about 10ppb or less, but I don’t know for sure.

There are a few different sources of diacetyl in beer, only two of which are widely discussed.  The first is during fermentation, where it’s created by the brewer’s yeast.   The other well-known source is from bacterial infection.  The third source is probably the least known:  beer aging.  Below, we’ll examine each of these sources.

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