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The Elemental Stench of Sulfur
A single element might seem an odd choice as a theme for a beer article, but this one is truly special. Of all the things in the universe our noses are capable of smelling, sulfur compounds are by far the stinkiest. Because of its importance in a host of biochemical processes, quite a few volatile sulfur compounds are found in beer. Many of them are detectable in astonishingly small quantities.
Eons ago, the coding for a unique protein containing copper found its way into the DNA of some distant aquatic ancestor, forming a chemical receptor. Because copper is highly reactive with sulfur, this receptor was exquisitely sensitive to many forms of sulfur.
Evolution quickly found many diverse uses for this capability. Sulfur compounds are found in many nasty situations including stagnant water, poison gases and mold. Our nose tells us in no uncertain terms to stay away. Odors of rotting meat and feces are also powerful, but may be either repellent or attractive, depending on your species and position in the food chain. Either way, these are substances you want to be aware of. Scent marking is another important tool largely based on sulfur compounds, useful for establishing territories and attracting mates, and is also an important tool prey species use to give their predators a wide berth. So powerful are these glandular secretions that they eventually came to be deployed as a weapon—in the case of the skunk. Coincidentally, that's an odor we can connect to beer.
A dramatic example of our sensitivity to certain sulfur compounds is the difference between ethanol, or ethyl alcohol, and a chemical called ethanethiol that has the same chemical composition save for a single atom of oxygen in ethanol that in ethanethiol is substituted by sulfur. The difference is dramatic. While ethanol is detectable only in very high concentrations—you can't really taste it in beer until it reaches six or seven percent—we are a hundred million times more sensitive to ethanethiol. As a consequence, it's the chemical used to add a noticeable odor to natural gas and propane, and it can also occur in beer.
The fact that we can detect a wide range of sulfur-containing chemicals down to low parts-per-trillion (or below!) in beer is a by-product of this evolution. Lucky for us, as these compounds can give us useful information about the state of the beer and the way it was brewed. They're rarely the most obvious flavors, but when you start looking for them in beer, you find them everywhere.
Some sulfur chemicals can be traced back to beer's ingredients. A common one is DMS (dimethyl sulfide), and it is known for a cooked- or creamed-corn character. It's formed during wort boiling from a precursor called SMM (s-methyl methionine), which is abundant in malt. If brewing is going as it should, the DMS is vaporized and carried away during boiling but under certain circumstances, the DMS can either remain in the wort or condense in the stack and drip back into the kettle. The old Pennsylvania regional beer, Rolling Rock, was famous for this flavor component, and when A-B shuttered the brewery and moved production elsewhere, they actually had a lot of difficulty recreating this "flaw" that Rolling Rock customers expected—and desired.
Other sulfur-containing chemicals can be found in kilned malt. Kilned flavors ranging from popcorn to nutty to coffee and roasty can be traced to a group of chemicals called heterocyclics that may contain sulfur. These are created during the Maillard browning that occurs when malt is kilned or roasted.
Hops also may contribute sulfur-based aromas. Most notable is the familiar skunky, rubbery, light-struck smell common in import beers bottled in clear or green bottles that offer no protection from the blue wavelength of light that causes the problem. The chemical, 3-Methyl-2-butene-1-thiol, is a powerful one, and often can be smelled while the beer is still on the table without bringing it anywhere remotely close to your nose.
Depending on the variety and growing conditions, hops can produce other sulfur aromatics as well. The old-time American hop, Cluster, as well as other US varieties, can have a cat-pee or blackcurrant-leaf aroma that is obviously not to everyone's taste. Other varieties can smell strongly of toasted onion or garlic. A hop called Summit is famous for this, but much of the 2014 crop of Citra was pretty well loaded with it, resulting in a lot of IPAs out there last year that drank pretty much like a nice hoppy beer with an onion bagel perched on the rim.
Because of its many biochemical processes, yeast also contains a number of sulfur compounds. Under certain conditions it can release a couple of fairly simple ones into the beer: sulfur dioxide and hydrogen sulfide. These are the two that tasters are most likely to describe as simply "sulfury," but they are quite different in character. Sulfur dioxide is the pungent odor of a just-struck match or spent gunpowder. Hydrogen sulfide is the rotten egg or volcanic gas aroma. Neither are particularly pleasant in beer, but may be acceptable as part of certain lager styles. Most common in lagers, both are strain-dependent and sensitive to yeast nutrition, especially when trace minerals such as copper or zinc are missing. One thing about these two is that they are extremely volatile, and can evaporate out of a beer and disappear after a few minutes. They also can saturate your receptors, so if you smell too deeply, you soon get to the point where you can't smell them anymore. So, these are best detected using the "drive-by" smelling technique, where the glass is passed beneath your nose over the course of a few seconds while gently inhaling through your nose rather than a full-on sniff.
During the course of fermentation, lager yeast produces a chemical called methyl thioacetate that has a flavor like cooked cauliflower that, believe it or not, is a positive flavor attribute in lager styles.
As the beer sits in the conditioning tanks, some of the yeast dies and spills its guts into the beer. While this occurs in all beers to some degree, it is a more common problem in lagers. A number of mercaptan-type chemicals including ethanethiol and methanethiol can contribute a subtle mess of aromas: rotten vegetables, leeks, drain-pipe gas or butane. Another yeast autolysis chemical called methional smells of mashed potatoes and is a breakdown product of the amino acid methionine. It may be released as beer ages, especially for extended periods of time, and is also common in over-pasteurized beers.
Musty and moldy aromas aren't very common in beer, but they can be extremely unpleasant and have amazingly low aroma thresholds: one drop can contaminate an Olympic-sized swimming pool of beer. They are unlikely to be produced during the brewing process, but are more often the result of ingredients that became wet at some point and were tainted by mold, or from ingredients stored in plastic bags or containers in wet, musty locations. We like to think of plastic as impermeable, but mold's signature chemicals can travel right through most plastic and impart their awful fragrance into the hops, malt or seasonings and thence to the beer. The presence of algae or other organisms upstream in the water supply can also contribute these nasty aromas.
I'm sorry if all this reads like a catalog of horrors, but these are the facts. It's no wonder that sulfur—in the form of brimstone—is often associated with the devil himself. While the individual flavor descriptors can be off-putting, many of them, when integrated into a complete and harmonious beer, can offer the depth and complexity we savor in beer, as we do in life.