Bumpin' "Gangsta's Paradise". If we could rewind the rap clock...that'd be greaaaaaat. #billlumberghofrapOther times, things crop up that are interesting and relevant. Fun things! Like this:
— Scott Spolverino (@InWithBacchus) February 15, 2013
@theladyblender @singleminded We should get Malt Maniac @davindek in on this one. Davin, sulphur is found in grain is it not!? ;)#whiskyIsn't the internet neat? Talking to the master blender for Morrison Bowmore about whisky? Social media is pretty boss. In conversation with others about whisk(e)y and sulfur, I realized that while it is widely discussed in the industry as a negative thing, many don't ask how it happens or what's done to take care of it. So, I've decided to do a bit of science jiggerypokery and present yet another super scientific case for all of you to enjoy. Presenting...The In With Bacchus Guide to Whisk(e)y: Sulfur and You!
— Andrew Ferguson (@scotch_guy) February 6, 2013
We have to start, really, at the root of the problem. And I mean this in the most literal sense. The sulfur problem begins in barley, specifically in the root system. Sulfates found in fertilizer and in the ground are taken up into the root system of barley. They don't mean to make your whisk(e)y smelly, they really don't! The problem is, sulfur is necessary for cellular activity in plants. Two amino acids, cysteine and methionine, are critical in growth of plants. These amino acids are key in protein building within a cell. The primary culprit is cysteine.
|From Wikipedia, because I'm too lazy to use ChemDraw.|
Ignore most of the other stuff here and focus on that sulfur down at the bottom. Bonded to hydrogen and another carbon chain (the little weird dot thing is a carbon chain that denotes its physical orientation in a 3d plane). It is bound to a Carbon and a Hydrogen. Sulfur is an anion; it wants to take electrons from other molecules to complete itself, which is known as an ionic bond. It's oxidation state (how many electrons it wants/gives to attain stability), is usually +2. In this state, it is useful for forming anti-oxidants (important in plants as they use CO2, not O2). However...it doesn't ALWAYS do that. SOMETIMES it will form a covalent bond (where electrons are shared, not taken) with itself, known as a disulfide bond. Two cysteine will come together, chuck off their hydrogen, and bond. The best way to describe this is as such. Let's say two couples are dancing on the dance floor. Normally, the hydrogen is content to let the sulfur lead and waltz around. But sometimes, the sulfur will ditch his hydrogen partner and begin dancing with another sulfur. In this case, no one leads, they just dance. That's a rough analogy between ionic and covalent bonds.
This means that, within a cell, a bunch of cysteine will bond together to become more stable (on a singular molecule level) and, in turn, link themselves. This makes it more stable (in terms of two compounds) and also allows it to be used more efficiently in the cells themselves (catalyzing important reactions in cellular parts, giving proteins rigidity). They also can be oxidized to form a variety of sulfur acids as well. Methionine is actually catalyzed using enzymes to BECOME cysteine. So no matter how you shake it, your barley needs sulfur to function. I've glossed over a lot of the biological minute because I'm a chemical engineer and I'm terrible at it. I probably got some of this wrong but it really just serves to illustrate the ORIGIN of sulfur and why its taken in in the first place.
So we have sulfur compounds in our barley. Can't do much about that. In fermentation, the heat used to activate the enzymes in barley (alpha/beta amylase, limit dextrinase) will cause the proteins to break down. The inclusion of yeast (which too contains cysteine) will catalyze the formation of sulfides, in the form of hydrogen sulfide. A nasty little bugger. It smells like sewer gas and rotten eggs and all sorts of delightful things. No bueno for good whiskey. However, we still have a few aces in our pocket here. We know that sulfur has gotten INTO the mash...but how do we get it out? This little beauty:
|From Wikipedia...because I don't have copper lying around my house. YET.|
Cu 2+ + H2S --> CuS+4 + H+
CuS4+ + 4H2O --> CuSO4 + 8H+
Beautiful, isn't it? A simple bit of chemistry saves us a majority of headache. Okay, fine, it may not be beautiful to you but it's BEAUTIFUL TO ME! The only problem is...this isn't ALL the sulfur compounds. Its not just H2S. There's others. I mentioned it awhile ago in my barrel aging post, actually. Take a look back at that cysteine structure. Remember how I told you about that weird dotted line thing being a carbon chain? A methyl carbon chain? Also remember how I said that they will bond together with others? Well...yeah. That becomes a problem. During the heat-related breakdown of cysteine, that covalent bond might break. As the molecule breaks apart due to the heat, pieces of it will start coming apart. Its theorized that the disulfur bond will break and a methyl group will replace the sulfur on one sulfur. It can also come from said methionine as well. It forms this chemical:
|Wikipedia. LAAAAAZY. Also, ChemDraw is annoying.|
Barrels. GOODIE! My favorite!
The boiling point of DMS is 99oF. Rick/rackhouses can get upwards of that temperature, even in Scotland. Even in colder temperatures it will evaporate as well. As I've said before, things from high concentration like to go to low concentration. Couple that with a low boiling point and you've pretty much always got a LITTLE bit of it in vapor form in the barrel. So eventually, it will evaporate out. IF you have good casks. Casks that breathe easily, are well stored with proper temperature fluctuations, and adequate airflow is important. For the trimethyl sulfides...it takes a bit longer because the boiling point is higher so less will be in vapor. I've heard it quoted that DMS evaporates in a year~ish while the higher polymethyls will evaporate in 2-3.
So sulfur. Inevitable but, with care, can be almost completely avoided. Thankfully the flavor threshold for humans on sulfur is pretty high so it'd have to be a glaring mistake in order to catch a whiff o' the old brimstone. Or cabbage. Or sewer. Either way, I think we're fine. If you have any questions, feel free to drop me a line.
EDIT (3:44pm EST): Had a question from Oliver Klimek of Dramming.com. His question is as such:
"Very interesting, Scott. But there is one thing about sulphur removal that isn't quite clear yet. Distillers tell me that for getting a more sulphury spirit they try to prevent the copper from oxidisiing because CuO supposedly is more efficient than elemental copper. You only describe sulphur removal by Cu alone."
This is true. The metals that we know and love (iron, copper, tin, aluminum) aren't readily reactive in their elemental state. They form complex crystalline matrices that provide balance and stability to the metal. Each chunk of metal you see is made up of layer upon layer upon layer of grid-like metal ions. That's why they're so good at conducting heat and electrcity: its easy for electrons to flow. Much like those desktop multi-ball novelties that have you pick up a ball on one end and it transfers momentum through the other balls to move the one on the opposite end, so too is kinda how metal in its elemental state works. The problem comes when you have activation energy. Activation energy is the energy needed to break the bonds of the metal to get it to react with another chemical. Think of a roller-coaster. That long, clanking chain that brings you to the top is the activation energy; once you hit the top you just saiiiiil down. So copper in its elemental state is not prone to spontaneously and violently reacting. However...CuO means that somehow...some way, the activation energy has been provided to destabilize the elemental copper and it is now polarized. When its in this stage, it takes far, far less activation energy to get it to react again. So for a more sulfur-y spirit, you'd want clean, clean copper. It takes more oomph for it to react. For a less sulfur-y spirit, you want that metal to react because it means that it takes far less energy for it to react with subsequent elements like the sulfur.