Sulfur

(This was written as part of Cory Cartwright's incredible 31 days of natural wine. Go check out the rest of the entries!)

We hear a lot about sulfur in the context of wine. Some wines have too much sulfur, some wines have too little; sometimes the sulfur 'blows off;' sometimes your friend gets a headache from the sulfites; it was wet at harvest, so the winemaker added sulfur to the must; I was hungry, so I ordered a large sulfur with pepperoni and onions. It can be hard to know what to believe...

Part I: Prerequisites

As the official chemist of "31 days...," I am here to demystify sulfur for you. However, before we begin, we need to review the difference between elements and molecules. (Yes, I know you hated chemistry in high school.) Sulfur is an element. The thing about elements is, they don't really exist. They're a bit like Platonic ideals. What do exist are molecules, which are made up of one or more elements. So when someone says "This smells like sulfur," what they really mean is, "This smells like a sulfur-containing molecule." Why is this important? Well, there are thousands of sulfur-containing molecules and they're all different. Some of the smell like shit (literally), some of them smell like garlic, some of them smell like a freshly lit match, and some of them don't smell at all. So which ones are which?

Part II: Bad Actors

SO₂: This is the big one, the one everybody talks about, at once superhero and super-villain. First, the super-hero. SO₂ can protect wine (and must) from oxidation. It does this through what I will call a "me-first" mechanism. The mechanism is very simple:

1. Evil oxygen enters your wine, due perhaps to a faulty cork or a sloppy winemaker. It looks around for a victim, eyeing your wine's vital flavor compounds.
2. A plucky SO₂ molecule jumps forward and says "Me first!" The SO₂ and oxygen molecules rip each others clothing off and hungrily devour each other. Meanwhile, the rest of your wine remains intact. Disaster averted.

SO₂ also interferes with the action of certain enzymes that cause wine to turn brown thereby helping to maintain color. Finally, SO₂ is an antimicrobial, which is to say that it kills bacteria, molds, and yeasts. Yeasts, you say? Don't we need those? Well, we need one of them, specifically saccharomyces cerevisiae, and that one just so happens to be resistant to SO₂. Thus, SO₂ is often used at crush to prevent any other yeasts from taking hold. (Where do you think Monsanto got the idea for Roundup Ready™ soybeans?)

As mentioned, however, there is a dark side; actually, there are several. The first, and easiest to understand, is the fact that SO₂ smells like burnt match, and so if you add too much of it to your wine, your wine will smell like burnt match, and nobody wants that. ("Burnt match," by the way, is more than just a metaphor. There's a bit of sulfur on the tip of your match and when it burns, it combines with oxygen in the air to make SO₂. Et voila! Burnt match.) Second, SO₂ can combine with certain flavor compounds present in wine and make them, well, no longer flavorful. Thirdly, SO₂ reduces the microbial complexity of fermenting wine, and there are those who believe that microbial complexity is directly related to flavor complexity. Finally, yours truly has collected unequivocal empirical evidence that some wines made with little or no SO₂ are unusually fresh, vital, and delicious (cf. Lapierre, Breton, Tissot...). It is for these reasons that many of those lunatic moon-worshippers known as natural winemakers will choose to use as little SO₂ as possible in their winemaking. I should add that even wines to which no SO₂ has been added will nonetheless contain just a little, as SO₂ is naturally produced during fermentation.

Finally, a technical note. You can skip this part, but you'll hurt my feelings. SO₂ is a colorless gas with the aforementioned odor of burnt match. When it is dissolved in water (and wine is 85% water), a small portion of it remains as SO₂, but much of it reacts with water to form a mixture of sulfite, SO₃^2-. and bisulfite HSO₃^-. Because no one likes dealing with smelly gases, winemakers do not work with SO₂ per se but rather KHSO₃, potassium bisulfite, which is an easy-to-handle white powder. It dissolves in water to produce bisulfite ion, just like SO₂, so the effect is the same. This is the reason why the terms SO₂ and sulfites are often used interchangeably, even though they technically refer to different chemical species.

H₂S: This is hydrogen sulfide, aka rotten eggs. It is also present in your farts. In fact, it is also present in yeasts farts, which is how it gets into your wine. For this reason, most fermenting wine smells like rotten eggs at one point or another. This is usually not a problem because H₂S is volatile. This doesn't mean that it has a bad temper, just that it evaporates quickly. If a winemaker is concerned about high levels of H₂S in a particular barrel, she may decide to rack the wine, i.e. transfer it from one barrel to another, in the hopes that the ensuing splashing will help get rid of the H₂S. This is a lot like shaking a bottle of Coke to get the fizz out. (Not that any of us drink Coke, of course...)

If H₂S sticks around, however, it can turn into sulfides and mercaptans, which is when things get really ugly.

Sulfides and Mercaptans: It's not too much of a simplification to say that sulfides and mercaptans are simply heavier cousins of H₂S. Because they are heavier, they are less volatile, and thus harder to get rid of. Like all of the compounds we have been discussing, they stink. However, they all stink in different ways, and, sometimes they even stink pleasantly. For example, the smell of freshly cut grapefruit is due to 1-p-menthene-8-thiol, aka Grapefruit Mercaptan. Some of the characteristic flavor of grilled and smoked meat is also due to mercaptans. And finally, the poster child for a benevolent sulfur, garlic is packed to the gills with all manner of delicious-smelling mercaptans, sulfides, and other sulfur-containing compounds.

An important characteristic of this class of compounds is its exceedingly low odor threshold (odor threshold is the lowest concentration at which you can still smell the compound). As an example, the aforementioned grapefruit mercaptan has an odor threshold of 0.1 parts per trillion, which is to say a little goes a long, long, long way. Another, somewhat related, issue is that these compounds can be perceived by your nose (and brain) very differently at different concentrations. What can be a pleasing, savory nuance at low concentrations can be positively revolting at higher concentrations. Anyone who's ever driven through Gilroy will know exactly what I'm talking about.

As it is with food, so it is with wine. Sulfides and mercaptans are an intrinsic part of the flavor of many, many wines. (Here is a paper on the importance of dimethyl sulfide to the flavor of Grenache and Syrah from the Rhone Valley.) However, when present in excess, they can impart an overpowering odor akin to cooked cabbage. It has become acceptable in wine circles to call this flaw reduction, although this usage makes most chemists cringe.

Part III: Thoughts

Sulfur gets a bad rap. If you take one thing from this post, I hope it is that there are thousands of sulfur-containing molecules and some of them are delicious. Of course, some of you will want to know where I stand on SO₂. To be honest, I have no desire to play backseat winemaker or to tell anyone what they should drink. Speaking only for myself, I have had several low-to-no SO₂ wines that have been almost shockingly alive and delicious, and I am grateful for those experiences. I have also had several that have been downright nasty. At the end of the day, litmus tests are for justices, not for wine...

Posted at 11:16 on Sun, 19 Feb 2012 in category wine | Comments (5)