Day 16: Sulfur

Why even try and talk about something scientific when you can get a PhD. from MIT to do it for you. Bay area chemist and Burgundy devotee Arjun Mendiratta gets technical with one of the most mysterious parts of any wine bottle, “contains sulfites” (what in god’s name are sulfites? and what do they have to do with natural wine? Well, keep reading…)

they don't tell you a lot of shit they put in wine

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:

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.
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, SO32-. 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.


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…

Follow day by day here:

Up next: Sommelier Linda Violago talks the past present and future of Spanish wine; or: I needed another reason to use this picture:

incredible indeed

~ by Cory Cartwright on July 6, 2009.

16 Responses to “Day 16: Sulfur”

  1. Great post: enjoyment and information is a nifty cocktail.

  2. Thank you, thank you, thank you… this a great post and should be required reading for the entire wine blogosphere… great great post!

    I’m still a little fuzzy, however, on:

    “This is the reason why the terms SO2 and sulfites are often used interchangeably, even though they technically refer to different chemical species.”

    I see that there is a misnomer in there somewhere but which one is it?

  3. Thanks for the kind words both of you.

    Regarding SO2/sulfites, there really is no misnomer. SO2, sulfite, and bisulfite are all in equilibrium. This means that they are always switching places while the overall proportions stay the same. Think of a group of hyperactive 7 year-olds playing some tag/hide-and-go-seek hybrid in your apartment. The kids are constantly running around, and yet you will always find one in the closet, one under the table, and two behind the couch. Same with SO2. There will always be some present as SO2, some as sulfite, some as bisulfite. Hope that helps.

  4. Arjun, it’s good to read an informed and even-handed post in regard to sulfur. Coincidentally, within the last two weeks, I’ve had a couple excellent bottles of Overnoy Chardonnay 2004 and tasted their Savagnin from the same vintage as well. Dani at Les Enfants Rouge in Paris has these wines and many other top selections on her affordable list. She is neither hip, dogmatic, or loud, but rather possesses good taste, knowledge, and an open mind.

  5. Let me humbly add my thanks as well. Too often wine talk is caught up in the subjective, and it’s very refreshing to break down some of the objective instead.

  6. Nice writeup, Arjun. So if my bottle of Touraine Cot reeks of rotten eggs, and the aroma doesn’t dissipate with even extended decanting (which should volatilize H2S), is this likely caused by a heavier sulfide compound?

  7. Slaton, I would say yes, although you might want to try the copper penny trick.

  8. My enologist friend insists the copper penny trick is hogwash.

  9. Interesting. I disagree; although I guess it would depend on the precise definition of hogwash.

  10. The copper penny in a glass of finished wine works fine with old pennies, even better if you scrub the penny with a ScotchBrite first. But it only works if the sulfide concentration is low. I have a bottle of dilute copper sulfate solution that allows me to check for and remove higher levels of sulfide. The copper reacts with H2S to make insoluble and non-volatile copper sulfide. Copper also reacts with more complex organic sulfides (mercaptans) to make compounds with varying degrees of solubility and volatility, but all of these compounds have a higher odor threshold than those not bound by copper. Disulfides and di-mercaptans are not reactive with copper. Wine naturalists will love to hear that addition of higher levels of SO2 (or worse from my perspective – ascorbic acid) can reduce disulfides and di-mercaptans back to their copper-reactive components.
    My panties get in a bunch when someone claims that racking and aerating a wine will remove H2S, sulfides, or other “reduction” flaws. Racking will clean up a wine if the source of the sulfide is lees, because you have removed the wine from the source of the sulfide – duh. Wines can be much more complicated and contrary – sometimes the source does not settle and you can rack all you want and sulfides come back as a new equilibrium with the source is established. Worse, aeration oxidizes sulfides to disulfides and di-mercaptans. The “reduction” seems to go away because the di-sulfides have higher sensory thresholds than the mono-sulfides. But they can and often do come back over time.

  11. John, that’s my understanding of the copper penny trick as well.

    Thank you for the clarification on racking for H2S. As a chemist but not a winemaker, my perspective is somewhat limited.

  12. [...] but one highlight for me (beyond Tracie B’s post on our visit to Joly, of course!) was Arjun’s treatise on sulfur and sulfites, a subject so hard to get a grasp on and so often misunderstood by wine [...]

  13. Great post. I spoke with Michael Stevenson of Panther Creek Cellars not long ago about the necessity to point out the sulfur content on the label, and I still wonder what the relationship between sulfur content and a headache is. I’m looking into the presence of cadaverine in wine now. Thanks again!

  14. Mattie – some people go into anaphylactic shock when they ingest SO2. For this reason (and the Food Allergen Labeling And Consumer Protection Act of 2004) “Contains Sulfites” needs to be on the label.

    If you want to know why you get a headache, look into cadaverine and other biogenic amines, and higher alcohols (3-6 carbons). Also, some people have a histamine reaction to some phenolic compounds. BTW – stressed “natural” ferments pump out biogenic amines and higher alcohols.

  15. [...] Chemist Arjun Mendiratta breaking down sulfur for the masses: [...]

  16. [...] Wine Chemistry 101 on Sulfur Posted on October 19, 2010 by cawel Cory Cartwright has written a nice dense post on sulfur. [...]

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