Two bottles of wine into dinner and already half a sheet to the wind, my friend’s husband turned to me and slurred, “You know, I’d really like to taste your wine without sulfur.” That, my friends, is called a buzz kill. While passionate debates comparing biodynamic versus conventional farming or organic versus non-organic wines are vitally important and even entertaining, spoiled wine is not. The fact remains that sulfur is the best weapon in our arsenal to combat wine spoilage from unwanted pests and untoward oxidation. Even the diehard champions of organic wine are lobbying to change the USDA certified organic wine label to allow more than 10 ppm/bottle (Grey Report). Maybe they finally got the memo that nobody likes brown wine, even the guy at Whole Foods Market with the dred-locks and hemp shirt. Snap! (Oh that wasn’t very nice). Used properly, sulfur dioxide is a really effective way to prevent spoilage and thwart browning. For instance, the “molecular” form decimates unwanted microbes whereas the “bisulfite” arrangement (HSO3-) is a valuable mop, binding up and incarcerating unwanted chemicals. Different sulfite configurations- as in bound or unbound- are suited to different jobs. As you might imagine, when sulfur’s bound, or stuck to other components, it’s preoccupied, unwilling to engage in entry-level jobs like killing microbes. So it’s the free conformation, commonly called the “active form,” that’s valued for its antimicrobial and antioxidizing power. Free sulfur dioxide comes in three flavors (1) molecular SO2, (2) bisulfite (HSO3-), and (3) sulfite (SO32-). In the winery, when I add it to grapes or barrels, it’s just a white powder, called “potassium metbisulfite.” When you mix the powder into acidic wine, it breaks up into these different incarnations. As I mentioned, the molecular form is the honed ninja, a fierce microbe killer. Since it actually splits disulfide bonds like a Kung Fu master breaking a wooden board (hiiiiii-ya!), one could carry the ninja analogy pretty far. Conveniently, the molecular form also de-activates hydrogen peroxide. Since hydrogen peroxide is itself a powerful oxidizing agent that induces browning and accelerates oxidation, it is best disarmed ASAP. Not only does sulfur dioxide combat both varieties of oxidation - chemical and enzymatic- but also it reverses certain oxidation reactions, restoring wine to its pre-oxidative state. A related form, the bisulfite ion is the mop. It cleans up all kinds of stuff, notably stinky smells. When it binds to acetaldehyde (smelly vinegar-y scent), the smell disappears. Imagine that Swiffer Wet Jet commercial where the grimy, floor muck leaps onto the Wet Jet pad- same idea as a nonvolatile hydroxysulfonate complex.
Unfortunately, waltzing with so many partners fills a dance card fairly quickly. In other words, over time, sulfur gets so bound up with tannins and other wine constituents that its antioxidant power dissipates. Whatever dose of sulfur dioxide we vintners add just before bottling slowly and progressively diminishes, like a slow air leak. When the sulfur can no longer withstand the inevitable force of enzymatic oxidation, you’re left with a “dead” bottle - brownish with that fruitless, oxidized sherry smell.
You could just add more right? Too much sulfur dioxide has sensory implications - kind of a distinct sharp, prickly feeling. The bisulfite mop also sticks to color molecules, bleaching color. Finally, rare individuals suffer from a sulfur allergy. Not a headache-y, “yuck, I drank too much” feeling but a life threatening anaphylactic reaction where their breathing shuts down. Fortunately, emerging research suggests that “sulfite allergies” may not even be triggered by sulfites after all. In a fascinating letter titled “Wine-Induced Anaphylaxis and Sensitization to Hymenoptera Venom,” Dr. Armentia describes five patients who suffered allergic reactions after drinking red or white wine (N Engl J Med 2007; 357:719-720). Challenging the patients with either the offending wine or bee/wasp venom induced identical allergic symptoms whereas re-exposure to sulfites did not. To test the sulfite allergy hypothesis, investigators employed both skin patch tests and bronchial and oral challenges; neither modality elicited an allergic response. Clearly the root cause of “sulfite allergies” is more complex than most consumers imagine.
Since sulfur additions have saved my ass before, cleaning up funky scents and preventing bacterial spoilage, I’m too spooked to give it up anytime soon. But I do buy organic peaches.