Few tools in the winemaker’s bag of tricks spark more discussion, controversy and confusion than sulfites. Most enologists trust the embattled molecules to safeguard their carefully crafted wines from the ravages of oxidation, microbial contamination and premature aging. Many consumers, on the other hand, know little of sulfites’ benefits, assume they’re unsafe and prefer their wines to be sulfite-free–often based on little more than the stuff’s undeservedly bad reputation.

Peter Granoff, master sommelier and managing partner of the Wine Merchant and Wine Bar stores in San Francisco and Napa, marvels at some of the myths circulating among consumers. He finds one oft-repeated canard, informally known as the European travel problem, especially ridiculous. “I’ve heard stories where consumers come back from Western Europe and have either talked to someone at a winery or a restaurant and have been told that sulfites are not used in Europe, but are added specifically for the American market.” That, Granoff says, “is complete poppycock.”

In fact, travelers would find that the same European wine they believed to be sulfite-free must wear a “contains sulfites” label in American stores, as required for export to the United States.

Further confusing the issue, U.S. winemakers can’t call their wine organic if they add sulfites, but Europeans can. “Rules differ depending on the wine’s country of origin,” Granoff says, “which only makes it that much more befuddling for the average individual.” To make matters worse, vintners who add elemental sulfur to grapes on the vine (mostly as a fungicide) can call them organically grown, but winemakers who add sulfites to organically grown grapes anytime during the winemaking process can’t call the wine organic.

When consumers seeking clarification go online (which they’re doing in increasing numbers, according to a recent Pew Research Center study), they’re bound to get even more confused, not to mention misinformed. They’ll read that sulfites—referring to sulfur dioxide and its derivative forms—are unnatural additives, cause allergic reactions, trigger migraines and destroy a wine’s sensory qualities.

Organic vs. unsulfited wines
Consumers who are worried about sulfites are showing more interest in organic wines. In a Nielsen Co. review of web-based chatter about why people bought organic wines last year, much of the “online buzz” focused on “less sulfites.” Europeans expressed a similar bias against sulfites in a 2006 survey funded by the European Union to identify consumer attitudes about organic wine. Most respondents worried about the health risks of sulfites, prompting the researchers to recommend that any proposed EU organic wine regulations require low maximum sulfite levels to appeal to health-conscious consumers.

Before U.S. law required winemakers to list sulfites, most consumers paid them little mind. The law (which required that any wines with a minimum 10 parts per million of total sulfur dioxide declare the presence of sulfites on the label) went into effect in 1987 and gave producers a year to comply. But the diverse nature of the wine market meant that wines destined for stores by the deadline—and thus required to carry the label—wound up next to unlabeled Bordeaux and other premium vintage wines bottled years before the rule went into effect. Naturally, consumers (incorrectly) concluded that one bottle contained sulfites and the others didn’t.

Concerns about sulfite safety first emerged in the early 1980s, when the FDA first noted a spike in consumer complaints describing adverse reactions to the additive. Most of the complaints cited salad bars and fresh fruits and vegetables. In response, the FDA contacted the Federation of American Societies for Experimental Biology, an independent scientific body, to evaluate sulfite safety. In 1985, when reports of adverse reactions peaked at 500 (including 13 deaths), FASEB concluded that, “Although sulfites are safe for most people, the additives pose a hazard of unpredictable severity to asthmatics and other sensitive people.”

But the words “contains sulfites” loom for the average consumer, unaware of the label’s intended audience: the sensitive few. And the sensitive few, researchers now know, typically have severe asthma. Of the estimated 22 million Americans who have asthma, according to the Centers for Disease Control and Prevention, about 20% have severe asthma. Of that subgroup, about 5%—or 220,000 Americans—are sulfite sensitive.

For sensitive individuals, an inadvertent encounter with sulfites might trigger anything from itchy hives and wheezing to shortness of breath and severe chest constriction. Only rarely has ingesting sulfites resulted in death—attributed to complications from asthma—but never from drinking wine.

Just as there’s no doubt that sulfites pose a real and potentially serious risk to these sensitive individuals, there’s no evidence that most people have anything to worry about.

A storied past
No one knows precisely when winemakers first discovered the means to preserve the fermented fruits of their labor, but ancient texts and archeological evidence suggest a very early appreciation of sulfur’s disinfectant properties. And though it’s often repeated that the Egyptians and Romans sanitized wine vessels with the fumes of burning sulfur candles, the evidence for this claim appears unsupported.

In his 1966 study of ancient wine, William Younger argues that Romans did not use sulfites. Longtime organic wine producer Paul Frey has scoured ancient and modern works for proof that early winemakers burned sulfur and insists that none exists. “You’ll read in a lot of important books that sulfites have been used since antiquity, but there’s no evidence for that,” says Frey, head winemaker at Mendocino’s Frey Vineyards.

Renaissance winemakers, however, likely burned sulfur before a German royal decree legalized the practice in 1487. In The Story of Wine, Hugh Johnson calls the directive the first document to outline the method, amount and reasons to add sulfur. Winemakers were permitted a maximum level of 18.8mg per liter (or 18.8 parts per million)—barely above the 10ppm that requires the label today—by burning a mixture of wood shavings, powdered sulfur, incense and herbs in empty barrels.

Unfortunately, fumigating barrels could over-sulfur the wines and produce that malodorous spawn of sulfur and fermenting grape juice, hydrogen sulfide. Advances in sulfur chemistry facilitated the synthesis of derivative forms of sulfur (such as potassium metabisulfite). When food-grade sulfiting agents became commercially available, winemakers, liberated from the unwieldy issue of the sulfur wick, gained far more control over this double-edged tool. And the food industry, at long last, gained the means to exploit sulfur’s diverse chemical properties.

With little evidence of toxic hazards tarring a long and storied service (aside from a few studies reporting vomiting and gastrointestinal distress at high doses), sulfites earned a slot under the U.S. Food and Drug Administration’s “generally regarded as safe” category in 1958, when the agency started regulating food additives. During the next two decades, manufacturers showed little restraint in incorporating sulfites into a staggering number of products.

A consumer would be hard-pressed to find sulfite-free processed foods in the 1970s and early ’80s. The extensive (but not inclusive) list of sulfited foods included fresh, canned, dried and dehydrated fruits and vegetables, fruit juices, molasses, maple and corn syrup, soup mix, fresh and frozen baked goods, gravy, vinegar, pickles and relish, chips, cookies, crackers, soft drinks, beer, wine, seafood, sugar, frosting and candy.

But it wasn’t the pantry or wine cellar that posed the greatest threat to sulfite-sensitive individuals. It was the restaurant.

Cause for concern
While enjoying a casual lunch of hamburger and fries on a European vacation in 1974, a 15-year-old American girl suddenly noticed an angry eruption of red welts colonizing her skin as she gasped for breath. This first brush with asthma sent her to the emergency room. Back home, she experienced similar fits during restaurant meals that again required hospital care.

During the next few years, the young woman experienced severe adverse reactions to two drugs used to treat her asthma and related symptoms. Sure enough, a series of controlled sensitivity tests revealed that sodium metabisulfite (bisulfite was the only chemical in both drugs) provoked the same symptoms. Reporting the case in the medical journal JAMA in 1982, the girl’s doctors called the findings “strong evidence that sodium bisulfite was the cause of the patient’s adverse drug and food reactions.”

The case might not have passed muster at such a high-profile medical journal had it not been for a groundbreaking study the year before. Working with four asthma patients who had reported sudden onset of severe wheezing and respiratory distress while dining out, Donald Stevenson and Ronald Simon tested likely triggers in a single-blind, placebo-controlled study. The researchers exposed their patients to potassium metabisulfite and placebos; only potassium metabisulfite produced the asthmatic symptoms. When other researchers replicated these results in larger, double-blind, placebo-controlled studies—the gold standard—it became clear that sulfites could indeed provoke asthmatic symptoms. But how?

Online references characterizing sulfite reactions as allergies abound. In a true allergic reaction, immune cells produce immunoglobulin E, a type of antibody that interacts with antigens (typically bits of protein), from the offending substance. Although a few cases in the scientific literature speculate antibody involvement, says Steve Taylor, a professor of food science and technology and director of the Food Allergy Research & Resource Program at the University of Nebraska, “certainly the vast majority provide no evidence of that at all. It’s not a true allergy.”

Exposure to sulfites aggravates existing asthma symptoms—though typically not headaches, another persistent myth. Researchers still don’t know how sulfites trigger adverse reactions in asthmatics.

One seductively elegant theory surfaced when a 1986 study found lower levels of sulfur oxidase—an enzyme that converts sulfur-containing compounds into a form that the body excretes—in the skin cells of sulfite-sensitive patients. But further investigation failed to implicate sulfur oxidase deficiency for most people, says Taylor. “It might be a multifactorial thing where a few people have this mechanism, and others have another mechanism.”

Since the FDA outlawed sulfite sprays on fresh foods and required labels on prepared products in 1986, it has received far fewer complaints of adverse reactions and no reports of fatalities. Taylor blames salad bars for most of the problems. “The severe episodes paralleled the development of these salad fresheners,” he says. “I think they’re the smoking gun.” Where food processors and winemakers added sulfites in prescribed, consistent amounts, levels at restaurants were large and uncontrolled, typically applied by an hourly employee. Restaurant workers, says Taylor, “were just throwing handfuls of this stuff in there.”

A molecular guardian
Not so long ago, with somewhat crude and imprecise methods for handling sulfur dioxide, winemakers often relied on trial and error to get the levels right. Hindsight suggests that tradition favored a heavy hand.

Back in the late 1970s early ’80s, people tended to use more sulfites as a general rule, says Doug Shafer, president of Napa’s highly regarded Shafer Vineyards. “When grapes would come in, you’d hit them with 100ppm sulfites at the crusher” to control wild yeasts and spoilage bacteria. Over time, people learned that just 20-30ppm would produce the same effect. Nowadays, Shafer says, the goal is to use as little as possible.

Microbiologist Linda Bisson from the University of California, Davis, thinks sulfite use was much higher several years ago, in part because sanitation wasn’t as good and people weren’t using refrigeration, “so they really had to knock out spoilage microorganisms.” But with technological improvements and a better understanding of what sulfur dioxide really does and when, “you don’t have to dump in the SO₂.”

“If you know your fruit, and your fruit is clean—and you know your winery, and your winery is clean—there’s no reason why you need to have a lot of SO₂,” says John Katchmer, enology consultant at Vinquiry, a private wine laboratory.

And if you’re trying to control undesirable organisms during fermentation, Bisson explains, simply inoculating with healthy yeasts can work wonders. In her undergraduate winemaking class at UC Davis, Bisson demonstrates sulfur dioxide’s antioxidant properties with a simple experiment. She has students add from zero to 200ppm sulfur dioxide with and without native yeasts and with and without inoculations of Saccharomyces, the yeoman’s yeast of efficient fermentations.

“They generally see that you don’t need the SO₂ if you’re inoculating, because you’re giving Saccharomyces a boost to take over. But if you don’t add SO₂ and you do a native fermentation, those often stick,” or stop fermenting, largely because the spoilage microbes hijack the initial fermentation. The experiment reveals that sulfites prevent the bad microbes’ enzymes from consuming the oxygen that Saccharomyces needs to do its job. “Saccharomyces is happier because it’s getting the oxygen and enzymes aren’t.”

It’s nearly impossible to find wines with absolutely no sulfites, because yeasts naturally produce SO2 during fermentation. And when it comes to adding sulfites, strategies vary by grape variety and winemaker style, yet some general principles apply. The U.S. legal maximum is 350ppm total—a fraction of the 2,000ppm limit for apricots, recommended by the World Health Committee’s food code committee. However, Katchmer says, “you usually only see that when someone really screwed up.”

Most winemakers usually add sulfur dioxide at the crusher at levels anywhere from 20ppm to 50ppm to suppress the natural flora on the grapes, Katchmer explains. By not adding SO2 up front, especially with red grapes that can sit in cold soaks for up to 96 hours, you’re risking bacteria that can create acetic acid (and its telltale taint of vinegar) or wild yeast that produce ethyl acetate (which smells like nail polish remover).

Doug Shafer learned the hard way what can happen when grapes go through the crusher without sulfur: “I had 3,000 gallons of Merlot go into spoilage in a heartbeat.” Although 97% of the grapes did just fine, one tank of Merlot did not. “Lactobacillus took over and outperformed our yeast. It was on its way to vinegar,” he recalls. Though Shafer managed to salvage the tank in the end, he learned his lesson, “and lessons like that stick with you for 35 years.”

Sulfur dioxide levels typically drop to negligible levels by the end of primary fermentation, so unless the winemaker plans a secondary or malolactic fermentation—typical for most reds and some whites—it’s time for a second dose. Katchmer adds, “What you’re looking for then is a certain level of free SO2,” which might require a few steps, because some sulfur will bind to other components in the wine.

In wine, sulfur dioxide exists in its free form as molecular bisulfate and sulfite, depending on the wine’s acidity. And at wine’s pH, the most dominant species by far—from 90% to 98%—is bisulfite. But it’s the molecular form that is antimicrobial, so winemakers can spend a lot of time measuring free vs. total SO2 levels to make sure they’re on track.

After fermentation, one of the primary reasons to add sulfur dioxide is to control spoilage microorganisms. Barrel-aged red wines are particularly susceptible to Brettanomyces, a ubiquitous yeast known for its funky, barnyard aromas. Adding sulfur dioxide at strategic intervals during the course of barrel aging can either kill or inhibit the odiferous microbe.

By the time the wine is ready for bottling, Katchmer says, most wineries are looking at free sulfite levels around 25-35ppm—levels that only the sulfite-sensitive individual would ever notice—and possibly as low as 5-10ppm after a few years in the bottle. Red wines usually need less sulfites than whites, because their tannins help protect the wine from turning brown and stale.

Still, for the average consumer who can’t shake an aversion to sulfites, no one says it’s impossible to make wine without them. “Their job is a just a lot tougher,” says Katchmer. And if stray lactic acid bacteria—unchecked by sulfites—gain a foothold just before bottling, they can produce compounds (called biogenic amines) that researchers think trigger bad reactions in humans, including, it seems, the headaches so many pin on sulfites.

Liza Gross is a San Francisco-based science journalist and senior science writer and editor at PLoS Biology. Her features have appeared in the science journal PLoS Biology, High Country News, Tikkun and Sierra.