Inside Wine: Sulfites

These controversial preservatives are critical to winemaking
Feb 19, 2003

Spoilage. Mere mention of the word to food purveyors evokes images of recalls, lost revenues and shattered reputations. Winemakers suffer similar anxieties because wine is, after all, a food product that's susceptible to a variety of spoilage problems both during production and after bottling.

So winemakers need to mount a strong defense. The most common and effective technique is the addition of sulfites, sulfur-based compounds that can take the form of sulfur dioxide gas (SO2), potassium metabisulfite powder or a solution made by bubbling SO2 gas through water. Depending on the goals of the winemaker and the type of wine being made, sulfites can be added at practically any stage of the production process, from the moment the grapes arrive at the crusher to just before bottling (some grapegrowers also spray vines with sulfur, as a fungicide).

Food purveyors and vintners have used sulfur since at least classical times. Today, sulfite additions preserve a host of products, such as fruit juices, dried fruit, fried potatoes and pickled foods.

Without sulfur, it's extremely difficult, if not impossible, to make top quality wines that can withstand the rigors of transportation and that reward long-term cellaring. "If you asked most winemakers to stop using sulfites, they'd become gas station attendants. They just wouldn't know how to do it," says Gordon Burns, technical director of ETS Labs in St. Helena, Calif., which analyzes about 300,000 wines per year.

In other words, the overwhelming majority of winemakers consider sulfites essential. But during the past two decades, there's been a trend toward smaller additions. In excess, sulfites impart a distinctive burnt matchstick aroma. And some winemakers suggest that excessive doses impede tannin maturation, which is essential in quality red wines. "People once added SO2 with abandon. Then 20 years ago, we got away from using so much," says David Ramey, owner and winemaker of Ramey Cellars in Healdsburg, Calif.

Winemakers distinguish between "free" and "total" sulfites. Free sulfites, which have not yet bound with oxygen or other components in the must or wine, are still active as preservatives. Total sulfite levels are the sum of the free and bound sulfites. Most wines get bottled with about 25 to 40 milligrams per liter (mg/l) of free sulfites. The level of total sulfites is usually about double that.

Since the 1980s, the use of sulfites has come under increased scrutiny due to potential health concerns. United States Food and Drug Administration regulations require food and wine producers to indicate "contains sulfites" on the label of any product that has at least 10 mg/l (10 parts per million). These regulations, which went into effect in 1986, were instituted because sulfite-sensitive individuals can experience allergic reactions.

Some FDA officials estimate that approximately 500,000 people in the United States have some degree of sulfite sensitivity. While some wine producers claim that figure is grossly exaggerated due to anti-alcohol sentiment, there's no disputing the seriousness of severe reactions. A small group (about 5 percent) of asthmatics are most susceptible, and the bronchial constriction experienced during severe attacks can be life-threatening.

Unfortunately, some of the research behind the regulations is sketchy. It's not known what quantity of sulfites typically triggers allergic reactions in sensitive individuals (the FDA selected 10 mg/l as their regulatory limit in 1986 because that was the diagnostic threshold).

Although sulfites are formed naturally by yeast during fermentation, vintners still make additions to get the desired results. The sulfites serve two essential functions: By binding with oxygen, sulfites prevent or minimize oxidation, which browns wine and imparts stale, flat flavors; they also inhibit undesirable (or untimely) microbial activity. (Sulfites should not be confused with sulfides, a different class of compounds that cause a variety of "off" aromas, such as the rotten-egg smell of hydrogen sulfide.)

There's no textbook on the timing and quantity of sulfite additions. Michael Brajkovich, winemaker and co-owner of Kumeu River Winery in Auckland, New Zealand, adds about 30 to 50 mg/l to his Merlot grapes at the crusher. But he'll double that amount if he notices rot, which imparts undesirable oxidative enzymes.

It's also a question of style. "It's a fine line. We don't want fruit to oxidize, but we don't want the sulfites to inhibit fruit expression," explains Rainer Lingenfelder, winemaker and owner of Weingut Lingenfelder in the Pfalz region of Germany. "It seems that earthier Riesling styles from the Mosel or Rheingau tolerate more SO2."

But oxidation isn't the only potential pitfall circumvented by sulfites. Many vintners prefer fermentation with so-called native yeast, which grows naturally in the winery and on the grapes. But unlike cultivated yeast, which gets introduced in such massive quantities that it quickly launches into fermentation, native yeast can take three or four days to get going. During that time, unwanted bacterial competitors can prematurely kick in, exhausting essential nutrients.

After the completion of primary fermentation, some of the sulfite that was added at the crusher (or press) stage is depleted, having oxidized, vaporized or bonded with components in the wine. That's ideal, because the next step for most wines is malolactic fermentation, in which bacteria convert tart malic acid into creamier lactic acid.

Malolactic bacteria are extremely sensitive to sulfites, so even modest quantities hinder them. However, oxidation due to low sulfite levels isn't a problem at this stage because malolactic bacteria generate a blanket of carbon dioxide that essentially seals off the wine from oxygen.

And many wines are better without malolactic fermentation. Some vintners, such as Lingenfelder, want the crisper style that comes from preserving malic acid. After primary fermentation is complete he adds enough sulfite (about 70 to 80 mg/l) to inhibit the malolactic bacteria in his Riesling.

Barrel-aging, which gradually exposes wine to air, is part of the program for many of the world's most reputable wines, including Bordeaux, Napa Cabernet, and red and white Burgundy. So vintners constantly monitor and tweak free-sulfite levels to ensure that oxygen exposure remains controlled, rather than excessive. "Every time we rack, top off or mess with the barrels, we analyze SO2 and add enough to increase it to 20 ppm [parts per million], maybe more," says Steve Test, winemaker at Merryvale Vineyards in St. Helena.

Brawny Cabernets tend to spend about two years in barrel. When it comes time to bottle, vintners usually want about 25 to 40 ppm of free sulfite. That's enough to slow oxidation and inhibit unwanted microbial activity, such as brettanomyces, a spoilage yeast, which can impart pronounced leather and barnyard character.

At bottling, different wines require sulfites for different reasons. Reds, due to their tannin and pigmentation compounds (anthocyanins), are more resistant to oxidation than most whites; but reds are more susceptible to microbes because they usually have less acidity. Late-harvest wines demand the highest levels, typically more than 50 mg/l free sulfites (sugar tends to bind with sulfites, and vintners need an ample sulfite supply to shut down microbes, which might otherwise wreak havoc gorging on the residual sugar).

Whether introduced during fermentation, barrel-aging or bottling, sulfur is an integral part of the winemaking process at the vast majority of wineries. Although its power and utility in stabilizing wines is unmatched, a few hardy producers embrace a noninterventionist philosophy that frowns on sulfite additions.

Amity Vineyards in Yamhill, Ore., offers an assortment of organic wines, one of which is also made without added sulfites. Called Eco-Wine, it's a Pinot Noir with a youthful, fruity style.

"Asthmatics are some of our most ardent customers. They'll buy 15 cases," says Myron Redford, owner and winemaker at Amity. "But they're a very small number. There is also a huge number of people who think they're allergic to sulfites, who blame sulfites for any reaction to wine."

Redford started the Eco-Wine label in 1990, at which time sulfite additions were not permitted for organic certification (current regulations allow a maximum of 100 mg/l added sulphites, with no more than 35 mg/l of free sulfites permitted at bottling).

Winemaking without sulfites can be a zero-sum game. Redford must take special care when handling the Eco-Wine: it can't withstand any time in barrel without oxidizing, and in order to eliminate potential microbial problems, he performs a tight filtration, which can reduce richness and complexity.

The rare cases in which individuals suffer allergic reactions to sulfites notwithstanding, most vintners agree that eliminating sulfite additions is both unwise and unnecessary. "There's a tendency to polarize these issues, to make it black and white," says Ramey. "But it's not; it's a question of being judicious. Let's not throw out the baby with the bath water."

—Daniel Sogg