Nobody really knows when sulfur dioxide (SO2) was first used in winemaking. Most texts suggest that the Romans were the first to add it to wine. “Yes, the Romans burnt sulfur, allowing the fumes to clean amphorae and help preserve the wine stored in them,” says Paul Lukacs of Loyola University Maryland, and author of Inventing Wine. “Pliny the Elder mentions this. They also added sulfur, along with resin and all other sorts of things to their wines. But, as best I can tell, they did this only with their most valued wines. They made little special effort to cleanse containers or preserve ordinary wines.”
Professor Patrick McGovern, perhaps the leading authority on ancient wine, is less sure. “To my knowledge, while there are mentions of sulfur-cleansing of houses, I know of no definitive text for it being used to fumigate wine vessels.” McGovern suggests that the first clear-cut evidence of its use is from 1670, while others cite mentions of its use from Germany in the 15th century. Its widespread use seems to coincide with the advent of the wine bottle, and the associated need for more stable wines that were likely to be kept for longer before drinking.
The current situation is that SO2 is almost universally added to wine as a preservative. But in the last few years its use has come under the spotlight, with the rise of the small but vocal natural wine movement. The goal of ‘natural’ winemakers is to add as little as possible to their wines, and manipulate them less both physically and chemically. Allied with this is the concern on the part of many consumers about the more general use of preservatives and ‘chemical’ additions to their foods.
What’s it all about?
SO2 is a winemaking wonder molecule, and it's really hard to make good stable wine without it. It does two main things. First of all, it is antimicrobial, and deters the growth of unwanted bacteria and rogue yeasts. Secondly, it counters oxidation: it is not itself an antioxidant, but it limits the damage that oxygen causes, and mops up some of the products of oxidation, preventing them from causing further damage to the wine.
The traditional way of adding SO2 to wine was to burn wicks of sulfur in wine containers before filling them. But while sulfur wicks are still used in some very traditional wineries, they are an imprecise way of delivering a dose of SO2. For this reason, winemakers usually add SO2 in the form of potassium metabisulfite, usually in powder or liquid solution form.
There are laws governing how much SO2 may be added to wine; the actual figure depends on the country and the wine style. These limits have fallen with time. At the beginning of the 20th century the limit was 500 mg per litre (this is the same measure as parts per million). In the EU, the limit for red wines is now 150 mg per litre while that for dry whites and rosés is 200 mg per litre. For sparkling wines and sweet wines the limits are higher. Elsewhere these limits differ. In reality, most modern wineries find it easy enough to stay well under these limits, but this hasn't always been the case. In the past many wines had levels high enough that when they were tasted young, you could catch some of the acrid smell of SO2 when you drank them.
Sulfur, sulfites, sulfides
There's widespread misuse of some of the terminology concerning sulfur compounds. Sulfur is the element, and this is often used as a fungicide in the vineyard in powder form, but it is not added to wine. Then there are sulfites (the term used for SO2 in wine), but these are different to sulfides, which are a group of volatile sulfur compounds implicated in reduction; the matchstick/struck flint reductive note found in some wines has nothing to do with sulfites or SO2. The rules state that if SO2 is present above 10 mg per litre then the label must declare ‘Contains sulfites’. Interestingly, yeasts themselves produce SO2 during fermentation, and sometimes can produce enough that the wine has to be labelled this way even if no SO2 was added during winemaking. Some strains, particularly some of the new H2S-producing strains, produce substantial amounts.
So how might SO2 be used by a winemaker? “I work in a sympathetic, low-interventional way where SO2 plays a crucial part in defining quality,” says consultant winemaker Sam Harrop. “It is not all-important, but it is an important tool. I believe in SO2 use with all varieties/styles pre- and post-ferment to safeguard site expression and purity in the final wine — whatever the price point. I believe in balanced (not excessive) use to protect wine quality.”
Harrop, like many winemakers, will make the first SO2 addition at the juice stage. He says that the purpose of this addition is primarily to rid the must of any non-Saccharomyces (wild) yeasts. “A small amount of SO2 at this stage will also provide a little bit of anti-oxidant activity.” He then inoculates with a yeast strain that he feels will suit his understanding of the terroir he is working with, and aims for a slow fermentation, followed by keeping the wine cool in the barrel. He leaves the wine on its gross lees without SO2 additions until one to two months before bottling. “When the blend is made I might add 40 ppm SO2. This will come down to 20 to 25 ppm free SO2 quite quickly.” After cold stabilising and fining the wine he'd be looking for a free SO2 level of 30 ppm with a total level of 60 to 70 ppm.
Chris Mullineux of South African winery Mullineux & Leeu Family Wines also tries to minimize SO2 usage. “Andrea and I try to be as pragmatic about it as we can,” he says. “We aim to use as little as possible as it does dumb down the wine when young but at the same time it is of course super important for us as we add nothing else to our wines and we want them to be stable and ageworthy.” Their target is 25 to 30 free ppm for both reds and whites at bottling, with totals generally between 60 and 80 ppm. “We work really carefully and somewhat obsessively clean in the cellar and wait until our wines are stable before bottling, so we don’t need to filter or add tons of SO2,” says Mullineux. “One thing we have learned to help our wines age better, and therefore for us to be able to use less SO2, is to keep an eye on the dissolved oxygen before and during bottling. If your wines are clean and healthy and your dissolved oxygen is low you can get away with lower SO2.”
The sulfur debate
There are three hotly-debated questions concerning SO2 usage. If it is seen as desirable to work with lower SO2 levels, does this mean that wines with low SO2 taste different, and if so, how? If SO2 is almost universally used in winemaking, is it possible to make wines that are sound without it, or with extremely low levels?
Sam Harrop doesn’t think so. “I love the concept of zero-added SO2 wines, and have consumed some nice ones over the years, but I don't think they are compatible with a terroir-focused winemaking programme,” says Harrop. “They can be fascinating, delicious wines, but offer a different and more philosophical drinking experience that not all consumers care for.” He continues, “too much SO2 is still used in commercial-scale wineries. At the other end of the price spectrum I see a concerning trend, following the success of the natural wine movement, to use less and less SO2, that can result in bottle variation, increased fault rates and premature ageing issues. I accept there are some lovely SO2-free wines available, but in my opinion the chances of making wine that reflect site without the use of SO2 pre- and post-ferment are slim if not impossible.”
Others disagree. Doug Wregg, a buyer for Caves de Pyréne, a UK-based importer specialising in natural wines, thinks natural winegrowers have been misunderstood. “I can hardly think of a single natural winemaker who is dogmatic about the subject,” says Wregg. “Most will do what is necessary to preserve the integrity of the wine or add a really tiny amount at bottling. I think it is like the chef who tastes the food before adding seasoning.” He says that the best farming practices and terroir will yield grapes with great potential. “These grapes contain the very means for their own survival and if you are a skilful vigneron you can use the existing material without giving a big helping hand (or crutch!),” he says. “The less-manipulated wines for me contain life, energy, verve, drinkability. When wines are made without sulfur (and if they are good) there is nothing that is extraneous to the wine, even the little quirks and flaws — if there are any — fit into the whole.”
South African winemaker John Seccombe, of Thorne & Daughters, has done some research on oxygen and wine. “SO2 doesn’t do exactly what people think it does,” he says. “If you have a fragile wine that is then exposed to oxygen you can get quite a bit of oxidation when you add SO2.” He thinks that it acts like a sink for oxygen.
“Working without the safety net of SO2 makes the winemaker operate in a different way,” says Wregg. “You have to maintain a scrupulously clean winery. You have to be vigilant, constantly tasting.” He says it may require using CO2, controlled oxidation, and skin/stem contact, and that vignerons who don't use sulfur tend to explore the wilder side of winemaking, along with traditional methods, such as open-top ferments, whole bunch, skin contact for whites, zero filtration, or the use of amphorae or cement. “It is this cocktail of approaches that changes the feel and the very shape of the wine.”
Wregg thinks that natural winemaking has improved enormously over the past five years. “Experience is the great teacher,” he says. “Once you understand the risks involved in working without the safety net, you are that much more careful.”
What are the alternatives?
Ascorbic acid is an antioxidant that is commonly used in white wines, but only as a supplement to SO2. If it is used without adequate SO2, it can actually cause oxidation. And it has no antimicrobial activity. Chitosan is a new product that can be effective against Brettanomyces, but doesn’t have all the benefits that SO2 has. And there is lysozyme, which is an enzyme that kills bacteria, but this is an expensive product, and alone it won't have all the effects that SO2 has. SO2 is inexpensive and its use in wine has been proven over many years. It looks like it is here to stay. The issue is using less of it by using it more smartly. Its efficacy is increased if it is added at the right time in the right doses, and the wine is protected from unwanted oxygen exposure, especially at bottling.