Sulfur dioxide, also referred to as sulfites (SO2 is the name of the active ingredient before it is added to wine), is an additive that plays several roles in modern winemaking and has done so since ancient times.
The practice of burning sulfur wicks inside maceration and fermentation tanks and casks, to prevent them from being spoiled by wood fungi, molds and other unwanted microorganisms, has long been known.
This technique has been used for centuries and is still used today in wineries that do not have more modern and efficient methods.
The gas is obtained either by burning sulfur to oxidize it, by roasting sulfide minerals, or through the reaction of sulfuric acid with elemental copper. In the food industry, sulfur dioxide is known as additive E220, which acts as an antioxidant and preservative, so it is used in fresh juices, jams and, of course, wine.
Sulfur Dioxide in Wine
Although sulfur dioxide can be added to wine in several ways, once in the wine it can be present in several forms:
- As molecular sulfur dioxide
- As free sulfur dioxide
- As bound sulfur dioxide
The proportion of these three forms (which we will discuss in a future post) tells us how effective the product will be for each of its properties. The properties that are most useful in wine include:
- Fungicide: Molecular sulfur dioxide (SO2) is highly effective at controlling all kinds of fungi, both those found in wine and those found in wood (remember that a whole range of the microorganisms that affect the characteristics of wine are, in fact, fungi). In wine, the proportion of molecular sulfur dioxide tends to be relatively low and depends heavily on the wine’s pH. Free sulfur dioxide also has an effect on the growth of yeast and other fungal populations, though not as strong as molecular sulfur dioxide. Bound sulfur dioxide, on the other hand, has practically no effect on fungi and yeasts, so it is of no use in controlling these pathogens.
- Bactericide: As with fungi, molecular sulfur dioxide (SO2) is highly effective at controlling almost all known genera of bacteria that attack wine. Free sulfur dioxide has a marked effect on the growth of bacterial populations, though not as strong as molecular sulfur dioxide. Unlike with fungi and yeasts, bound sulfur dioxide has been observed to have slight activity against some genera of bacteria, especially lactic acid bacteria. This is thought to happen because, when these bacteria break down the molecule to which the SO2 is bound in order to use it as an energy source, they release the SO2, leaving it in its free, bactericidal form.
- Antioxidant: In their molecular (SO2) and free forms, these compounds have a strong affinity for the oxygen dissolved in wine. They therefore act as antioxidants by “stealing” the oxygen in the wine and preventing the oxidation of other substances we may consider more valuable. Bound sulfur dioxide has no antioxidant activity because, being bound, it is for practical purposes as if it were already oxidized.
Conclusions
We should remember that wine, harsh as it may sound, is the product of the partial biological decomposition of the fruit of the vine, carried out by selected microorganisms. However, since this process of decomposition has not finished, substances remain on which other microorganisms can survive, such as spoilage yeasts and the bacteria responsible for turning wine into vinegar. It is therefore essential to have a strategy for controlling these microorganisms in order to preserve the quality of the final product.
Although some people may find it disturbing or unsettling to add substances that do not come from the grape to a food product such as wine, history has made it very clear that this compound is vitally important in making quality wines. It makes it possible to carry out a range of winemaking processes safely, preventing microbiological or chemical spoilage and preserving the quality of the wine. While some wineries have chosen to make wines without it, they must still use other strategies to control microorganisms and the oxidation of wines and musts, since both are generally inherent to winemaking.