Making Wine: Sulfur Dioxide

 Sulfur dioxide certainly is one of the most debated and detested chemical component used in enology. Despite the use of chemical substances in the production of wine is pretty frequent, when the subject of chemistry in enology is brought up, sulfur dioxide is the most frequently associated element, as if this would the only one to be used in the production of wine. Indeed, sulfur dioxide is just one of the many chemical elements used in enology, certainly the most common and important one. Sulfur dioxide - whose chemical formula is SO₂ - is a colorless gas, with the typical pungent and harsh smell, irritating the eyes and the respiratory tract. The inhalation of this gas can cause pulmonary edema and the prolonged inhalation of high doses can also cause death. Despite the quantities used in enology are minimal and far lesser than the level causing such effects, it is always advisable to take proper precautions and to avoid as much as possible the inhalation of this gas during its use in the many phases of wine production.

Salts of metabisulfite potassium

 Sulfur dioxide is used in enology for its important antioxidant, preservative and antiseptic properties, effects which are indispensable for the health, stability and quality of wine. Despite sulfur dioxide is among the most detested chemical components used in enology, up to now was not possible to find alternative components in order to provide effective antiseptic and preservative like sulfur dioxide, as well as more healthy for man. It should be remembered sulfur dioxide is one of the most common preservatives used in the food and beverage industry, therefore its use is not limited to enology and wine only. European Union identifies and codifies additives and chemical substances used in food industry, with numeric codes preceded by the letter “E” and which must be mentioned in the ingredients list. Codes ranging from E220 and E229 are classified as “sulfurs” - such as sulfur dioxide (E220) and potassium metabisulfite (E224) - all of them having preservative, antiseptic and antioxidant properties. These substances are considered as possible cause of allergic reactions in sensitive or intolerant subjects.

 Sulfur Dioxide is used in enology - as already said - for its concurrent antioxidant and antiseptic effects. Despite these positive effects on wine, it must always be remembered its use must always be limited, both because of the negative effects on health as well as for organoleptic reasons. The maximum allowed quantity in enology are set by specific laws in force in every country. As for the European Union, the maximum allowed quantities are 160mg/l for red wines and 210mg/l for white and rose wines. There are dispensations allowing member states to increase this value for a maximum of 40mg/l in case of unfavorable vintages. As sulfur dioxide has toxic effects on the human body, the World Health Organization (WHO) has defined the maximum daily intake in 0.7mg/kg of body weight, whereas the lethal intake is defined in 1.5g/kg of body weight. On this regard, it should be remembered sulfur dioxide can cause hemicrania and other disorders in sensitive and intolerant subjects. It should also be remembered a proper oxygenation before consumption - an operation which can be done by simply swirling the glass - frees about 30-40% of the sulfur dioxide contained in a wine.

 The rational and intelligent use of sulfur dioxide in wine making, it is not only suggested by healthy issues or “ethical” issues about the limited use of chemistry; its excessive use can drastically compromise the quality of wine. Excessive quantities of sulfur dioxide can in fact give the wine unpleasing flavors and aromas or may favor the wine to become cloudy during its keeping. Sulfur dioxide is not naturally present in the grape, however it can be produced by some yeast species which are naturally found in the must - from few milligrams to more than 50mg per liter - therefore it can be considered as a natural byproduct of wine. Also for this reason it is always recommended to do a proper selection of yeasts in order to ensure a better fermentation process while limiting the production of negative elements which could compromise both the organoleptic quality and stability of wine. Thanks to its antioxidant and antiseptic qualities, the use of sulfur dioxide in wine making is today considered indispensable.

 In wine making, sulfur dioxide is used since the very first phases of the production, from the must to bottling. In using sulfur dioxide, it is useful to know a part of this gas gets combined with some elements found in the must and wine, whereas the remaining part remains free, that is, non combined. It will be the free part to have antioxidant and antiseptic effects: for this reason it is indispensable sulfur dioxide gets combined for the least possible part. Combined sulfur dioxide is however useful, because in case the free part it is lost - during the operation of decanting, for example - a small part of the combined one gets free and replaces it. It should however remembered this phenomena is pretty limited, therefore it is always indispensable to add sulfur dioxide every time the wine comes in contact with the oxygen, as in the case of decanting, filtering and bottling. The quantity of free sulfur dioxide plus the quantity of the combined part gives the total quantity of sulfur dioxide.

 Sulfur dioxide is mainly used in the following cases:

• in the must of white wines, in order to avoid the activation of alcoholic fermentation and to allow the decanting of solid parts
• before the start of alcoholic fermentation in order to select yeasts and, in case of red wines, to favor a better extraction of color and tannins from the skins
• every time the wine comes in contact with the air - such as decanting, clarifying, filtering and bottling - therefore avoiding oxidation and development of unwanted bacteria or yeasts

 As the keeping of wine always represents a critical factor and the preservative effects of sulfur dioxide are always ensured by the free part, in a correctly kept wine must always be present some quantity of free sulfur dioxide. The use of sulfur dioxide in enology can be done in many ways. In home wine making it is always recommended to use potassium metabisulfite also because of its simplicity of use. Potassium metabisulfite is in fact sold in form of salts, it can be easily weighed and can be kept for a long time in an airtight container away from light sources. Potassium metabisulfite contains 55% of sulfur dioxide, therefore each gram contains 550mg of SO₂. When the correct quantity has been determined, potassium metabisulfite can be directly added to the must or wine, while stirring the mass with a stick of proper length such to allow a safe distance from the container therefore avoiding the inhalation of the gas which develops during the operation.

 The effects of sulfur dioxide can be classified in four categories: antioxidant, stabilizer, solvent and effects on taste. In the must and in the wine are found many substances which get easily oxidized, therefore changing both their appearance and taste. The use of sulfur dioxide prevents the oxidation of these substances and in particular of coloring substances, tannins, aromas, alcohol and iron. The risks of oxidation during the production of wine are pretty high, a process which begins since the very moment the bunch is harvested and moved to the winery. Moreover, every time the wine undergoes any phase of production, the chance of getting in contact with oxygen is always high, a risk which is further increased when the must or wine is rich in enzymes or molds - such as Botrytis Cinerea - or catalyzing metals, such as iron and copper. For these reasons, the use of sulfur dioxide can limit the effects of oxidation, while ensuring a higher quality and keeping of wine.

 The stabilizing and antiseptic effect of sulfur dioxide is very important and contributes to a better keeping of wine. The stabilizing effect is also used for the must obtained from white grapes, as it delays the start of fermentation, therefore allowing the decanting of solid parts and favoring the must to become limpid. Sulfur dioxide destroys or momentarily stop the development of bacteria responsible for malolactic fermentation (generally avoided in white wines) and those causing grave faults to wine, such as acescence or volatility as well as lactic acidity. Moreover, the selective action done by sulfur dioxide in yeast species naturally found in the must is very important as well. Every type of yeast is characterized by its own activity during fermentation and behaves differently. In order to ensure a better and more homogeneous fermentation, sulfur dioxide is useful in this case as well. Some yeasts and many bacteria are particularly sensitive to the effects of sulfur dioxide which will then do a proper operation of selection.

 Some kind of yeasts, which are not very active in the fermentation process and which produce secondary and unwanted substances for the quality of wine, are very sensitive to the effects of sulfur dioxide, whereas the others having a beneficial action during the fermentation, and in particular Saccharomyces Cerevisiae, are more resistant. Thanks to sulfur dioxide it is therefore possible to eliminate unwanted yeasts and bacteria, while keeping the yeasts having a positive role in the alcoholic fermentation. Sulfur dioxide has a solvent property and favors the extraction of some substance found in the skins of the grape. During the maceration of skins of red grapes in the must, sulfur dioxide favors the extraction of coloring substances and tannins. For this reason it is always recommended to avoid the adding of sulfur dioxide in white grapes as this would cause the must to have a deep yellow color and too much tannins. In the must produced with white grapes, the adding of sulfur dioxide is always done after the separation of solid parts. Among the other solvent effects, sulfur dioxide favors the extraction of mineral substances and acid.

 Sulfur dioxide has also a positive effect on wine flavors and aromas. According to an organoleptic point of view, it avoids the oxidation of aromas, in particular fruit aromas typical in young wines, attenuates the tastes of rot and mold. In order to get these positive effects, sulfur dioxide must be added when the alcoholic fermentation is completely done. In case it is added too early, when the wine is still fermenting, that is when the temperature is too high, it can happen the development of unpleasing tastes and aromas, such as sulfur dioxide, mercaptan and rotten eggs. Sulfur dioxide also has a bland clarifying effect, as it favors the coagulation of colloidal substances found in wine and in the must, therefore favoring the spontaneous precipitation of lees. Sulfur dioxide, added in high quantity to the must, is used to obtain the so called “mutage on must”, that is a non fermentable must, because of the interruption of yeasts activity.

 Despite the effects of sulfur dioxide in wine making are indispensable and important, it is however and always recommended to limit its use and to add the least possible quantity, first of all, in order to reduce the effects on the health of the subjects particularly sensitive to this gas. This precaution is particularly important in the production of wines requiring high quantities of sulfur dioxide, in particular in sweet wines and in wines having a high content of residual sugar, wines to which can be added, also according to law, higher quantities of SO₂. In any case, after having added sulfur dioxide, it is indispensable to stir the wine or the must homogeneously, while trying to be as much as possible precise in the preparation of the quantity: always and however the least indispensable. Finally, it should be remembered it is always preferred to add sulfur dioxide the less frequent possible although at higher quantities. The frequent and repeated adding of small quantities causes in fact the increasing of total sulfur dioxide. The antioxidant and stabilizing effects on wine and on the must - it should be remembered - are uniquely done by free sulfur dioxide and not by the combined part.