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Sulfur dioxide in Oenology
Sulfur dioxide is used in oenology - as already mentioned - for its contemporary antioxidant and antiseptic actions. Despite these positive effects on wine, it is good to remember that its use must be limited in any case, both for the negative effects on health and for organoleptic reasons. The maximum quantities allowed in oenology are established by specific laws in force in each country. As regards the European Union, the maximum limits allowed are 160 mg / l for red wines and 210 mg / l for white and rosé wines. Exemptions are foreseen which allow member states to raise this value to a maximum of 40 mg / l in unfavorable years. Since sulfur dioxide has toxic effects on the organism, the World Health Organization (WHO) has defined the maximum daily dose in 0.7 mg / kg of body weight, while the lethal dose is defined in 1.5 g / kg of body weight. In this regard, it should be remembered that in those predisposed and sensitive to sulfur dioxide, this can be a reason for migraines as well as other ailments. It should also be remembered that proper oxygenation before consumption - an operation that can also be carried out by swirling the glass - releases about 30-40% of the sulfur dioxide contained in the wine. In oenology, sulfur dioxide is used from the earliest stages of wine production, starting from must to bottling. When using sulfur dioxide, it is advisable to know that a part of this gas combines with some components of the must or wine, while the remaining part remains free, that is, not combined. The free part will play the important antioxidant and antiseptic effects: for this reason it is essential that sulfur dioxide combine as little as possible. The combined sulfur dioxide is still useful, since in the case in which the free fraction is dispersed - during the transfer operations, for example - a small part of the combined fraction is freed by replacing it. It should however be noted that this phenomenon is rather limited, therefore it is always essential to add sulfur dioxide in all cases in which the wine comes into contact with lossigenous, such as in the case of decanting, filtering and bottling. The amount of free sulfur dioxide added to the combined amount determines the amount of total sulfur dioxide.
Sulfur dioxide and wine (source: www.civiltadelbere.com)
The chemical forms of sulfur dioxide
The sulfur dioxide contained in wine can be present in different forms, not all equally interesting from an oenological point of view. The term free sulfur dioxide indicates the forms which can be released by acidification, namely:
- H2SO3 or molecular sulfur dioxide (non-dissociated fraction);
- HSO3 or bisulfite ion (semi-dissociated fraction);
- SO3 or sulfite ion (completely dissociated fraction).
Instead, when we talk about combined sulfur dioxide we indicate that part of sulfur dioxide linked in a more or less stable way with some wine compounds such as acetaldehyde, sugars, ketone acids, uronic acids, oxidation products of sugars and anthocyanins. Depending on the stability of the bond, a further distinction is made between:
- Combined SO2, permanently linked with acetaldehyde;
- SO2 deposit linked to compounds with medium or weak affinity and which can, by dissociating by heating, originate free SO2.
Finally, the term total sulphurous refers to the sum of the two free and combined forms.
Sulfur dioxide (source: www.my-personaltrainer.it)
Sulfur dioxide is mainly used in the following cases:
in the must for white wines, with the aim of avoiding the start of alcoholic fermentation, allowing the settling of the solid parts;
before the beginning of alcoholic fermentation, with the aim of selecting the yeasts and, in the case of red wines, to favor a better extraction of color and tannins from the skins;
in all operations involving the contact of wine with air, such as decanting, clarification, filtering and bottling, thus avoiding lossidation and the development of unwanted bacteria or yeasts.
For a correct use by SO2 in the microbiological stabilization of wine it is therefore necessary to know:
- the quantity of free SO2;
- the pH of the wine;
- the temperature of the wine;
- the alcoholic degree;
- the amount of molecular SO2 present;
- the quantity of "paralyzing" molecular SO2 that is necessary to block the growth of microorganisms present in wine.
Since the conservation of wine is always a critical factor and the preservative effects of sulfur dioxide are carried out by the free fraction, a properly stored wine must always have a certain quantity of the same.
Forms of use
The use of sulfur dioxide in oenology can be carried out in different forms. Once the correct dose has been determined, the potassium metabisulphite can be added directly to the must or to the wine, mixing the mass with a stick of a length that allows a certain distance from the container thus avoiding breathing the fumes that develop during the operation .
Sulfur dioxide can be used in oenology in different forms: depending on the annual consumption, the preparation of the cellar staff and the moment of use of sulfur dioxide, one form may be preferable to the others.
Liquid sulfur dioxide
This indicates the gaseous sulfur dioxide stored in cylinders at the pressure of -3 atmospheres and which, therefore, at room temperature, is in the physical state of liquid. It can be introduced into the wine directly through a tube immersed at least halfway up the tank to be treated.
THE advantages related to the use of liquid sulfur dioxide are:
- it is a pure product that does not undergo alterations;
- it has a double sulfur yield compared to potassium metabisulfite;
- it does not add other substances to the wine and does not increase its ashes;
- it costs less than the other sulphurous forms.
On the other hand, the disadvantages I'm:
- it is a very dangerous product to handle that requires trained manpower;
- the smaller cylinder contains 58 kg of sulfur dioxide;
- the holding in the cellar of quantities greater than 75 kg requires specific authorizations.
These are aqueous solutions based on potassium bisulfite containing 150 g / l of SO2 (Sulfosol A) or based on ammonium bisulphite at concentrations of 150 g / l of SO2 (Sulfosol M), 400 g / l SO2 ( Supersolfosol) or 630 g / l SO2 (Neosolfosol C). Unlike the sulfur solutions prepared in the cellar by dissolving liquid sulfur dioxide or potassium metabisulfite in water, produced industrially, if properly stored in closed containers and rooms not subject to strong temperature changes, they are stable for rather long times. Normally they are used as they are; further dilutions may be necessary only if automatic dosing equipment is used.
The big guys advantages related to the use of sulfur solutions are:
- ease of dosing: it is sufficient to have a graduated container like a cylinder to measure exactly the quantity to be added to the wine;
- greater operator safety: although these are always dangerous products, the level of risk for those who use them is much lower than that of liquid sulfur dioxide.
Disadvantages: compared to liquid sulfur dioxide, the cost per gram of SO2 is certainly higher.
The potassium bisulfite solution can be used in every phase of the refinement. It can be dosed in wine in a single solution, and in this case a homogenization reassembly of the mass is required, or it can be introduced continuously using a Venturi tube or automatic dosing units. In this second case, if all the volume to be treated comes into contact with the solution, the homogenization reassembly is superfluous.
Potassium metabisulphite powder
In home wine production it is preferable to use potassium metabisulphite also for its simplicity of use. Potassium metabisulphite is in fact sold in the form of salts (the only sulfur salt allowed for oenological use), it can be easily weighed and can be kept for a long time in airtight containers protected from light. Potassium metabisulphite contains 55% sulfur dioxide, therefore each gram contains 550mg of SO2. Slightly soluble in cold water and insoluble in alcohol, before its use it must be completely dissolved in lukewarm water or wine. The solution thus obtained is not very stable and must be immediately introduced into the mass, which must be subsequently stirred or reassembled to have a homogeneous distribution of the sulfur dioxide.
Advantages of potassium metabisulphite:
- although it is always a dangerous product, there are few risks for the operator;
- low cost, second only to sulfur dioxide.
Disadvantages: need to weigh the product.
Finally, potassium metabisulfite products have been developed both in effervescent tablets and in effervescent granules.
Actions and properties
- Antiseptic action: inhibits the development of the microbial flora. The antiseptic efficacy of a given dose of sulfur dioxide is enhanced if the starting microbial population is reduced, for example by filtration. During the storage of wines, sulfur dioxide inhibits the development of all microorganisms, avoiding cloudiness due to the presence of yeasts or the refermentations of sweet wines. The antiseptic action occurs against both yeasts and bacteria; however, bacteria are sensitive to lower doses than yeasts and therefore sometimes alcoholic fermentation can occur and malolactic fermentation cannot. The tolerance to sulfur dioxide by yeasts is greater in the must than in wine, because in the latter there is a high concentration of alcohol, also antiseptic. Also important is the selective action carried out by sulfur dioxide in the strains of yeasts naturally present in the must. Each type of yeast responds to its own characteristics and behaves differently during fermentation. With the aim of ensuring better and more homogeneous fermentation, sulfur dioxide is also useful in this case. Some yeasts and many bacteria are particularly sensitive to the effects of sulfur dioxide which will therefore carry out an appropriate selection operation.
- Antioxidant action: the antioxidant effect, on chemical reactions, is used throughout the duration of the process, both in winemaking and in conservation. In particular, SO2 preserves wines from excessive oxidation of phenolic compounds and some aromatic substances, coloring substances, tannins, alcohol and iron. The risks of oxidation during wine production are quite high: from the moment the bunch is collected from the vine and transported to the cellar there is always the danger of going through oxidation. In addition, every time operations are carried out on wine, the possibility of contact with oxygen is always very high, a risk that increases further in the event that the must or wine is rich in enzymes and molds - such as Botrytis Cinerea - and metals catalysts, such as iron and copper. For these reasons, the use of sulfur dioxide can limit the effects of oxidation, thus ensuring greater quality and conservation of the wine. SO2 also performs an antioxidant action: it protects musts and wines from enzymatic oxidations, instantly inhibiting the oxidase enzymes (tyrosinase, laccase) and, if necessary, allowing their subsequent destruction.
- Solubilizing action: sulfur dioxide has a solvent effect favoring the extraction of certain substances present in the grape skins. During the maceration of the red grape skins in the must, sulfur dioxide favors the passage in solution of the coloring substances and tannins. For this reason it is always preferable to avoid the sulphiting of white grapes since this would lead to yellowing of the must and enrichment of tannins. In white grape musts, the addition of sulfur dioxide is always carried out after the separation of the solid parts, that is, after draining. Among other solvent effects, sulfur dioxide favors the extraction of mineral substances and acids.
- Action on the organoleptic characteristics: sulfur dioxide also has a positive effect on the taste and aromas of wine. From an organoleptic point of view, it avoids the loss of aromas, in particular the fruity ones typical in young wines, eliminates the so-called "vanished taste", reduces the tastes of rotten and moldy. To achieve these positive effects, sulfur dioxide must be added when the alcoholic fermentation has completely ended. If it is added too early compared to the end of the fermentation, that is when the temperature of the wine is still too high, unpleasant aromas and flavors of sulfur dioxide, mercaptan and rotten eggs can develop. Finally, sulfur dioxide has a mild clarifying action, since it promotes the coagulation of the colloidal substances present in wine and must, thus promoting the spontaneous precipitation of the lees. Sulfur dioxide, added in large quantities in the must, is used to obtain the so-called "mute must", that is, non-fermentable, due to the blocking of the activity of the yeasts.
Although the effects of sulfur dioxide in winemaking are indispensable and important, it is however always advisable to limit its use and use the lowest possible doses, above all to limit the health effects of subjects particularly sensitive to this gas. In any case, after adding sulfur dioxide, it is always advisable to mix the wine or must in a very homogeneous way, trying to be as precise as possible in the preparation of the dose: always and in any case the bare minimum. Excessive additions of this compound to wine can lead to an accumulation of acetaldehyde and a production of hydrogen sulfide and mercaptans, with consequent anomalous odors.
- Scotti B., Use of sulphurous in aging and new forms of employment, Vinidea.net - Rivista Internet Tecnica del vino, 2004, N .1 / 2