Medium-strength acid (international name dihydrogen sulphite), which is formed when sulphur dioxide is introduced into water. Particularly in a warm environment, it tends to oxidise into the much more toxic and stronger sulphuric acid. The salts and esters of sulphurous acid are called sulphites or neutral or secondary sulphites, as well as hydrogen sulphites or formerly bisulphites or primary or acid sulphites. When sulphur dioxide is added to mash, must or wine, most of it is converted into sulphurous acid, with only a small proportion remaining as sulphur dioxide. The sulphurous acid immediately begins to split into sulphites and hydrogen sulphites. Both form compounds with various substances in the wine. Only when these processes are complete is the so-called free sulphurous acid present. The amount depends on the temperature and the pH value, but is usually around 20%. This means that around 80% of the sulphurous acid is present in the wine in bound form.
Antimicrobial and reducing effect
The sulphurous acid or sulphur dioxide has an antimicrobial effect, particularly against Acetobacter (acetic acid bacteria), lactic acid bacteria and wild yeasts. The hydrogen sulphites bind aroma-disrupting substances, including acetaldehyde (alcohol precursor). The sulphites react with the dissolved oxygen to form sulphates and have a reducing effect. The sum of free and bound sulphur dioxide (sulphurous acid) gives the total sulphur (also total sulphur dioxide or total sulphurous acid). The proportion in wine is measured using iodometry (free) and acidimetry (bound) and is given in milligrams per litre (mg/l). Excessive amounts of sulphur in wine are harmful to health, which is why there are legal limits. Even if these limits are adhered to, however, improper dry preservation of empty wooden barrels or over-sulphurisation of the wine can lead to the wine defect sulphuric acid firn.
Human organism
Sulphur is an important component of the human body and is involved in a number of metabolic processes. It is one of the most important functional carriers in the organism, is necessary for skin, hair and nails, keeps the tissue soft, reduces stress, has been proven to help with asthma, arthritis, rheumatism, internal and external inflammation, back and muscle pain and all allergies. Sulphur baths are generally recognised as beneficial to health and are also used by many people. The body of an adult with an average weight of 70 kg contains around 150 g of sulphur. The daily requirement is estimated at just under 900 mg. Some sulphur compounds are toxic (poisonous) in higher doses. Sulphites, for example, can destroy the vitamins of the B group, inhibit the mode of action of enzymes and increase the effect of carcinogenic substances. Foods rich in sulphites should therefore only be consumed in moderation.
Permissible quantities in wine
The WHO (World Health Organisation) has defined 0.7 mg per kg of body weight as the safe daily intake limit (ADI) for sulphur in the human body. Sulphur in food can trigger allergies or other undesirable reactions. In the case of allergies to sulphites, even much smaller quantities than those listed below can trigger so-called "sulphite asthma". For this reason, since 25 November 2005, EU regulations have made it mandatory to label wines with a sulphur content of more than 10 mg/l (incidentally, this value is exceeded in almost all wines). The two texts "Contains: Sulphur dioxide" or "Contains: Sulphites" (however, the use of the formula "SO2" is not permitted). This provision applies to all products, i.e. grape must, wine, sparkling wine and vinegar (wine vinegar). The proportion of residual sugar is also decisive. The values since the EU wine market regulation valid since August 2009:
For the production of organic wine, significantly lower quantities up to a maximum of two thirds of these maximum quantities are assumed. Individual organic associations oblige their members to do so. For example, Bioland stipulates a maximum of 110 mg/l (red wine) and 140 mg/l sulphur dioxide (white wine) for red and white wines with less than 5 g residual sweetness per litre. Wines with more than 5 g residual sweetness per litre may not contain more than 140 mg/l (red wine) and 180 mg/l (white wine) sulphur dioxide. See also tabular lists under the keywords total extract (wine ingredients) and wine-making agents.
Further information
For the production of alcoholic beverages, see Champagne (sparkling wines), distillation (distillates), speciality wines, spirits (types), winemaking (wines and wine types) and wine law (wine law issues). All work and aids in the vineyard during the vegetation cycle are listed under vineyard care.
