Sulphurization

The Sulfur (also sulfurization ) is a method of preservation and is used mainly for extending the shelf life of wine , dried fruit and horseradish applied. The sulfur dioxide or the sulfite formed in aqueous solution has a preservative effect .

Sulphurization has been known since ancient times. It was temporarily banned due to the excess of sulfur. Emperor Maximilian I allowed sulphurisation for wine again and prescribed a maximum value (approx. 40 mg / l).

The sulfur requirement during winemaking depends on the grape variety and the time of harvest, white wines generally require more SO ₂ than red wines , quality wines require less SO ₂ than quality wines , as these often have a higher proportion of noble grapes (legal maximum values: for wines below 5 g / l residual sugar, red: 150 mg / l, white and rosé: 200 mg / l; for wines with more than 5 g / l residual sugar: red: 200 mg / l, white and rosé: 250 mg / l; late harvest : 300 mg / l; Auslese: 350 mg / l; Beeren-, Trockenbeerenauslese, Eiswein: 400 mg / l).

There are several ways to sulfurize a product:

• Burning of sulfur in empty wine barrels to gaseous sulfur dioxide SO ₂ ( g )
• Addition of potassium disulfite K ₂ S ₂ O ₅ , which splits off sulfur dioxide SO ₂ in acidic solution
• Addition of liquefied sulfur dioxide SO ₂ ( l )
• Addition of sulphurous acid solution H ₂ SO ₃
• Addition of sodium hydrogen sulfite NaHSO ₃
• Addition of sodium sulfite Na ₂ SO ₃

These methods differ in terms of safety and dosage. Burning off sulfur is a historical method and is generally no longer used today because the dosage is too imprecise. In addition to sulphurising the wine, burning sulfur also has the task of preserving empty wooden barrels that are damp on the inside. In this process, sulfur is burned inside the barrel until the combustion ends due to lack of oxygen. An alternative barrel preservation is to fill the barrel with sulphurized water.

Mode of action

• Antimicrobial: Like other preservative acids , only the undissociated acid molecule can pass through the cell membrane of microorganisms. When sulphurizing, only the physically dissolved (SO ₂ + H ₂ O) acts , which irretrievably inhibits the metabolism and damages the cell membrane.
• enzyme deactivating: Various enzymes , including those that promote oxidation, are inhibited to different degrees by SO ₂ . This inhibition can take place by destroying the secondary and tertiary structure of the enzymes.
• Reducing: Components of the sulphurized product that have been oxidized by oxygen can be reduced to sulphate ions (SO ₄ ²⁻ ) by oxidation of the sulphurous acid dissolved as bisulphite ion ( hydrogen sulphite , HSO ₃ ^- ) . The bisulfite ion can be oxidized by free oxygen and thus protects the product from oxidation.
• sensory: an additional benefit arises in winemaking, as SO ₂ binds to methanal ( formaldehyde ), it suppresses the undesirable influence of this by-product of alcoholic fermentation on the taste.

Acetaldehyde problem

The main purpose of sulphurisation is to bind acetaldehyde , without which no wine can do. However, the quantities are small: between 10 and 30 milligrams of SO ₂ per liter. White wines require a little more sulfur dioxide because of their increased susceptibility to oxidation, red wines a little less. Sulfur dioxide does not only bind the acetaldehyde. It also reacts with other ingredients in wine, such as pyruvic acid , ketoglutaric acid and glucose . Sulfur dioxide changes and thus affects the taste of the wine. For this reason alone, the producers of fine wines try to keep the sulfur content as low as possible. Insofar as the sulfur dioxide has been chemically reacted with ingredients ("bound sulfur"), it is not perceptible to the senses and has no health significance. The so-called “free sulfur” is different: it is present in wine as sulfite, i.e. H. as the salt of sulphurous acid. It is this free sulfur that can possibly be smell and cause health problems if the wine is too sulphurous.

Methods of determination

The sample is acidified ( phosphoric acid ) and heated. The resulting sulphurous acid is in equilibrium with its anhydride sulfur dioxide SO ₂ . The expelled SO ₂ (g) is oxidized with hydrogen peroxide H ₂ O ₂ to sulfuric acid H ₂ SO ₄ . This can be titrated with a base. Volatile acids (e.g. acetic acid) are also measured and must be distilled off from the hydrogen peroxide solution and measured. This content is subtracted from the sulfur dioxide content.

Expel

${\displaystyle \mathrm {Na_{2}SO_{3}+H_{3}PO_{4}\longrightarrow H_{2}SO_{3}\rightleftharpoons SO_{2}+H_{2}O} }$

Catch

${\displaystyle \mathrm {SO_{2}+H_{2}O_{2}\longrightarrow SO_{3}+H_{2}O\longrightarrow H_{2}SO_{4}} }$

Titration

${\displaystyle \mathrm {H_{2}SO_{4}+2\ NaOH\longrightarrow Na_{2}SO_{4}+2\ H_{2}O} }$

Declaration obligation

For all wines that have been marketed in the Federal Republic of Germany since January 1st, 2006, the sulphurisation must be shown on the label; the label "Contains sulphites" is sufficient. Wines that were bottled before the cut-off date may also be sold without the notice.

There is no need to declare if a wine contains less than 10 mg / l sulfur.

swell

• Swiss Food Book (chapter on wine, vinegar, dried fruits and fruit juices)