SO₂ and sulfites in wine

   During the fermentation process in the manufacture of wine, SO₂ or K₂S₂O₅ is added to the initial wine pressings to kill microorganisms, the presence of which results in spoilage of the wine. Molecular SO₂ is only used for large scale wine production, while K₂S₂O₅ is the common additive in small scale production. In acidic solution, [S₂O₅]^2- undergoes the following reactions:

The overall equilibrium system for aqueous SO₂ is:

 The position of equilibrium is pH-dependent; for the fermentation process, the pH is in the range 2.9–3.6. Only molecular SO₂ is active against microorganisms. The first (i.e. yeast) fermentation step is followed by a bacterial fermentation step (malolactic fermentation) in which malic acid is converted to lactic acid. After this stage, SO₂ is added to stabilize the wine against oxidation.   Adding SO₂ too early destroys the bacteria that facilitate malolactic fermentation. Malolactic fermentation is usually only important in red wine production.   The addition of SO₂ to white and red wines is handled differently. Red wines contain anthocyanin pigments, and these react with [HSO₃]^- or [SO₃]^2- resulting in a partial loss of the red coloration. Clearly, this must be avoided and means that addition of SO₂ to red wine must be carefully controlled. On the other hand, significantly more SO₂ can be added to white wine. Red wine, therefore, is less well protected by SO₂ against oxidation and spoilage by microorganisms than white wine, and it is essential to ensure that sugar and malic acid (food for the microbes) are removed from red wine before bottling. Red wine does possess a higher phenolic content than white wine, and this acts as a built-in anti-oxidant.

     Wines manufactured in the US carry a ‘contains sulfites’ statement on the label. Some people are allergic to sulfites, and one possible substitute for SO₂ is the enzyme lysozyme. Lysozyme attacks lactic bacteria, and is used in cheese manufacture. However, it is not able to act as an antioxidant.

   A possible solution (not yet adopted by the wine industry) would be to mount a combined offensive: adding lysozyme and a reduced level of SO₂.