SO2 is used as a preservative to protect grape juice, must or wines from oxidation and microbial spoilage. When SO2 is added, part of it reacts with different components present in the grape juice/must or wine to bound and become inactive as an antioxidant and antimicrobial agent. What remains is the Free SO2 and is in two parts "bisulfite" (HSO3-) and molecular SO2. The molecular SO2 is the active form against microorganisms and unwanted bacteria. SO2 additions are made at different stages of the wine-making process. The most practical usage of sulfur dioxide addition is by adding liquid SO2 solution, sodium or potassium metabisulfite.
SO2 liquid solution can be prepared by bubbling gaseous SO2 into water. The amount of SO2 added as a liquid solution depends on the strength of that solution, and can be calculated from its concentration, normally expressed as % weight/volume.