Sodium Metabisulphite or Sodium Pyrosulfite is an inorganic compound of chemical formula Na2S2O5. It is a white crystalline or powder solid with a slight sulphur odour. The substance is sometimes referred to as disodium metabisulfite and is used as a disinfectant, antioxidant, and preservative agent.
In winemaking, this basically has two purposes: to kill wild yeast and for use as a sanitizing agent. Use it before to clean bottles, carboys, and to soak corks in a solution before bottling.