Sodium hydrogen sulfite,
Sulfurous acid, sodium salt
Clear or milky white liquid with a sulfurous odor.
Sodium bisulfite is used in almost all commercial wines, to prevent oxidation and preserve flavor.
Sodium bisulfite releases sulfur dioxide gas when added to water or products containing water. The sulfur dioxide kills yeasts, fungi, and bacteria in the grape juice before fermentation. When the sulfur dioxide levels have subsided (about 24 hours), fresh yeast is added for fermentation.
Sodium bisulfite (usually with an acid like citric acid to make it produce gas faster) is used to sterilize winemaking equipment.
It is later added to bottled wine to prevent oxidation (which makes vinegar), and to protect the color of the wine from oxidation, which causes browning. The sulfur dioxide displaces oxygen in the bottle and dissolved in the wine. Oxidized wine can turn orange or brown, and taste like raisins or cough syrup.
In fruit canning, sodium bisulfite is used to prevent browning (caused by oxidation) and to kill microbes.
sodium bisulfite: InChI=1/Na.H2O3S/c;1-4(2)3/h;(H2,1,2,3)/q+1;/p-2/fNa.O3S/qm;-2
sulfur dioxide: InChI=1/O2S/c1-3-2