Sodium metabisulfite is a white crystalline powder with a pungent sulfur odour. Its main uses are as an antioxidant or chemical sterilant.
 
Uses in Wine: Sodium metabisulfite is a common wine or must additive, where it forms sulfur dioxide gas (SO2). This both prevents most wild microorganisms from growing, and it acts as a potent antioxidant, protecting both the color, and delicate flavors of wine. Without it most wines would taste like a rather poor sherry, vinegar, or have some other equally disgusting flavor.