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.
Typical dosage is 1/4 tsp Sodium metabisulfite per 5 gallons of must (yielding roughly 75 ppm of SO2) prior to fermentation, and 1/2 tsp per 5 gallons (150 ppm of SO2) at bottling.
Winemaking equipment is sanitized by spraying with a 1% SO2 (2 tsp Sodium metabisulfite per L) solution.
Uses in Beer: Sodium metabisulfite is sometimes used in the brewing industry to inhibit the growth of wild yeasts, bacteria, and fungi. This is called stabilizing. It is also used to neutralize chloramine that has been added to tap water at the source as a disinfectant. It is used both by homebrewers and commercial brewers alike. It is not used as much for brewing beer, because the wort is almost always boiled, which kills most microorganisms anyway. It can also be added to strike water (the water used to mash the barley) in order to remove chloramines which can cause phenolic off flavors in beer (TCP).