Used to retard yeast activity and clean equipment. Also effective as an anti-oxidant. The primary usage of sodium metabisulfite is addition to the grapes or wine. Sodium meta is generally less expensive than potassium meta and so is used for cleaning, and potassium meta is used in the wine for its health reasons. Both are broken down to bisulfite when added to water, which in turn liberates sulphur dioxide gas (SO2). This SO2 is the active chemical that provides the cleaning and protective properties (Burning sulphur sticks also produces SO2).
There are three common form of usage; a stock solution, pure powder, and Campden tablets. A 10% stock solution is made by adding 1 ounce (by weight) of the powder to 8 fluid ounces of water. Mix well and store in a tightly capped bottle. One teaspoon (or 5mL) of this solution provides 75 ppm of SO2 in 1 gallon of wine. It will last for a few months if kept closed. Pure powder can be used when the large quantity of grapes or wine makes a stock solution impractical. Use _ oz. (slightly less than 1 teaspoon) to 10 gallons of must or wine to obtain 75 ppm of SO2.
After addition of sulphite to must, wait 12-24 hours before adding the yeast so the SO2 can dissipate. Sulphite powder and solution can be used effectively as long as it gives off a strong sulphur smell when mixed with water. Old sulphite (more than 1 year) should generally be replaced. General recommendations are for 50-100 ppm for fresh grapes, and 30-50 ppm for storage and bottling. For use as a cleaning solution for equipment, mix 1-2 oz. per gallon of water.