What Is SO2?:
Potassium Metabisulfite, (often referred to as "SO2", "sulfites" "meta", or "meta-bi") has several uses in winemaking. At the crush, sulfites are generally used to help control the spoilage bacteria and indigenous yeast that may already be present both on the fruit and in the winery (i.e. on the picking bins, processing equipment, tanks, tubing, etc). The amount generally used is enough to inhibit most of the unwanted organisms but not enough to hinder a cultured wine yeast, which has a higher tolerance to sulfites than most of the indigenous organisms do. This inhibition effectively "wipes the slate clean" for the cultured wine yeasts to step in and rapidly colonize the must so that it can dominate the subsequent wine fermentation. In addition, sulfites also help to inhibit the enzymatic browning of both fermenting wine and finished wine so that all of their delicate complexities can be preserved. Later, during storage and in the bottle, sulfites at the proper levels will further protect a wine by continuing to inhibit spoilage organisms, as well as by scavenging oxygen.
Historically the most common form of Metabisulfite is as a powder. 1/4 tsp adds 50ppm to 5 gallons of must. Used Metabisulfite loses potency with time and should be replaced every year to assure that you are getting correct levels. You can use old Metabisulfite to make solutions for sanitizing equipment.
A more modern approach to sulfite use is the Efferbacktol powder packets. They disperse SO2 in a self-dissolving format that reminds us of alka-seltzer. We use them at crush as we are filling bins (we add them as we fill the container, bin or tank) and during storage. They are the most convenient when used in oak wine barrels as a 2 gram packet adds 9ppm to a 60 gallon barrel and a 5 gram packet adds 18ppm to a 60 gallon barrel. Keep them on hand in the cellar and your more likely to keep proper sulfite levels in your wine.
Some Useful Tips:
Here are some important elements about working with SO2 to keep in mind:
The exact amount needed to effectively do the job is determined by the pH of the wine.
Free SO2 levels fall faster in wood cooperage than in glass or stainless, so if you are using an oak wine barrel you will most likely need to manage sulfite levels more closely.