Potassium Metabisulphite, (often referred to as "SO2", "sulphites" "meta", or "meta-bi") has several uses in winemaking. At the crush, sulphites 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 yeast, which has a higher tolerance to sulphites than most of the indigenous organisms do. This inhibition effectively "wipes the slate clean" for the cultured yeast to step in and rapidly colonize the must so that it can effectively dominate the subsequent fermentation.
In addition, sulphites also help to inhibit the enzymatic browning of both musts and finished wines so that all of their delicate complexities can be preserved. Later, during storage and in the bottle, sulphites at the proper levels will further protect a wine by continuing to inhibit spoilage organisms, as well as by scavenging oxygen.
Note that the exact amount needed to effectively do the job is determined by the pH of the wine. Refer to our MoreManuals! on Red or White Winemaking or one of the winemaking books that we offer for a complete explanation on how to properly manage sulphites.
In addition, it's important to keep in mind that free SO2 levels fall faster in wood cooperage than in glass or stainless, so if you are using a barrel you will most likely need to manage sulphite levels more closely.
The most common form of Metabisulphte is as a powder. 1/4 tsp adds 50ppm to 5 gallons of must. Used Metabisulphite loses potency with time and should be replaced every year to assure that you are getting correct levels. You can use old Metabisulphite to make solutions for sanitizing equipment.
A convenient way to add sulphites is to use pre-mearsured Effervescent SO2 packets. The granules are effervescent, much like Alka-Selzer, which helps them to dissolve. The granular composition is varied, allowing the product to dissolve at multiple levels in the tank or barrel.
The 2 gram size will add 9 ppm sulphites to a 60 gallon barrel or 18 ppm sulphites to a 30 gallon barrel. The 5 gram size will add 22 ppm to a 60 gallon barrel.