From Jack Kellers website:
Many of the recipes call for using one or more crushed Campden tablets while others do not. Some recipes call for the use of potassium metabisulfite instead. So why is this? Indeed, all recipes should use potassium metabisulfite (or Campden), but some authors list it and others don't, and some of the recipes on my site were developed by others -- but even I often leave it out of my recipes if it is not added initially. It is just something you should know you should add without being told. It kills almost all wild bacteria and fungi that ride in with the raw ingredients your wine will be made from, inhibits the early viability of wild yeast so that your cultured wine yeast can get a head start, and deters both the browning the oxidation of wines for a considerable period. But this compound is so strong that only 1/4 teaspoon is sufficient for treating 5 gallons of wine. Campden tablets contain a rather large amount of inert binding material and an appropriate amount of potassium metabisulfite (or sodium metabisulfite, which I don't recommend you use) for treating one gallon of wine. Use very finely crushed Campden tablets (you'll have to do the crushing yourself), dissolved in a little water, juice or must, for one gallon batches. Use potassium metabisulfite for 5-gallon batches and larger. If you can divide 1/4 teaspoon of the pure compound into 5 equal parts, then by all means use the potassium metabisulfite for 1-gallon batches instead of crushed Campden tablets.
Add the Campden or potassium metabisulfite (pot meta for short) when the fruit is crushed, unless you are going to use boiling water to extract the flavors, color and juices of the base. The boiling water will kill off the bacteria, fungus and wild yeast, but when you rack the wine you should add the appropriate dose of crushed Campden or pot meta. Some of the sulfur in the dose will bind with other components of the wine but some will exist as unbound sulfur (also called "free sulfur") in the form of a dissolved gas called sulfur dioxide, or SO2. This gas is the sanitizing and antioxidizing agent. As time progresses, the gas is slowly released into the atmosphere or breaks down and the sulfur in it binds with other components created as the wine develops and ages. Thus, the dose of SO2 must be regenerated periodically. If you add the Campden or pot meta to the must at the beginning, add another dose at the 2nd, 4th, and 6th rackings and just before bottling (it must be added at the same time as potassium sorbate when stabilizing a wine, as potassium sorbate alone will not stabilize the wine against malolactic bacteria, something you do not want to take up residence in your bottled wine). If you add Campden or pot meta at the time of the 1st racking, add it again at the 3rd and 5th rackings and before bottling (when stabilizing the wine). This should be done whether the recipe mentions it or not.
