What is the purpose of using Campden tablets?
Initially, Campden tablets are used to kill off any potentially harmful bacteria that may may be present in the base ingredients used in winemaking, and to discourage any wild yeast from gaining a foothold. Campden will not kill yeast, but it creates an environment inhospitable to them. As sulfur dioxide (SO2) is released into the must and the atmosphere above the must dissipates, the environment inside the fermenter slowly changes and the yeast can grow, but by then our cultured wine yeasts, which are more tolerant of SO2, have gotten a good start and “crowd out” the wild yeasts, use up all the dissolved oxygen, and consume all the fermentable sugar and nutrients. In the end, there is nothing left for the wild yeasts and they die without propagating.
When we rack, we add oxygen to the wine and expose it to airborne microorgamisms we’d rather keep out of the wine. This causes the wine to oxidize if antioxidants are not present to prevent it from happening. SO2 is a powerful antioxidant but it dissipates over time, so we add more Campden to the wine when we rack to prevent oxidation. The same antioxidant protects the wine from airborne bacteria and molds.
Campden is also added to the carboy at the time of the 1st racking, the 3rd racking, the 5th racking, and so on. We skip the 2nd, 4th, 6th, etc. rackings unless one of them is the last racking before bottling. We always add Campden (or metabisulfite salt) at the last racking because the wine is particularly vulnerable to infection (from the air, inside the bottle or on the cork) when bottling it.