Sulphur compounds are used sometimes in the vineyard to prevent powdery mildew and a bit at the pressing for antioxidant capacity. Problems if there are not enough amino acids or nitrogen in the juice, yeast will create, during the fermentation, volatile sulfur compound (sulphides, H2S) as a by-product of amino acid metabolism. More they stress, more they produce! No one want sulphides. Its is creating bad smells and pre-age the wine, and what a process to get rid off it when you have them! The wrong solution can even make it worst….
Four types exist. First H2S. It is the easiest to get rid off, as it is volatile: a bit of oxygen would be enough. This action has to be soon as the rotten egg smell appear. If you leave it for too long, it could be already transformed to the second type: Mercaptans. Then, the oxygen would « feed » it and it will become the evil and sneaky di-sulphide. Why don’t you want di-sulphides? Because it’s harder to get rid off it. There are 2 sub-types. The type DMDS, once comfortable in the bottle, it will express some onion and cooked cabbage smell. The other one, DEDS, will have more burnt rubber and garlic notes. Lovely! Dimethylsulphides (DMS)are the last sulphides of the list. You can not get rid off it and it get most of the time detected at the final stage. The consuming stage! we can find it in aged wines. It can be nice at small dose as it can bring some blackcurrant and jam caracteres for reds, but it can also bring unplaisante vegetals test (asparagus, cooked corn, etc)
So if mercaptans are already in the wine, a copper addition will do. However, if it is too late and di-sulphides are already in the place, copper won’t change anything…. To « reverse » di-sulphides to mercaptan, it needs first ascorbic acid addition, and then, treat it with copper.
To be sure to know which type of sulfides you have in your wine, it’s better to do a trial with different fining agents: Copper, cadmium, ascorbic+copper.
I find the chemistry of those compound incredible, how we can go forward, and reverse… and forward again… but until a certain point. Sparging O2 or using too much of ascorbic acid can oxidase the wine. Copper also has to be manipulate with care and be added the best and minimum amount to don’t spoil the nice aromas existing.
However, if it’s a bad year, and sulphures will be in quantity from the start. There are few alternatives to prevent all this « hide and seek game». Commercial yeasts produced less HO2 than some wilds. Feeding the yeast of DAP and nutriment at the right time of the fermentation if YAN is low. Control the temperature for a fermentation nice and slow and not stressed yeast. Be sure than the fermentation is finish when you add SO2. As sulphides are sometimes in the lees, racking early can allow decrease the amount in the wine too… if too early, maybe clean lees from somewhere else can be used instead.