Volatile sulphur compounds (also called reductive characters), and the use of ascorbic acid and copper sulphate (CuSO4) to treat them, are again in our spotlight as we continue to receive samples for copper addition recommendations.
Volatile sulphur compounds may well be considered a microbiological fault, as their origins are in the alcoholic fermentation, when yeast produce H2S under conditions of stress. If fermentation yeast are given too much, too little, or an unbalanced ratio of ammonia/amino acid nitrogen, or if they are stressed in any other way, H2S may be produced. And this H2S, through a series of subsequent redox reactions, may result in the presence of volatile sulphur compounds that may only be detected long after fermentation. These compounds exist in a reductive-oxidative equilibrium which shifts in a more or less smelly direction, depending on the reductive or oxidative environment of the wine. The compounds have odours of rotten egg, canned or cooked vegetables, cooked cabbage, tinned asparagus, onion, garlic, burnt rubber, burnt match and wet wool, and are definitely considered a fault. Vinlab is able to quantify a comprehensive range of these volatile sulphur compounds by GC-MS.
Traditionally winemakers have used ascorbic acid (a strong anti-oxidant) and CuSO4 (a strong oxidant) to treat these nasty compounds. However ….. the latest message in treating wines with these faults is that adding ascorbic acid and CuSO4 some months after fermentation, and especially before bottling, may potentially make the situation significantly worse in the long-term. The redox equilibrium WILL shift over time, due to the nature of bottle ageing, and because there will always be residual copper in the wine that has had copper additions, so that what we might smell as an improvement today, could well end up causing more damage in six months time.
So the best solution is to get it right at fermentation – measure YAN, treat your yeast nicely, and choose a low H2S-producing yeast strain. And in the event of fermentations with unplanned H2S formation, an emergency action plan may be to add a very small amount of CuSO4 towards the very end of fermentation at RS +/- 5g/L, so that fermentation is not inhibited and that any residual copper will precipitate out with the yeast lees.
(p.s. never add copper to sluggish or stuck fermentations)