Copper Addition Details

Copper Sulfate (CuSO₄) reacts with sulfur compounds such as Hydrogen Sulfide (rotten egg smell) and mercaptans (burnt match, rubber, cooked cabbage, canned corn, etc.) These aromas can arise during fermentation due to nutrient limitations and they can come about also throughout the aging process through breakdowns of sulfur-containing amino acids.

Dealing with these objectionable aromas early, either by adding CuSO₄ or by nutrient supplements during fermentation, is important to minimize the amount of intervention required to fix the wine. Sulfides left untreated can form disulfides which not only sound twice as bad but are not eliminated by CuSO₄ treatment alone.

Splashing wine to eliminate sulfide aromas can encourage the formation of disulfides, which have a lower sensory threshold than sulfides (so the wine will not smell as bad as before). These disulfides can serve as a reservoir of stink from which sulfides can be re-formed under reductive conditions, bringing you back to square one.

If your wine stinks you should do a copper trial to find the right treatment, then add CuSO₄.