Copper additions to wine

Copper is added to wine in the form of Copper Sulfate pentahydrate (CuSO₄ • 5H₂O), a blue crystalline powder. The active ingredient in Copper Sulfate is the copper ion.

The addition calculations on this site all assume you're using Copper Sulfate pentahydrate, as opposed to metallic copper or straight Copper Sulfate (the green stuff).

So when people talk about a copper addition rate or copper solution they are talking about the rate of the copper ion (Cu⁺²), not the entire CuSO₄ molecule. CuSO₄ is about 25% copper by weight, which is why you have to add about 0.4 ppm of CuSO₄ to get 0.1 ppm of Cu⁺².

Why add copper?

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.

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Why bother with copper solutions?

Copper solutions are a great way to add copper to barrels or to lab samples.

Typical addition rates for copper are below 0.5 ppm, which makes for a small amount of CuSO₄ to weight out for a lab sample, and a tedious chore for a barrel group.

Also, having a stock copper solution on hand makes it easy to diagnose sulfide problems and determine the correct treatment level with a lab trial.

Note: Copper solution calculations on WineAdds are in terms of copper ion concentration (Cu⁺²), not CuSO₄ concentration.

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Why bother with copper trials?

Lab trials are always a good idea before making additions to wine lots. This is especially true with copper additions as you want to make sure your addition is going to have the desired effect.

Foul-smelling sulfur compounds can be either sulfides or disulfides, and copper alone is only effective against sulfides. If you prepare a copper trial and you don't see any improvement, then your wine has a disulfide problem and you'll need to add Ascorbic acid with your copper and wait several months for the stink to go away. If you see your copper trial improving the wine, then you have caught your sulfide problem in time.

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