Is there too much residual copper in your wine?
The use of copper sulfate for fining in wine is well established and widely used. The level of residual copper is highly regulated and, yet, does not appear to be regularly checked by many winemakers. Based upon the results of our testing for residual copper this year, we outline why some caution is needed with this additive.
Copper fining of wine has been employed for many years as a way to remove sulfides from wine. An aqueous copper sulfate solution is typically used as a source of copper ions – these copper ions react with the sulfide ions in the wine to form insoluble copper sulfide, which is then removed by settling or filtration. This is a very effective way of removing the stench caused by sulfide ions from the wine. The use of soluble copper can cause one problem though – residual copper ions in the wine after the copper fining treatment.
Prevention of hydrogen sulfide by good nutritional management
The main source of hydrogen sulfide in wine is known to be from yeast fermentation when yeast-assimilable nitrogen (YAN) is a limiting nutrient during primary fermentation (Jiranek et al. 1995; Jiranek et al. 1991).
It is possible to limit the amount of sulfides formed in the wine; this is a preferable path to follow than to add excess copper to the wine to remove the sulfides. Therefore, a greater awareness of adding nitrogen nutrients, such as diammonium phosphate (DAP) or blended complex commercial nutrients is needed. Awareness and improvements in yeast nutrition, including the wider use YAN measurements, have occurred over the past several years.
Screwcaps and the increase in copper doses
Over the past decade, screwcaps have taken over from traditional cork closures in Australia, primarily because cork was causing an unacceptable amount of cork-tainted wine. This large and rapid changeover in Australia was interestingly not followed in other wine-producing countries, such as the US or France.
The widespread use of screwcaps, while virtually eliminating cork taint, did create one minor issue: it was found that many wines had a much more ‘reduced’ character and a greater level of sulfide taint was noticed in the early days of screwcap adoption.
While copper fining had been widely practised in Australia for many years, it was suddenly found that a more rigorous application of copper was required. From anecdotal evidence, it seems most winemakers started to use a larger dose of copper to ensure the ‘reduced’ or sulfide character in wine under screwcap did not persist in the bottle.
Toxicity of copper sulfate
Copper sulfate is a poison. If you have a bottle on your lab shelf or in your winery store check the label. It will (or should) say ‘Poison’ in large letters on the label.
This chemical is widely used in the wine industry, albeit in small quantities, and as such it appears that it is sometimes assumed to be an innocuous material. It is not.
The LD50 (the median lethal dose) of copper sulfate is 300mg/kg in rats (MSDS). That is, an average dose of only
0.03g is needed to kill a 1kg size rat, So, please be aware copper sulfate is a material that deserves respect and that should be used very carefully and always the minimum dose should be employed.
Warning: Do not pipette copper solutions by mouth; always use a safety bulb pipette for additions.
Copper content of copper sulfate
The common form of copper sulfate has the formula CuSO4.5H2O and has a molecular weight of 249.68. The copper ion itself has an atomic weight of 63.55; therefore the proportion of the commercially available pure salt that is copper is 25.5%.
This ratio is used in a practical sense by many winemakers by using a 400ppm solution of CuSO4.5H2O and assuming it gives a 100ppm solution of copper ions (if accurately done it would actually be 102ppm).
This 100ppm copper ion solution (400ppm copper sulfate) is then used in bench trials using 1mL of copper stock solution in 100mL of wine to give a 1ppm copper treatment (Rankine 1989). Smaller or larger rates are made by using different addition rates, e.g., 0.5mL of stock gives a 0.5ppm addition rate. A number of trials can then be done easily to find that minimum level of copper that removes all trace of sulfides in the wine.
Performing these trials is a better process to follow than adding a standard addition to all wines and having the risk that excess copper is added when it is not necessary.
These fining trials are typically done by winemakers after primary fermentation and, then, at the pre-bottling stage as well. Usually there are two instances where copper is added to wine.
Residual copper content of wine
Some of the copper added to wine does remain in the wine solution after the sulfides have been precipitated. The amount of copper in the wine prior to fining can also add to the total amount left in solution. For example, if copper- based sprays have been used in the vineyard then some residual copper can be in the juice prior to ferment.
As copper is a toxic heavy metal there are legal limits for the amount of copper that can be present in various foods and drinks. The copper limit in wine varies from one country to another and the limits vary widely. There is no longer a specific limit in Australia for the amount of copper in wine (Food Standard 4.5.1).
There is a limit in the US of 0.5mg/L and in the EU of 1mg/L (see Wine Australia website).
Of the wines that we have tested in our labs so far this year, 11% of wines were above the 0.5mg/L limit and 7% were above the 1.0mg/L limit. Obviously some of these wines are at risk if exported to the US and EU, and are also at risk of copper casse.
As well as being undesirable due to its toxicity, copper in wine is also unwanted due to its propensity to cause a fault known as copper casse (from the French noun ‘casse’ meaning flaw). The casse, or haziness, is only noticed in white wines and is caused by high copper levels and related to protein and sulfide levels. The amount of copper required to cause copper casse is quite small – the minimum amount suggested is 0.5mg/L (ppm) (Rankine 1989).
This problem in wine is not very common but if it does occur it can be very difficult to remove. Blue fining using cyanide compounds must be used to remove excess copper. Needless to say, this treatment is not particularly nice to use. A low copper level in the first instance is a much better way to go.
The use of copper sulfate in removing sulfides from wine is done regularly by most winemakers. Copper sulfate is a toxic material and should be treated with caution. The residual amount of copper in wine is tightly regulated and should be checked on a regular basis to ensure that regulatory requirements of importing countries are met. Of the wines tested in our laboratories this year, 11% were above the US regulatory limit of 0.5mg/L.
Jiranek, V; Langridge, P; Henschke, P.A. Applied
Environmental Microbiology, Feb 1995, 461-467
Jiranek, V; Henschke, P.A. Assimilable nitrogen: regulator of hydrogen sulfide production during fermentation. Australian Grapegrower and Winemaker, April 1991, 27-30
MSDS, Ajax Finechem, Taren Point, NSW
Rankine, B; Making Good Wine, 1989, Pan Macmillan
Food Standard 4.5.1, Food Standards Australia New
Article from the Australian and New Zealand Grapegrower and Winemaker, Issue 580, 2012
Page Number(s): 74-76
Author: Greg Howell
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