The chemistry of copper in wine is both complex and fascinating. Fascinating to me perhaps because of the many years that I have spent working with the measurement of copper and trying to relate this to its role as a mediator of chemical reactions. When I first started this research, the question was copper’s role as a mediator of oxidation. Winemakers were adding, and many still do add, copper(II) sulfate to remove any hydrogen sulfide off-odour. Internal winery communication did lead to an intriguing mistake in one case. The winemaker asked the cellar hand to add 2 mg/L ‘copper’, or ‘2 parts’ in wine industry jargon, to the wine. As the mass ratio of copper to copper sulfate is 1:4, the winemaker meant ‘add 0.5 parts Cu as 2 parts CuSO4’. Of course, the cellar hand assumed that ‘2 parts Cu was 8 parts CuSO4’ resulting in a considerable excess of residual copper. Fortunately, yeast cells are effective bioremediators of metals, including copper, so most of the excess copper could be removed in this way.
It is now apparent that copper may also mediate the development of reductive processes by leading to an increase in H2S. Trying to interpret the onset of either oxidative or reductive processes using the total copper concentration has not been helpful. Critical values of total copper between 0.2 and 0.6 mg/L have been proposed as the reported threshold concentration for the onset of wine spoilage. Copper speciation is potentially a better way forward.
Two general strategies for speciation have been examined. One utilises the electrochemical technique of stripping potentiometry (SP) and the other uses a solid phase fractionation method. Much of this work has been carried out by Dr Andrew Clark and Dr Nikos Kontoudakis and colleagues at the National Wine and Grape Industry Centre (NWGIC) at Wagga Wagga in association with the Chemistry Group at the Australian Wine Research Institute. Andrew in his PhD thesis used SP for measuring the total copper concentration in wine along with some initial studies of copper speciation. He became so ‘infected’ with the technique that he has continued using it with marked success in developing meaningful speciation measurements.
The initial strategy was to use SP in combination with medium exchange (ME); that is, an enrichment or electrodeposition step in wine and then a manual transfer of the electrode still under potentiostatic control to a clean,
non-wine solution for stripping using a constant current. The time for re-oxidising the deposited copper is proportional to concentration. The fraction of copper that was deposited on the electrode and then stripped is the ‘labile’ fraction, with the difference between the total copper concentration and the labile fraction commonly referred to as the non-labile fraction. The labile fraction consists of copper ions plus weakly bound copper that dissociates during the electrodeposition step, while the non-labile fraction refers to ‘tightly bound’ copper.
The NWGIC group subsequently modified the manual ME strategy by using a flow cell. Here, the stripping solution is pumped over the electrode surface during stripping. As the process can be automated, it is more efficient than the classic batch method of physically transferring the electrode from one solution to another.
The surprising outcome of the flow cell ME procedure was that the labile concentration was generally lower than assessed by the batch method (Talanta 2016, vol. 154, pp. 431–7). This led to a study of potential binding agents for copper that could influence the labile/non-labile ratio. Compounds used included polysaccharides, tannins and proteins previously extracted from wine as well as catechin-type compounds and sulfur compounds that are known to exist in wine. Perhaps not surprisingly, hydrogen sulfide was the only additive found to decrease significantly the labile fraction (Food Res. Int. 2017, vol. 98, pp. 95–102). A solid phase extraction technique showed that the hydrophobic fraction was specific for sulfide-bound copper (Food Chem. 2019, vol. 274, pp. 89–99) and this correlates with the non-labile form assessed by SP.
The SP labile/non-labile measurement gives detailed insight into the copper activity. In the Talanata study, the authors found that of the 52 wines examined, only three had labile concentrations above 0.015 mg/L, the practical limit of quantitation, with the average as a proportion of the total being 11%. This is considerably lower than the 53% labile found in an earlier study using the manual batch method.
When a wine sample was left exposed to air, the labile concentration actually increased to the extent that after two months, the measured labile concentration equalled the total. This would explain in part why the batch study, with its agitation due to stirring with the sample exposed to air, gave higher labile concentrations. When sulfide was added to the oxidised wine, the labile concentration was found to decrease.
There is clearly a set of complex reactions/equilibria occurring in wine that lead to the loss of sulfide via binding to copper and then its release over time. By providing insight into the changes occurring in the non-labile/labile distribution, the flow-based SP methodology will allow an assessment of the rates of release and binding of copper to sulfide. All part of the on-going story.