Is adding copper the solution? (Part 2)

by | Nov 1, 2021 | Oenology research, Winetech Technical

The aim of this Winetech-funded project is to improve the understanding of winemakers of the influence of copper fining on a wine quality.

 

Introduction

Young white wines made from Sauvignon blanc and Chenin blanc grapes can attribute their signature tropical aromas of “guava”, “passion fruit” and “grapefruit” to the high natural presence of varietal thiols.1,2 Winemakers are aware that varietal thiols can decrease in the presence of oxygen and therefore to preserve varietal thiols, the uptake of oxygen is often minimised from grape crushing, through vinification and wine storage.1 These reductive winemaking techniques, if not carried out optimally (e.g., incorrect yeast nutrient management during fermentation), can sometimes result in the production of unwanted reductive off-odours, resembling “boiled-egg” and/or “cabbage”.3 They are the two most common off-odours appearing in reductive wines.

“Boiled-egg” off-odours are derived from the presence of excess hydrogen sulphide (H2S) and “cabbage” off-odours are derived from high levels of methanethiol (MeSH).4 Copper fining is usually done to remove excess reductive off-odours from wines before bottling. The “boiled egg” off-odour is rapidly removed after the copper addition, due to copper binding to H2S rendering it non-volatile. The copper-H2S complex is not completely removed from wine by precipitation or filtering, as it was previously believed, but it stays in wine.5 On the other hand, removing “cabbage” off-odour present in wines after bottling from wine by copper addition, is only partially effective.

Unfortunately, copper present in wines after bottling can also partially remove varietal thiols (e.g., 3-Mercaptohexan-1-ol, shortened 3MH).6 In the previous article, we have reported that in our experiment the addition of copper to Chenin blanc wine, decreased 3MH levels after six weeks, but only slightly decreased the “passion fruit” aroma (when compared to non-treated control wine). This article reports on the chemical and sensorial long-term effects of copper fining on reductive Chenin blanc wines after one year of wine storage.

 

Materials and methods

  • For this experiment, Chenin blanc 2019 from Worcester (South Africa), naturally high in varietal thiol levels was used as a base wine.
  • The wine was sparged with nitrogen to reduce dissolved oxygen (to <1%) and free SO2 was adjusted to 38 – 40 mg/L (the wine had a pH of 3.4 and an alcohol content of 12.73 vol %).
  • Various combinations of H2S, MeSH and copper (in the form of copper-sulphate solution) were added to the base wine, resulting in eight different treatments. The concentrations of reductive compounds used in this experiment were at the high range of the spectrum normally found in wines:4 40 µg H2S/L and 20 µg MeSH/L. The wines were treated with 0.3 mg/L copper.
  • Wines were bottled in a reductive (anoxic) environment, under crown caps, and stored for 24 hours, six weeks and one year at 15°C.
  • After each time point, a sensory analysis with a trained panel and a chemical analysis of the bottled wines were performed.

 

Results

 

Reductive odours

When the one-year wine storage data was compared to the previously reported six-week period, the bound H2S concentration levels were further increased in all the wine samples with added copper. This bound H2S does not contribute to reductive or “boiled egg” off-odours in wines. In the wine samples spiked with H2S, but without added copper, free and bound H2S were present at very low levels. This is most probably due to the wine matrix (e.g., quinones) binding to H2S, which also occurs under the regular winemaking conditions.7

The results indicate that copper takes a longer time to effectively react with MeSH and remove the “cabbage” off-odour from the wine. Our six-week wine storage data showed that copper fining did not result in a big decrease in free MeSH. The action of removing free MeSH with copper has only occurred after six weeks and was confirmed after one year of wine storage. The sensory analysis has confirmed a decrease in “cabbage” off-odour in wine spiked with MeSH followed by the copper treatment. However, even after one year of wine storage, the free MeSH and corresponding “cabbage” off-odour was still present in the wine and only decreased to around 50% of its initial value. Contrary to the H2S results, levels of bound MeSH have increased in all the wine samples after one year of storage, whether the wine was copper fined or not.

 

Varietal thiols

The overall concentrations of 3MH increased and 3MHA decreased after one year of wine storage. This change is due to the already known process of hydrolysis of 3MHA to 3MH occurring during wine ageing. Interestingly, a decrease in 3MH levels was seen in all the samples with added copper (Graph 1). That is a change from the previously reported data after six weeks of wine storage, which showed a 3MH decrease only when copper was added to the wine without reductive compounds present. Therefore, copper addition had prolonged detrimental effects on 3MH, even after one year of wine storage. On the other hand, 3MHA levels were not affected by the copper additions even after one year. However, due to the majority of 3MHA being converted to 3MH during the wine storage, the newly created fraction of 3MH could also be affected by copper fining.

 

 

Sensory results after one year showed a decrease in “guava” aroma in Chenin blanc wine treated with copper (Graph 2). On the other hand, when reductive compounds H2S and MeSH were present in the wine, the decrease was less apparent. Interestingly, the aromas of “peach” and “banana” were elevated in the samples with added copper after one year (Graph 3). Researchers have previously published results that showed when varietal thiols are present in higher concentrations, esters present in wines boost the varietal thiol aromas, while ester aroma perception is decreased.8 However, when esters are present in higher levels, the varietal thiols enhance the perception of esters (e.g., “banana” aroma derived from isoamyl acetate). This might apply here, as the decrease of varietal thiol 3MH concentration by the copper addition could give esters and other aromatic compounds enhanced sensorial perception.

 

 

Conclusion

In Part 1 of this series, we suggested that to preserve varietal thiols, the minimum required amount of copper additions should be made to remove H2S off-odours from wine. After one year of wine storage, we have seen that the wines that contained copper, also went through changes in the aromatic profile compared to non-copper-treated wines. Besides a decrease in levels of varietal thiol 3MH in copper-treated wines, the increase in aromas of “peach” and “banana candy” was noticed. The copper addition altered the aromatic profile of Chenin blanc wine used in this experiment. However, we should not forget that “boiled egg” off-odour derived from excess H2S is far more detrimental to the perceived wine quality than a slightly altered aromatic profile of a wine.

 

Abstract

In Part 1 of this article series, we have shown the results of the short-term impact (after 24 hours and six weeks) of copper fining on a reductive Chenin blanc white wine that was naturally high in varietal thiols. In this second part, we report the long-term effects observed after one year of storage. Using chemical and sensorial wine analysis, researchers from the South African Grape and Wine Research Institute (SAGWRI) (Stellenbosch University) carried out this novel project in collaboration with the researchers from the Australian Wine Research Institute (AWRI) in Adelaide.

 

Contributing researchers: Marlize Bekker and Allie Kulcsar.

 

References

  1. Coetzee, C. & Du Toit, W.J., 2012. A comprehensive review on Sauvignon blanc aroma with a focus on certain positive volatile thiols. Food Research International 45(1): 287 – 298.
  2. Wilson, C., 2017. Chemical evaluation and sensory relevance of thiols in South African Chenin blanc wines.
  3. Kreitman, G.Y., Elias, R.J., Jeffery, D.W. & Sacks, G.L., 2019. Loss and formation of malodorous volatile sulfhydryl compounds during wine storage. Critical Reviews in Food Science and Nutrition 59: 1728 – 1752.
  4. Siebert, T.E., Solomon, M.R., Pollnitz, A.P. & Jeffery, D.W., 2010. Selective determination of volatile sulfur compounds in wine by gas chromatography with sulfur chemiluminescence detection. Journal of Agricultural and Food Chemistry 58(17): 9454 – 9462.
  5. Clark, A.C., Grant-Preece, P., Cleghorn, N. & Scollary, G.R., 2015. Copper(II) addition to white wines containing hydrogen sulfide: Residual copper concentration and activity. Australian Journal of Grape and Wine Research 21(1): 30 – 39.
  6. Ugliano, M., Kwiatkowski, M., Vidal, S. et al., 2011. Evolution of 3-mercaptohexanol, hydrogen sulfide, and methyl mercaptan during bottle storage of Sauvignon blanc wines. Effect of glutathione, copper, oxygen exposure, and closure-derived oxygen. Journal of Agricultural and Food Chemistry 59(6): 2564 – 2572.
  7. Nikolantonaki, M. & Waterhouse, A.L., 2012. A method to quantify quinone reaction rates with wine relevant nucleophiles: A key to the understanding of oxidative loss of varietal thiols. Journal of Agricultural and Food Chemistry 60(34): 8484 – 8491.
  8. King, E.S., Osidacz, P., Curtin, C., Bastian, S.E.P. & Francis, I.L., 2011. Assessing desirable levels of sensory properties in Sauvignon blanc wines – consumer preferences and contribution of key aroma compounds. Australian Journal of Grape and Wine Research 17, 169 – 180.

 

– For more information, contact Matija Lesković at matija@sun.ac.za.

 

Article Archives

Search for more articles

Generic selectors
Exact matches only
Search in title
Search in content

Stay current with our monthly editions

Share This
Shopping cart
There are no products in the cart!
Continue shopping
0