It can be! Copper instabilities are one of the most common metal instabilities, largely because of the low concentrations of copper required to cause instability. Copper is also a catalyst for oxidation and reduction reactions and again, low concentrations can have an impact. Furthermore, copper reacts with thiols, so it can affect the varietal aromas of wines, especially those varieties where thiols play a major role, such as Sauvignon Blanc and Cabernet Sauvignon.
A portion of the copper present in grape juice originates from the soil, but it is difficult to know how much given that copper-containing fungicides can be used which also contribute to the copper content. In fact, copper-based agrochemicals are generally the major source of copper in grape juices. In the past, contact of juice with unprotected brass or bronze winery fittings might also have been a source of copper.
If no copper-based vineyard sprays are used, then the level of copper in the juice is likely to be less than 0.5 mg/L. If copper-based sprays are used, then the level of copper can vary depending on the number of applications, total dose applied and the time between the last application and harvest. The copper content of the juice could therefore range from less than 1 mg/L to higher than 15 mg/L.
Tromp and de Klerk (1988) found that 10–15 mg/L of copper inhibited fermentation and that the resulting wines were browner than controls due to increased oxidation. Investigations conducted at the AWRI have also shown that must copper concentrations less than 10 mg/L have no effect on the rate of fermentation. Consequently, levels of copper approaching 10 mg/L are considered to be unlikely to affect the rate of fermentation. However, if other inhibitory factors are also present (e.g. agrochemical residues, acetic acid, high ethanol etc), then it is possible that such levels of copper might influence fermentation rate.
Luckily, most of the copper present in juice is removed during fermentation via formation of sulfides and through binding to yeast and removal with the lees. However, steps should be taken to ensure optimal fermentation conditions to minimise the risk of a sluggish or stuck fermentation. This can be done by:
• choosing a robust yeast
• ensuring good yeast preparation (see Cowey 2014)
• measuring yeast assimilable nitrogen (YAN) and adjusting with diammonium phosphate (DAP) if it is too low (i.e. <150 mg/L for whites and <100 mg/L for reds) • using yeast hulls or proprietary inactivated yeast product, • maintaining steady fermentation rate by minimising sharp or large temperature fluctuations, and • keeping yeast in suspension by warming and/or agitation.
Ferments are typically treated with copper sulfate or aerated (red ferments) to remove hydrogen sulfide (H2S) odour. However, if mercaptans are also present, aeration can lead to their oxidation to disulfides.
Using copper during fermentation is generally considered to be ‘safe’ because any residual copper tends to be removed with the yeast lees. However, copper can react with other wine constituents or mediate oxidation reactions that can decrease the intensity of wine aroma. For example, copper can react with the fruity thiols in Sauvignon Blanc and related varieties which can sometimes lead to loss of varietal characters.
Yeast strain is one of the most important factors in limiting H2S production during fermentation. Therefore, it may be beneficial to use a low H2S-producing yeast strain. This may reduce or eliminate the need for copper fining during fermentation.
The depletion of YAN is a common cause of H2S production, especially that released during the early stages of fermentation when yeast growth is active. H2S produced during the early to mid-stages of fermentation can often be ameliorated by the addition of DAP or proprietary fermentation nutrient preparations that contain nitrogen. However, DAP additions are often ineffective against H2S produced during the late stages of fermentation. In this situation, it might be best to perform a copper trial towards the end of fermentation when most of the sugar has been utilised, but when there is also a significant number of yeast cells still present to bind any excess copper.
The AWRI has always advised against adding copper just before bottling if a wine does not actually exhibit any reductive character. This advice was initially based on the resulting increased risk of post-bottling copper haze formation and due to the fact that copper is a catalyst for oxidation reactions. However, there is also another reason to avoid this practice, as Ugliano et al. (2011) and Viviers et al. (2013, 2014) reported increased accumulation of H2S and methane thiol (methyl mercaptans) during bottle ageing of wines when copper sulfate was added at bottling.
Cowey, G. 2014. Ask the AWRI: Top tips for a successful yeast culture. Aust. N.Z. Grapegrower Winemaker 600: p. 42.
Tromp, A., de Klerk, C.A. 1988. Effect of copperoxychloride on the fermentation of must and wine quality. S.A. J. Enol. Vitic. 9: 31–36.
Ugliano, M., Kwiatkowski, M., Vidal, S., Capone, D., Siebert, T., Dieval, J.B. Aagaard, O., Waters, E.J. 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. J. Agric. Food Chem. 59(6): 2564–2572.
Viviers, M. 2014. Effects of metals on the evolution of volatile sulfur compounds in wine during bottle storage. Aust. N.Z. Grapegrower Winemaker. 600: 49–51.
Viviers, M. Z., Smith, M. E., Wilkes, E., Smith, P. 2013. Effects of Five Metals on the Evolution of Hydrogen Sulfide, Methanethiol, and Dimethyl Sulfide during Anaerobic Storage of Chardonnay and Shiraz Wines. J. Agric. Food Chem. 61(50): 12385–12396.