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88 p r a c t i c a l w i n e r y & v i n e ya r d M ay 2 0 1 4 w i n e M a K i n g Copper still important The influence of the concentration of cop- per and other metals in wine on H 2 S pro- duction was reinforced in another recent AWRI trial where different levels of cop- per (0, 0.2, 0.5, 0.75 and 1.5 mg/L) were added to a Chardonnay (both bentonite- fined and unfined) that was bottled under commercial conditions. After six months, the wines with a 0.5 mg/L or greater copper addition had sig- nificantly higher H 2 S concentrations than the lower dose wines (Figure 2). The dos- ing relationship was especially apparent for the wine that had not been bentonite- fined for protein stability. The difference in results for fined and unfined wines led to further investiga- tion of these wines. which revealed that they had significantly different iron lev- els (0.2 mg/L for the unfined wine and 0.5 mg/L for the bentonite-fined wine). The extra iron was added through the bentonite-fining process. When the levels of H 2 S formed after six months were plotted against the copper:iron ratio for each dose rate, a significant linear trend was apparent for copper additions below 1.5 mg/L (Fig- ure 3). This clearly shows the impor- tance of the copper:iron ratio in the formation of H 2 S. The non-linear results seen at the highest dose rate suggest that once this high level of copper was reached the chemistry followed a differ- ent pathway. It is not just "academic" These effects are not just limited to aca- demic trials. The metal concentrations in 144 wines rejected from a 2011 interna- tional wine show because of reduced char- acters were analysed and compared to the metal concentrations of another 514 wines that were rejected for other faults unlikely to be influenced by metal content. Comparing the concentrations of met- als for wines that were shown to have above threshold levels for MeSH (more than 1µg/L) to those that did not, it was apparent that higher levels of iron and magnesium and lower levels of aluminum were associated with the reduced wines compared to the non-reduced wines. This study did not show any impact of copper, probably because the majority of wines tested in this study had a relatively low copper concentration (median less than 0.1 mg/L for both reduced and non- reduced groups). Metal effects can be reversible In the large multi-metals study, some metal treatments were initially associated with decreases in volatile sulfur com- pound concentrations, but these effects were shown to reverse after four months of anaerobic storage (Figure 4). At day 1, when samples were analyzed directly after metal spiking, when DO concentrations were high (0.15 – 1.50 mg/L), the three metal treatments (cop- per, iron and Cu*Fe) decreased the H 2 S concentration compared with the control. After 12 months of anaerobic storage, the samples treated with copper and with Cu*Fe displayed significant increases in H 2 S concentration. In the Chardonnay and Shiraz samples, the same reversible effect was observed for MeSH as was seen for H 2 S. At day 1, no MeSH was measurable in Shiraz sam- ples with or without added copper, but after one month of anaerobic storage the samples not treated with copper showed increases in MeSH compared to the sam- ples treated with copper. However, as the wine consumed all available oxygen, the MeSH concentra- tion in the copper-treated wines slowly increased. After 12 months of anaerobic storage the MeSH concentration in all samples with added copper had reached an average of 6.39 µg/L, substantially higher than its odor threshold value of 1.8-3.1 µg/L. 3 These results show that the formation of MeSH is not only influenced by the presence of metals, but that the oxygen concentration in wine also signif- icantly affects its evolution. Be conscious of what you add to your wine Winemakers should pay attention to this observation of reactions reversing over time, particularly when consider- i ng rout i ne add it ion s of copper to remove unwanted sulfur aromas from wine. Copper additions are effective if made early in a wine's life, preferably around the end of fermentation, when yeast cells are still available to help "mop up" resid- ual metals. If, however, copper is added later, significant amounts can remain in the final wine, greatly increasing the risk of developing reductive aroma com- pounds in the bottle. This study has demonstrated the importance of keeping metal concentra- tions as low as possible in wine, as the metals can act singly or in combination to greatly influence evolution of unwel- come reductive aromas. PWV This text was edited from original publica- tion in the January/February 2014 Wine & Viticulture Journal, with permission of the publisher, winetitles. Acknowledgements This research was facilitated using infra- structure provided by the Australian gov- ernment through the National Collaborative Research Infrastructure Strategy (NCRIS). The Australian Wine Research Institute (AWRI) is a member of the Wine Innovation Cluster in Adelaide, South Australia. The work was supported by Australia's grape- growers and winemakers through their investment body, the Grape and Wine Research and Development Corporation, with matching funds from the Australian government. Ella Robinson is thanked for her editorial assistance. Bibliography 1. Lachner, R., and G. 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