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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 85 w i n e M a K i n g W inemakers are very familiar with adding copper sulfate to wines that show rotten egg or other reductive char- acters when in tank. Many have also seen these reductive characters disappear from their wines in the short term, only to have them return at a later date, sometimes after bottling. By exploring the chemistry of volatile sulfur compound formation and the important role played by metals, these common winemaking observations can be better understood, potentially leading to recommendations of ways to reduce the risk of unwanted reductive aromas. Where do reductive aroma compounds come from? Volatile sulfur compounds responsible for reductive aromas in wine are mainly derived from yeast metabolism. They can also form via the degradation of sulfur- containing amino acids and sulfur-con- taining pesticides. One of the important factors that influ- ences their production is the amount of oxygen a wine is exposed to post-bot- tling, with wines exposed to very low levels of oxygen more likely to develop reductive aromas. 4 Recent reports have shown that H 2 S, MeSH and DMS concentrations can increase in wine post-bottling and that lower post-bottling oxygen exposure results in greater increases. 2,5,6 Metal ions are naturally present in grapes and wine and, in trace amounts, metals are important in fermentation. They can also be introduced into wine by human activity, both through direct addition and as a byproduct of other winery processes. Metals such as tungsten (W), zinc (Zn), copper (Cu), cobalt (Co), iron (Fe), nickel (Ni) and manganese (Mn) all have the ability to catalyze oxidation and reduc- tion reactions, but of these only Fe, Zn, Cu and Mn are likely to be present in wine at concentrations high enough to have a significant effect. 1 Aluminium (Al) has also been shown to be important in limiting oxygen consumption in wine. 7 Several of these metals have been associ- ated with undesirable effects in wine. A multi-metal experiment To investigate the effects of metals on the formation of the reduced aroma com- pounds MeSH, H 2 S and DMS during bottle aging, a large experiment was designed where five metals (Cu, Fe, Mn, Zn and Al) were added to Chardonnay and Shiraz wine samples singly and in all possible combinations (31 metal treat- ments, one control). 8 The metals were present at two levels: a low level equivalent to the concentra- tion of the metals already present in the base wine, and a high level that was spiked to approximately 10 times the concentration of the metals measured in the base wine. Role of trace metals in wine 'reduction' Marlize Viviers, Mark Smith, Eric Wilkes, Paul Smith and Dan Johnson, The Australian Wine Research Institute, Glen Osmond, SA, Australia BY AT A GLANCE • Reduc ve aromas in wine are caused by vola le sulfur compounds including hydrogen sulfide (H 2 S), methanethiol (MeSH) and dimethyl sulfide (DMS). • H 2 S is described as having a ro en egg odor, MeSH as rubber or natural gas and DMS in high concentra on as canned corn or vegetal aroma. • Metal ions have been shown to affect the forma on and release of these un- pleasant aroma compounds in wine. • Some reac ons of metal ions with vola le sulfur compounds are reversible – with metal ions ini ally decreasing their concentra on but later resul ng in high concentra on. • To reduce the risk of reduc ve aromas, grapegrowers and winemakers should aim to minimize the metal concentra ons in grapes and wine. • Copper addi ons are most effec ve if made near the end of fermenta on, when yeast cells are s ll available to remove residual copper ions. Copper salts have traditionally been added to wines to remove unpleas- ant volatile sulfur aromas. However, investigations of interactions between metals and volatile sulfur compounds are now revealing that metals can also promote the formation and release of these unwanted aroma compounds, particularly in low-oxygen storage environments. Winemakers can mini- mize the risk of reductive aromas by managing when to make any copper additions and taking steps to minimize metal concentrations. 12.0 10.0 8.0 6.0 4.0 2.0 0.0 Time Points H 2 S – Chardonnay D1 M1 M10 M12 Odor threshold 1.1 – 1.6 µg/L µg/L Control Zn Mn*Zn*Al Cu*Fe*Mn*Zn 12.0 10.0 8.0 6.0 4.0 2.0 0.0 Time Points MeSH – Shiraz D1 M1 M4 M6 M12 Odor threshold 1.8 –3.1 µg/L µg/L Control Cu*Mn*Zn Cu*Zn*Al Cu Figure 1: Line graphs show three of the metal additions that were associated with the largest increase in H 2 S concentration at month 12 for Chardonnay samples (left), and three of the metal additions associated with the largest increase in MeSH concentration at month 12 for Shiraz samples (right).