Role of trace metals in wine 'reduction'

(Wines&Vines) - Copper salts have traditionally been added to wines to remove unpleasant 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 minimize the risk of reductive aromas by managing when to make any copper additions and taking steps to minimize metal concentrations.

Winemakers are very familiar with adding copper sulfate to wines that show rotten egg or other reductive characters 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-containing pesticides.

One of the important factors that influences their production is the amount of oxygen a wine is exposed to post-bottling, with wines exposed to very low levels of oxygen more likely to develop reductive aromas.