Cu fractions in Shiraz and Pinot Noir wines during bottle aging: Rates of changes and capacity for conversion

The Cu fraction in wine associated with Cu(II)-organic acid complexes can suppress detrimental aromas attributed to hydrogen sulfide and methanethiol. However, the long-term stability of this Cu fraction (known as Cu fraction I) during bottle aging of red wine is not well understood. This study utilised Pinot Noir and Shiraz wines containing 0.43 ± 0.01 and 0.56 ± 0.02 mg/L respectively of total Cu, to which 0, 0.3 or 0.6 mg/L Cu(II) was further added immediately before bottling. The bottles were then stored at 14 °C for 1 yr. Cu fraction I concentrations were measured using two independent methods: i) stripping potentiometry, and ii) ICP-OES analysis of filtrate after diatomaceous earth depth filtration. Within the first 6 months of storage, Cu fraction I was found to decrease in all wines. Using stripping potentiometry, the first-order decay rates were found to be 0.012 ± 0.001 day-1 and 0.010 ± 0.001 day-1 for Pinot Noir and Shiraz respectively, corresponding to half-lives of 55 ± 4 and 67 ± 9 days. The decay rates for Shiraz versus Pinot Noir were similar for wines with different Cu addition rates, or when rates were determined using the different analysis techniques. Both wines had a high capacity for conversion of Cu fraction I to Cu fraction III during the 1-yr storage period, with 0.4-0.8 mg/L Cu forming Cu fraction III in the Pinot Noir, and 0.3-0.6 mg/L in the Shiraz. These conversion capacity ranges are higher than the typical Cu(II) additions made to wine during production. Overall, the results show that red wine has a large capacity for enabling the sulfide-binding of Cu-organic acid complexes during bottle aging and the conversion occurs at a relatively uniform rate with the concentration halving approximately every 2 months.