Hydrogen sulfide and methanethiol are recognised as two of the most significant contributors to reductive off-flavours in wine. Cu(II) in wine is known to interact with both sulfhydryl compounds, lowering the concentration of their aroma-active forms while transitioning Cu(II) to a sulfhydryl-bound form. Both hydrogen sulfide and methanethiol can form during the aging of wine in low oxygen conditions, such as during bottle aging, and
their production is known to be accelerated by wine storage temperature. Consequently, the protection offered by Cu(II) to inhibit accumulation of the reductive aroma compounds during bottle aging will be limited by the Cu(II) concentration at the bottling and of rate sulfhydryl compound formation. Although insights have been made on the typical rates of binding of Cu(II) in wines in cellar conditions (i.e., 14 °C), the impact of elevated storage
conditions is not certain, but likely to influence the duration of time that Cu(II) can inhibit reductive wine characters. This study determines the rates of Cu binding during the storage of wine at variable temperatures.
Four red and three white wines were bottled with low (< 0.2 mg/L) and high (0.6 mg/L) Cu(II) concentrations. The wines were stored at 14 °C and 40 °C and measured at 0, 1, 3, 7 and 12
months, and 0, 3, 5 and 12 days, respectively. The different forms of Cu were quantified by colorimetry for white wine, and stripping potentiometry for red wine, and enabled calculation of Cu(II) binding rates during wine aging and estimation of activation energies for binding. The formation of free and salt-releasable hydrogen sulfide and methanethiol were determined by gas chromatography with chemiluminescence detection.
The results showed that the rate of Cu(II) binding was dramatically higher at 40 °C than 14°C, with complete binding of Cu(II) in the order of 3 – 8 months and 1 – 5 days, respectively, for the wines bottled with high Cu(II). The relative order of Cu(II) binding rates amongst wines was temperature dependent, whereby Cu(II) binding rates became more uniform across wines when stored at the higher temperatures. This implied limitations in utilising high storage temperatures to predict Cu(II) binding at lower storage temperatures. The accumulation of the aroma active forms of sulfhydryl compounds, that is, the free hydrogen
sulfide and free methanethiol, only occurred after Cu(II) was converted to its bound form and then also required a further lag time. These results provide critical insights into the time-line of protection afforded to wines by Cu(II) against the emergence of reductive characters during bottle aging.
Authors: Zhang Xinyi1, Langford Kylie2 and Clark Andrew C1
1Gulbali Institute, Charles Sturt University, School of Agricultural, Environmental and Veterinary Sciences, Charles Sturt University, Wagga Wagga, NSW 2678, Australia
2Treasury Wine Estates, 97 Sturt Highway, Nuriootpa, SA 5355, Australia
*corresponding author: xizhang@csu.edu.au
keywords: Copper, reduction, hydrogen sulfide, methanethiol, bottle-aging
