## 3 Results and Discussion
### Evolution of the Concentrations of Cu and SO\({}_{2}\) during Wine Production
The SO\({}_{2}\) treatment to the juice significantly (\(p\leq 0.05\)) decreased the concentrations of MeSH in Chardonnay after fermentation (PAF), but this influence was not detected in any measurements after cold stabilization (PCold for Chardonnay, Figure 1C). In contrast, in Shiraz, the SO\({}_{2}\) treatment did not display any significant (\(p\leq 0.05\)) influence until 15 Mo, with increased MeSH concentrations in both of the two SO\({}_{2}\) treated wine samples at 15 Mo (HSO2 and LSO2 treatments, Figure 1C,D). The presence of increased concentrations of SO\({}_{2}\) in the Shiraz must may have led to increased precursors for MeSH, such as methionine, which may have been degraded during bottle adding to produce MeSH [55]. Impact of Cu or SO\({}_{2}\) Addition to Juice/Must on Total Concentrations of Volatile Aldehyde Compounds in Wine  
The addition of Cu or SO\({}_{2}\) to grape juice/must influenced several of the measured volatile aldehyde compounds in the finished and bottle aged wines (Table S2), including certain Strecker aldehydes (e.g., 3-methylbutanal, phenylacetaldehyde, and methional) and hexanal. The effects of the added Cu or SO\({}_{2}\) on these aldehyde compounds were different for the Chardonnay and Shiraz wines. Most aldehyde compounds showed increasing concentrations in the Chardonnay wines throughout the wine production process (from PAF to PB), while a negligible change or a decrease occurred in the Shiraz wines (Table S2). Examples included 2-methylpropanal, 3-methylbutanal, phenylacetaldehyde, hexanal, benzaldehyde, and methional. [PERSON] et al. [56] reported a negative relationship between the accumulation of certain volatile aldehyde compounds in bottle aged red wines with the concentration of aldehyde-reactive phenolic compounds. The latter phenolic compounds included anthocyanins and procyanidins. The different aldehyde concentration changes observed for white and red wines were most likely a consequence of the lower aldehyde-reactive phenolic concentration and higher free SO\({}_{2}\) concentration in the white wine compared to red wine. This would allow aldehyde compounds to accumulate more in white, albeit in their hydrogen sulfite-bound form. Although the SO\({}_{2}\) concentrations in the Chardonnay and Shiraz wines were adjusted to similar concentrations after fermentation (Section 2.2), the free SO\({}_{2}\) measured in red wines includes the portion that is actually bound to anthocyanins. Cu and SO\({}_{2}\) additions to the grape must, regardless of the magnitude of addition, increased the concentration of hexanal after fermentation in Shiraz wines (Figure 2B), while no significant (\(p\leq 0.05\)) effect was observed in Chardonnay wines (Figure 2A). Somewhat surprisingly, in the Shiraz wines, the influence of Cu was more significant than SO\({}_{2}\), despite SO\({}_{2}\) having the ability to act as a binding agent for aldehyde compounds. However, it should be noted that the higher SO\({}_{2}\) concentration in the juice/must may also stimulate the production of acetaldehyde by _S. cerevisiae_[57]. The generated acetaldehyde would bind SO\({}_{2}\) more strongly than hexanal and hence limit the efficacy of SO\({}_{2}\) to trap hexanal. If not bound to SO\({}_{2}\), the hexanal present during fermentation may be more readily metabolized by yeast (e.g., reduced to hexenol) or react with flavonoid compounds, and thereby decrease in concentration. The role of Cu in increasing hexanal concentration is unclear. It is possible that Cu could influence the oxidative processes (i.e., chemical oxidation of lipids), particularly if oxygen concentrations increased toward the end of fermentation. However, although the effects of Cu and SO\({}_{2}\) treatments were evident on hexanal concentrations in Shiraz wines at PAF, these differences had largely disappeared by bottling (PB).