## 3 Results and Discussion
### Evolution of the Concentrations of Cu and SO\({}_{2}\) during Wine Production
As bentonite removes proteins, which may act as a potential source for sulfide in wine, this may explain why significantly (\(p\leq 0.05\)) lower H\({}_{2}\)S concentrations were measured in several bentonite-treated wines after 15 months of bottle aging. Additionally, the elevated Fe concentration (Table S2) in bentonite-treated wines [35; 42] may have accelerated wine oxidation, and contributed to an increased oxidative loss of H\({}_{2}\)S. Apart from Cu treatment, SO\({}_{2}\) addition to the grape juice did not significantly (\(p\leq 0.05\)) alter the H\({}_{2}\)S concentration in Chardonnay (Figure 1A). The concentrations of H\({}_{2}\)S in the Shiraz wines were predominantly higher than the equivalent treatments for the Chardonnay wines, which may have been induced by a lower yeast assimilable nitrogen (YAN) concentration in the Shiraz must (180 mg/L) compared to the Chardonnay juice (340 mg/L) (Figure 1B). Additionally, in contrast to the trends observed in Chardonnay wines, the  
Figure 1: Concentrations of low molecular weight sulfur compounds (LMWSCs) in Chardonnay and Shiraz wines during production and bottle aging process. The X-axis of the figure represented sampling time points: J/M, juice/must; PAF, post-alcoholic fermentation; PCold, post-cold stabilization; PB, pre-bottling; 15 Mo, 15 months after bottle aging. (**A**) H\({}_{2}\)S in Chardonnay wines; (**B**) H\({}_{2}\)S in Shiraz wines; (**C**) methanethiol (MeSH) in Chardonnay wines; and (**D**) MeSH in Shiraz wines. Note that different scales were used on the concentration axes of A–D to aid the clarity of differentiating the sample treatments within each graph. Shiraz wines at PAF had a progressively lower H\({}_{2}\)S concentration in the samples with increasing Cu addition to the must (Figure 1B). However, the measured bound Cu showed no decrease with Cu treatments from post-fermentation (PAF) to 15 months of aging (15 Mo) (Table 2). Hence, the apparent decrease in H\({}_{2}\)S concentration may have been due to an incomplete dissociation of the copper sulfide complexes during quantification [48]. Again, in contrast to the Chardonnay wines, the elevated SO\({}_{2}\) concentration in the grape must significantly (\(p\leq 0.05\)) increased the H\({}_{2}\)S concentration in the Shiraz wines (Figure 1B). SO\({}_{2}\) is a potential source of sulfur for the yeast during fermentation and can contribute to the H\({}_{2}\)S production [22], especially when nitrogen availability is low [17]. The Shiraz juice YAN of 180 mg/L is at the lower level of the suggested YAN concentration range for fermentation [53]. Therefore, in combination with elevated SO\({}_{2}\), it may have facilitated the accumulation of H\({}_{2}\)S in the Shiraz wines. Cu addition to the grape juice/must lowered the concentrations of MeSH after fermentation (PAF) in both wine matrices (Figure 1C,D). Similar to H\({}_{2}\)S, the concentrations of MeSH decreased significantly (\(p\leq 0.05\)) after the cold stabilization in both Chardonnay (PCold) and Shiraz (PB) wines, and any differences that were previously evident were largely eliminated by PB (Figure 1C). This decrease in concentration was most likely caused by the oxygen exposure to the wines during production [52]. Bentonite treatment had little influence on MeSH concentration. After 15 months of bottle aging, the influence of juice/must Cu treatments to lower MeSH concentrations was evident and more prevalent for Shiraz than Chardonnay wines (Figure 1C,D). The relationship of increased Cu concentrations with lower MeSH concentrations during bottle aging has been previously described in wine [54]. Consequently, the presence of Cu in the juice/must created long-term MeSH concentration differences in both Chardonnay and Shiraz wines, despite this effect being not evident for Shiraz wine at bottling.