Production Method and Wine Style Influence Metal Profiles in Sparkling Wines

Discussion

Overall, the metal levels detected in our samples were comparable to those reported in the existing literature on sparkling wine (Table 1). Notably, our maximum concentrations of Al, As, Fe, Mn, and Zn exceeded previously reported maximum levels. However, only single wines in our analysis showed higher mean levels of Al, Fe, and Mn. For As, concentrations were approximately three-fold higher than the reported literature values in 48% of our Niagara sparkling wines. In comparison, mean Zn concentrations were higher than the literature values in 52% of our evaluated wines, with the maximum levels approximately four-fold higher than previously reported.

Conversely, maximum Cd concentrations in our Niagara sparkling wines were ~30-fold lower than previously reported maximum concentrations in the literature. Cd content is primarily related to fertilizer use, as prolonged or intensive application of high Cd fertilizer leads to substantial accumulation of Cd in the soil that is subsequently challenging to remediate (Reilly 1980a). This may indicate that the comparatively young Niagara Peninsula viticultural area has lower Cd accumulation than France and Spain, where considerably higher Cd levels were reported in sparkling wine (Jos et al. 2004a). Further assessment is necessary to validate this hypothesis. In addition, elevated levels of heavy metals including Cd, As, and Pb have been linked to the use of diatomaceous earth as a filtration aid during wine and beer production (Redan et al. 2019).

Our reported Fe, K, and Na levels for Niagara wines appear more similar to those of sparkling wines produced in the Southern hemisphere (e.g., Brazil and Argentina) than to those of European origin (e.g., France and Spain) (Table 1). The reason for this association is unclear.

It is of note that single bottle analysis does not capture bottle variation, particularly in TM sparkling wines, in which individual secondary fermentation is carried out in each bottle. Although single-bottle analyses have been used in previous studies of wine metal composition (Cabrera-Vique et al. 1997, Teissedre et al. 1998, Jos et al. 2004b, Paustenbach et al. 2016, Gajek et al. 2021), this source of variation warrants further research.

PCA. PCA included the metals found in all of the wines—namely B, Ca, Mg, Mn, K, Na, Sr, and Zn (Figure 4)—with samples identified according to production method (Figure 4A) and wine style (Figure 4B). Of the metal ions for which significant differences were observed between mean values for production method and wine style, only B, Sr, and K were included in the PCA due to >LOD values in all wines. Higher mean B levels were identified in TM wines than in CM wines, which is in agreement with the biplot for production method (Figure 4A). The B vector loading in the negative quadrant of PC1 and the positive quadrant of PC2 appear to be strongly associated with many TM wines but no CM wines. Higher mean Sr levels were identified in CM wines compared to TM wines, and the Sr vector loading in the positive quadrant of PC1 and the negative quadrant of PC2 appears to be strongly associated with several CM sparkling wines but few TM wines.

The PCA biplot of sparkling wine production style (Figure 4B) shows that many rosé wines are isolated to the positive half of PC2, with two exceptions. Interestingly, only one rosé wine appears to be negatively related to K content, which agrees with our finding of higher mean K concentrations in rosé wines compared to non-rosé sparkling wines. Although our wines were collected from a relatively small geographic area of the Niagara Peninsula, elemental composition may also be affected to varying degrees by vineyard origins and the specific winemaking facility (Hopfer et al. 2015).

Elemental composition by production method. Differences in certain elemental concentrations were observed between CM and TM products. Higher mean levels of Cr, Ni, and Sr were identified in CM wines (0.021 ± 0.008 mg/L, 0.018 ± 0.004 mg/L, and 0.32 ± 0.07 mg/L, respectively) compared to TM wines (0.015 ± 0.004, 0.016 ± 0.011, and 0.24 ± 0.09, respectively). Additionally, higher mean B concentrations were observed in TM wines (3.4 ± 0.8 mg/L) compared to CM wines (3.0 ± 0.6 mg/L). Although Cr and Ni may be partially derived from the vineyard soil, they are also used in the production of stainless steel as an electroplated coating to resist oxidative damage (Reilly 1980b). Cr is extracted into foods and beverages during manufacturing in a pH-dependent manner, with increased extraction under acidic conditions (Reilly 1980b). Due to the low pH (3.0 to 3.2) of sparkling wines, transfer of Cr from stainless steel to wine during the prolonged tank contact in CM production is possible. Furthermore, a study by Cabrera-Vique et al. (1997) demonstrated that Cr content increased with bottle age for various vintages of red wine produced by the same process, vineyard, and winery. The authors suggested that this was due to the extraction of Cr from stainless steel during production, as well as leaching of chromium oxides from glass pigments in the bottle during aging. In a study of metal content in still wines from Greece, Cr and Ni had a low positive correlation (Spearman ρ = 0.421; p < 0.05), which was suggested to be associated with stainless steel fermentation vessels used in production (Skendi et al. 2020). Based on our results, higher mean levels of Cr (p < 0.05) and Ni (p < 0.01) in tank-fermented CM wines could potentially be linked to stainless steel processing equipment used for the second fermentation and could therefore be useful indicators for authenticating sparkling wine production methods. Further research is required to establish Cr composition in relation to the tank fermentation method.

Other identified elements with significant mean differences between production methods were B and Sr, which are primarily derived from the vineyard environment, where they are transported into the plant via the root system during growth. Grapevines (genus Vitis) are perennial plants and uptake a significant amount of nutrients between the energy-intensive cycles of bloom and veraison (berry ripening) (Moyer et al. 2018). B is essential for vine growth. However, its relationship to higher content with the TM production method (p < 0.05) remains unclear and may instead be an artifact of the relatively small Niagara Peninsula growing region, which has ~17,000 acres of land under vine (Vintners Quality Alliance Ontario 2020).

Similarly, Sr shows no clear link to production method, and its higher levels in CM wines require further investigation. Heavy metals such as Sr have been used to authenticate wines due to their natural occurrence in soil, air, and water sources. In a multielement analysis of Canadian wines, Sr content was highly effective in discriminating between geographic origins for Canada’s two major wine-producing regions, the Niagara Peninsula and the Okanagan Valley (Taylor et al. 2003). In another study, Sr levels varied significantly between grape varieties (Gajek et al. 2021). Although Sr levels may vary during winemaking, the stable ⁸⁷Sr/⁸⁶Sr isotopic ratio remains unchanged from the vineyard environment through to the finished wine. Thus, the ratio can be used alone or in combination with other heavy metal isotopic information for geographical traceability (Bora et al. 2018). Future research into the Sr isotopic ratios of CM and TM wines may elucidate possible contaminants in the winemaking process and provide information about the relationship between Sr content in finished wine and in vineyard soil.

Elemental composition by wine style. The rosé-style sparkling wines included in our analysis contained higher mean levels of K (613 ± 153 mg/L) compared to non-rosé wines (466 ± 158 mg/L), which is presumed to be a result of maceration in rosé wine production (p < 0.01). K is the most abundant element in wine and is considered a major metal, along with Ca, Na, and Mg (Pohl 2007). K is concentrated in grape skins and seeds, with a lesser amount localized in the pulp (Pérez Cid et al. 2019), and has been shown to increase in final wines with prolonged maceration during skin contact in white wine production (Darias-Martín et al. 2000). In red still wines, K has been identified at higher levels than in white still wines (Mitić et al. 2014). Therefore, the observed increase of K in rosé sparkling wines is likely due to the maceration of grape skins and seeds with juice prior to fermentation. The origin of K is primarily soil derived, although fertilizers rich in K may also affect these levels (Mpelasoka et al. 2003). The impact of high K levels on organoleptic qualities of wine is principally associated with potassium bitartrate precipitation, leading to decreased tartaric acid and titratable acidity (g/L) levels and increased pH in wine (Ough et al. 1969, Ferreira et al. 1995).

The Spearman matrix for rosé-style wines (Supplemental Table 8) indicated that K is moderately correlated with both Ca (ρ = 0.541; p < 0.05) and Mn (ρ = 0.559; p < 0.05). Although K is found at several-fold higher concentrations than Ca or Mn, they are all components of grape berries. Thus, maceration in rosé sparkling wine production may also extract Ca and Mn to a lesser extent. Notably, high K levels may also be partly attributed to nonproduction factors, including rootstock, grape variety, soil composition, canopy management, and irrigation (Mpelasoka et al. 2003).

Additionally, Cu was significantly higher ( p < 0.05) in non-rosé sparkling wines (0.080 ± 0.091 mg/L) compared to rosé samples (0.034 ± 0.036 mg/L). However, there is no clear association between Cu levels and winemaking processes for this style. The origins of Cu are primarily associated with various viticultural aspects, including environmental pollution and the use of Cu-based vineyard sprays as pesticides or fungicides (Wilkes 2018). In wine, residual Cu has been implicated in oxidative and reductive spoilage, protein instability, and the development of colloidal haze, while also inhibiting microorganisms and thereby impacting wine fermentation (Clark et al. 2015, Claus 2020). During fermentation, Cu is used as a yeast micronutrient, but its levels decrease substantially postfermentation, when it is reduced to insoluble sulfides and precipitated with flocculating yeast cells (Tariba 2011). However, Cu content may increase as a result of added copper sulfate, which is commonly used as a treatment to remove hydrogen sulfide-related reductive off-flavors in wine (Pohl 2007, Fabjanowicz and Płotka-Wasylka 2021). The concentration of Cu has also been reported to be higher in grape skins than in pomace, but this does not explain the higher levels measured in non-rosé style sparkling wines.

Although the reason for higher Cu levels in non-rosé compared to rosé style sparkling wines remains unclear, it is notable that Cu(II) ions can catalyze the oxidation of polyphenols through the formation of reactive oxygen species, thereby leading to premature browning, as well as astringent and metallic tastes (Pohl 2007). The threshold for Cu ions impacting wine sensory characteristics has been reported at 0.5 mg/L (Morozova et al. 2014), and all detected values in the Niagara sparkling wines were below this level. Nonetheless, it is important to note that Cu, particularly in tandem with Fe, plays an important role in wine oxidation and browning reactions, with direct ties to flavor development. According to a study by Morozova et al. (2014), Riesling still wines with 0.5 mg/L Cu and 1 mg/L Fe showed enhanced fruity, fresh, tropical, and citrus aroma qualities under low oxygen (<1.2 mg/L) bottling conditions (Morozova et al. 2014). However, these metal levels in elevated oxygen conditions reportedly led to undesirable and oxidized aromas, presumably due to Cu- and Fe-catalyzed radical reactions (Morozova et al. 2014). Moreover, wine browning reactions can occur via enzymatic or nonenzymatic pathways, and Cu and Fe are also implicated in initiating these processes (Li et al. 2008). Although enzymatic browning occurs primarily in the grape must during processing, nonenzymatic browning reactions can take place at any stage during the wine’s lifetime, including during aging (Li et al. 2008).

Interestingly, in model systems, various metal ions have been shown to accelerate a specific subset of nonenzymatic browning activity called the Maillard reaction, which involves the condensation of amino acids and reducing sugars (Hyase et al. 1996, Rizzi 2008, Omari et al. 2021). Compounds associated with the Maillard reaction have been identified in aged sparkling wine, likely due to the abundance of precursors in this matrix, and contribute desirable roasted and toasted aromatic qualities (Keim et al. 2002, Marchand et al. 2011, Le Menn et al. 2017). There is currently no existing literature on the effects of metal composition on the Maillard reaction in sparkling wine. Therefore, the role of metals in the development of desirable aroma compounds during the aging of sparkling wine remains poorly understood.