# Free Monoterpene Isomer Profiles of _Vitis Vinifera_ L. cv. White Wines
## 4 Discussion  
Monoterpene concentrations varied substantially between varietal wines. The total concentration of monoterpenes in Muscat and Torrontes wines were about four times greater than in Gewurztraminer wines, six times greater than in Riesling and Viognier wines, and over 30 times greater than in Chardonnay and Pinot gris wines (Table 2). In addition, limonene and nerol oxide were newly reported in these varietal wines. Similar results have been found when investigating free terpene concentrations of linalool, \(\alpha\)-terpineol, citronellol, and linalool oxides [33; 34], Muscat monoterpene content was 10 times higher than in other varieties, including Chardonnay, Gewurztraminer, and Riesling [34]. Recent studies found that rose oxide was highly correlated with Muscat and Gewurztraminer aroma [35; 36]. [PERSON] et al. reported higher _cis_-rose oxide contents in Gewurztraminer wines as compared to our study in older vintages, 1994, 1995, and 1997 [37]. Moreover, (2\(S\),4_R)-(--)_cis_-rose oxide was found as the main stereoisomer with high enantiomeric ratios (higher than 90%) in all _V. vinifera_ L. cv. Morio-Muskat must [38]. In our study, the largest concentrations of rose oxide were found in Torrontes wines, followed by Gewurztraminer and Muscat wines (Table 2). However, lower enantiomeric differences (EF: 0.23-0.49) of (2\(R\),_4_S_)-(+)_cis_-rose oxide vs. (2\(S\),_4_R_)-(--)_cis_-rose oxide was detected in our study as compared to [PERSON] et al. 2006 (Table 3). This may be due to fermentation differences. (2\(S\),_4_R_)-(--)_cis_-Rose oxide is formed through a reduction pathway during yeast fermentation and which may alter the enantiomeric ratio, thus forming more (2\(R\),_4_S_)-(+)_cis_-rose oxide [39]. Low monoterpene concentrations in Chardonnay wines found in this study are in agreement with other research [40; 41; 42]. Specifically, that monoterpene content of Chardonnay must and juice makes up only 5% of the free volatiles [40; 41; 42]. This was anticipated as Chardonnay is considered more a neutral white wine grape and winemakers use oak and other techniques to increase the wines complexity [43; 44]. The dominant monoterpene compositions differed among varietal wines. Our research shows that total monoterpene alcohols were abundant in Gewurztraminer, Viognier, Muscat, and Torrontes wines (Figure 1). Monoterpene alcohols play an important role in the aroma of Muscat and other aromatic grape varieties, and are thought to contribute to the fruity and floral notes [36; 45]. Other researchers found that linalool was abundant in Muscat grapes with highly distinctive fruit character [7; 46; 47; 48]. Linalool and citronellol have often been considered as important contributors to the aroma of Gewurztraminer, known for its floral, spicy notes [49]. Linalool oxide and nerol oxide were dominant isomers in Sauvignon blanc, Pinot gris, Riesling, and Chardonnay wines (Figure 1). Other studies showed little to no terpene content in Chardonnay, Sauvignon blanc, and Pinot gris wines [8]. Our results, however, show that these wines are abundant in linalool oxide and nerol oxide isomers. These differences in monoterpene isomers may be due to different biosynthesis pathways in aromatic and less aromatic grapes..  
Monoterpene profiles have been used to characterize grape varieties and varietal wines in other research [3; 9; 50]. Based on our results, the wines can be grouped into three classes for the varietal wines (Figure 2), very similar to the classification determined by [PERSON] et al. [10]. The most aromatic varietal wines included Torrontes and Muscat (group 1), Gewurztraminer wines (group 2), and the less aromatic varietal wines, including Chardonnay, Pinot gris, Riesling, and so forth (group 3). Interestingly, Riesling was grouped into the less aromatic wines in our study although Riesling has been generally considered as an aromatic grape variety.