## 3 Results and Discussion
### Volatile Compound Composition
#### 3.1.2 Esters  
Esters have normally been considered the primary source of the fruity scents and these volatiles could contribute wine with the fruity aroma [7,43]. A total of 35 esters were found in both grape clone wines before aging (Table 2). However, content differences were observed for 10 individual esters. The wine made of clone 169 showed significantly (\(p\leq 0.05\)) higher concentrations of ethyl lactate, isoamyl lactate, and ethyl 9-decenoate, whereas ethyl  
Figure 1: Clustered heatmaps of volatile compounds in wine made of clone 169 and 191 aged in two types of oak barrel during the aging process. T-0, T-3, T-6, T-9, and T-12 represent before aging, 3-month aging, 6-month aging, 9-month aging, and 12-month aging, respectively. Y191 and Y169 represent wine made of clone 191 and 169 before aging, respectively. W191 and W169 represent wine made of clone 191 and 169 aged in Wolin oak barrel for 12 months, respectively. B191 and B169 represent wine made of clone 191 and 169 aged in Bordeaux oak barrel for 12 months, respectively. (**a**) C\({}_{6}\) alcohols and higher alcohols, (**b**) esters, (**c**) fatty acids, (**d**) terpen and norisoprenoids, (**e**) volatile phenols, and (**f**) furanic compounds, phenolic aldehydes and oak lactones, (**g**) other compounds.  
2-hexenoate, ethyl acetate, hexyl acetate and methyl salicate were present at significantly (\(p\leq 0.05\)) higher levels in the clone 191 wine before the aging process. The aging process significantly reduced the concentration of most of the esters in all wines (Figure 1b). The contents of ethyl 9-decenoate, ethyl decanoate, ethyl benzoate, 2-ethyl-1-hexyl acetate, and isoamyl acetate decreased in all wines by 72-81%, 50-51%, 18-32%, 85-90%, and 10-29%, respectively (Table 2). The decreased concentrations of these esters might mainly result from their hydrolytic reaction during the aging period [44]. Additionally, the oak barrel could absorb esters, resulting in a decrease in the content of these esters [45; 46]. It is worth noting that esters are the major volatiles that incorporate fruity and floral notes to the wine aroma, and their concentration decrease during aging might lower the fruitiness of aged wine [9]. In addition, during the aging period, esterification and transesterification also take place, which might lead to increased ester concentration in wine [29; 44; 47; 48]. In the present study, it was observed that seven esters showed an increase on their concentration in both clone wines during the aging process. Ethyl lactate, diethyl succinate, isoamyl lactate, and ethyl acetate increased their concentrations by 311-797%, 357-519%, 388-506%, and 87-100%, respectively. As shown in Table 2, there were significant differences (\(p\leq 0.05\)) in 18 esters between the clone 169 and 191 wines after an aging period of 12 months. Among these esters, the differences in ethyl acetate, ethyl butanoate, ethyl lactate, ethyl 9-decenoate, and ethyl 2-hexenoate between the two clone wines were consistent both before and after the aging process. Additionally, the aging process resulted in differences in 2-ethyl-1-hexyl acetate, isoamyl acetate, ethyl benzoate, and isoamyl octanoate between these two clone wines. Furthermore, the wine aged in the Wolin oak barrel had significantly (\(p\leq 0.05\)) higher concentrations of ethyl undecanoate and ethyl myristate than the wine aged in the Bordeaux oak barrel after aging.