## 3 Results and Discussion
### Volatile Compounds Analysis Using GC-qMS
#### 3.4.1 Esters  
Esters are mainly produced through the esterification between alcohols and acids during wine fermentation, and these volatiles have been confirmed to play vital roles in the determination of the wine's overall aroma [3]. Esters can be divided into ethyl esters, acetate esters and other esters according to their chemical nature [39]. Fatty acid metabolism can result in the formation of acyl-CoA, and acyl-CoA can be further converted into ethyl esters through ethanolysis [40]. These passion fruit wines were rich in ethyl esters (Table 3). Ethyl acetate has been reported to be a dominant ethyl ester in fruit wine, and its concentration directly affects the overall aroma of fruit wine. For example, the aroma complexity of wine could be enhanced with an ethyl acetate level below 150 mg/L in wine, whereas a concentration higher than 150 mg/L could bring an unpleasant aroma to wine [41]. In the present study, these passion fruit wine samples exhibited a similar ethyl acetate concentration, and its levels in these wine samples were all above its threshold, but below 150 mg/L. This indicated that ethyl acetate could provide these wine samples with fruity and floral scents. The other dominant individual ethyl esters in these wine samples included ethyl acetate, ethyl butanoate, ethyl hexanoate, ethyl caprylate, ethyl caprate, ethyl 9-decenoate and ethyl cinnamate. These ethyl esters had their OAV above 1, indicating that they significantly contributed to the overall aroma of these passion fruit wines. These volatiles were reported to bring fruity and floral aromatic notes [25; 31]. For example, ethyl caprylate possessed sweet, floral, fruity and banana scents, whereas ethyl butanoate and ethyl hexanoate had apple, cherry and banana notes [41]. Among these wine samples, the VI and CY-fermented wine exhibited the highest concentration of these dominant ethyl esters. The BV wine appeared to show the lowest content of these ethyl esters, indicating that the BV-fermented passion fruit wine might lack a floral and fruity aroma compared to the other strain-fermented wine samples. It has been reported that ethyl butanoate was the main component of the passion fruit aroma and showed the highest odoriferous importance for the characteristic aroma of passion fruit [2]. In our study, there was a significant difference in the content of ethyl butanoate among these passion fruit wine samples. The CY wine exhibited the highest concentration on ethyl butanoate, followed by the ES and VI wine. The BV-fermented passion fruit wine showed the lowest content of ethyl butanoate. It should be pointed out that some ethyl esters (such as ethyl laurate in the present study) with an OAV between 0.1 and 1 could also contribute their fruity flavor character to fruit wine due to the synergistic effect in the wine matrix [20; 25]. The rest of the ethyl esters found in these passion fruit wine samples had their concentration below their threshold, which indicated that they might not contribute to the overall aroma of the wine, although a concentration difference existed in these strain-fermented wines [42]. Regarding acetate esters, these wine samples fermented by different strains contained different acetate ester contents (Table 3). Isopentyl acetate appeared to be the major ethyl acetates in these wine samples. Isopentyl acetate was reported to exhibit banana flavor notes [21; 35]. In the present study, the concentration of isopentyl acetate was much higher than its threshold in these wine samples, indicating that its flavor scents significantly contributed to the overall aroma of the passion fruit wines. It should be noted that the concentration difference was also found in some other acetate esters among these wine samples. However, their level in wine was not above their threshold, limiting their contribution to the overall aroma in the passion fruit wine. It was observed that these wine samples contained 10 other esters (Table 3), and some of these esters showed a concentration difference among the wine samples.