## 3 Results and Discussion
### Analysis of Volatile Flavor Substances during the Fermentation of Passion Fruit Wine
#### 3.4.1 Volatile Flavor Substances during Fermentation

Table 2: _Cont._Esters are the main contributors to aroma quality during fermentation and are able to impart the fruity characteristics of passion fruit wine. The esters in CF and NF showed a trend of first increasing and then decreasing. At the late stage of fermentation, the content of esters in CF (13.85%) was higher than that in NF (11.95%). The contents of decanoic acid, ethyl ester, octaoic acid, ethyl ester, dodecanoic acid, ethyl ester, ethyl 6-methylpyridine-2-carboxylate, and methyl 5(3)-methyl-4-hydroxy-3(5)-pyrazolecarboxylate were relatively high among the 24 esters, and these mainly contribute special floral and fruity aromas. Among them, the content of methyl 5(3)-methyl-4-hydroxy-3(5)-pyrazolecarboxylate was the highest, and this may be the main special aromatic substance of passion fruit and passion fruit wine. Alcohols are mainly synthesized by amino acid metabolism, glycolysis and other pathways [50]. The content of alcohols in both CF and NF showed a trend of gradually increasing, and the contents of phenylethyl alcohol, cyclohexanol, 3-(acetyloxymethyl)-2,2,4-trimethyl-, 1-butanol, 3-methyl-, and benzyl alcohol were relatively higher among the 11 alcohols. Among them, phenylethyl alcohol presents a special rose fragrance, 1-butanol and 3-methyl- present an apple and brandy fragrance, and benzyl alcohol has a flavor similar to special bitter almonds. These substances contribute to the pleasant, soft and harmonious aroma of fruit wine. Some research shows that a small amount of aldehydes and ketones can provide special aromatic characteristics to wine [51]. The relative contents of ketones gradually decreased in CF and NF. The most prevalent ketones were 3-hexanone-2,2,4,4-d4,2-pyrrolidinone,1-methyl- and 4(3H)-pyrrolidinione,3-methyl-, among which 3-hexanone-2,2,4,4-d4 gives flavor to fruit wine, such as the grape flavor. The relative contents of 4(3H)-pyrrolidinione,3-methyl- were highest in both CF and NF, reaching 11.56% and 11.11%, respectively. Benzaldehyde has the aromas of rose, honey and lilac. The acids present are mainly volatile acids, which have an important impact on the overall aroma of wine [52]. 4-tert-Butyl-2-nitrophenol acetate is the main characteristic acid in passion fruit wine, and its content gradually decreased, which shows that the late stage of fermentation is a key period for the formation of some esters (acids + alcohols) and provides a rich basis for the formation of flavor substances in passion fruit wine. The contents of alkane and alkene compounds showed a trend of gradually increasing during CF and NF. Benzene ring compounds showed a trend of decreasing and then increasing in CF and NF; among them, the relative contents of 3-acetyl-2,5,6-trimethylhydroquinone, 1,4-di(methyl-d3)benzene-d4, and phenol were relatively higher, and the content of 2,4-di-tert-butylphenol was highest in passion fruit wine at different stages of fermentation, showing a trend of increasing after decreasing. At the same time, it is also a common volatile phenolic substance in fruit wines, such as grape wine [53]. Studies have shown that excessive volatile phenols will weaken the fruity aroma of wine, but they will increase the complexity of the aroma at low concentrations, such as in grape wine [54; 55]. The amines in CF and NF showed a trend of decreasing gradually, and the contents of (methylsulfonyl)amine, 4H-1,2,4-triazol-3-amine, and 4-methyl- decreased gradually with fermentation. Pyridines are substances with weak alkaline and special smells, and the contents of 2(1H)-pyridinone,3-hydroxy- and furan,2-methyl-5-(methylthio)- were higher in ICF, reaching 0.75% and 4.76%, respectively. Hydroxypyridines are similar to phenolic compounds in structure and properties (aromaticity), and tyrosinases can catalyze the oxidation of 2,3-dihydroxypyridine. The esters, alcohols, ketones, aldehydes, acids and other flavor substances are combined with eachother, giving passion fruit wine a full, mellow and harmonious flavor character.