## 3 Results and Discussion
### Effects of Shaking Process on Volatile Aroma Components  
A total of 26 different substances was shared in FL vs. S1, S1 vs. S2, and S2 vs. S3, including 5 alcohols, 3 aldehydes, 1 alkane, 2 alkenes, 4 heterocycles, 4 esters, 2 furans, 3 ketones, and 2 other classes (Table 2). Except for 1-propylbenzene, 1-ethyl-3-methylbenzene, and 1-penten-3-one, most of the components were upregulated and provided a green, sweet, floral, and cocoa odor [39].  
More specifically, benzeneacetaldehyde, (Z)-3-hexenol, 1,3-dimethylbenzene, dimethyl sulfide, (Z)-3-hexenyl butyrate, ethylbenzene, 1-penten-3-ol, (E)-4,8-dimethylona-1,3,7-triene, 3-heptanone, (Z)-3-hexenyl-\(\alpha\)-methylbutyrate, 2-decanone, and 2-butylfuran were shown significantly upregulated in FL vs. S1 group. Benzeneacetaldehyde was formed by the Strecker degradation reaction of phenylalanine, which exhibited a rose-like odor [26] and is a very important flavor compound in various teas, especially in oolong tea and black tea [40; 41]. Dimethyl sulfide showed a cabbage odor [37] and is considered a beneficial compound to the aroma of green tea and oolong tea [24]. Ethylbenzene represented an aromatic odor and was one of the most definite odorants contributed to the green tea with chestnut-like [42]. (Z)-3-Hexenol (green, grass, fruity odor) [40], (E)-4,8-dimethylnonadi-1,3,7-triene, 3-heptanone (green pepper) [43], and (Z)-3-hexenyl-\(\alpha\)-methylbutyrate (fruity odor like unripe apple and pineapple) [44] were the specific differential compounds only in FL vs. S1 and mostly showed green and fruity odor, indicating that the first shaking process was the key process for the accumulation of these substances. Studies have shown that benzeneacetaldehyde, dimethyl sulfide significantly increased during the shaking process of oolong tea [24; 45]. Moreover, it has been reported that the accumulation of (Z)-3-hexenol during the shaking process was a stable phenomenon, due to the de novo synthesis pathway, rather than enzymatic hydrolysis [46; 47].  
In the S1 vs. S2 group, six compounds were upregulated, including 2-ethylfuran, 2-methylbutanal, indole, nerolidol, (Z)-hexanoic acid, 3-hexenyl ester, and hexyl butyrate, and three compounds (1-penten-3-one, 1-ethyl-3-methyl-benzene and 1-propylbenzene) were downregulated. The upregulated compounds mainly exhibited sweet, floral, and fruity aroma [26; 27; 39]. Studies have shown that the accumulation of (Z)-hexanoic acid, 3-hexenyl ester occurred in the shaking process [48] and exhibited a fruity and green odor [25]. The results of this experiment were consistent with the literature [48], and further proved that the second shaking was the key process, which was conducive to the accumulation of (Z)-hexanoic acid, 3-hexenyl ester.  
In the S2 vs. S3 group, there were seven upregulated differential metabolites (pentanal, dimethyl sulfide, indole, (E)-\(\beta\)-farnesene, etc.) and one downregulated differential metabolite (1-ethyl-3-methyl-benzene). Among them, 1,1-dimethyl-cyclopropane, (E)-\(\beta\)farnesene, and (E,Z)-3,6-nonadien-1-ol (sweet, green, waxy, melon, fruity) [35] were the special metabolites. (E)-\(\beta\)-Farnesene exhibited floral odor [42], the content of which was 2.92-fold higher in S3 than that in S2.  
The leaves were the breathing tissue even after picked from the tea tree to the end of the shaking stage, which could respond to the stress (mechanical damage, dehydration) during tea processing. Indole was a characteristic aroma component of oolong tea, and previous studies have shown that the accumulation of indole occurred mainly in the shaking stage of oolong tea [12; 24]. This experimental study was consistent with the results of the existing literature [12; 24], and it was proven that the accumulation of indole was mainly in the second and third shaking stages (S2, S3).