## 3 Results and Discussion
### Evolution of the Volatile Compounds during Dehydration
#### 3.3.2 Volatile Compounds Evolution Expressed as ng/berry
We presume that this might be caused by the conversion of free terpenoids to bound forms under the catalyzation of glycosyltransferase during the dehydration process. As the dehydration concentration leads to high sugar levels and high hydrophilicity in the vacuolar, the cells need to convert the hydrophobic free forms of terpenoids with increasing content (\(\mu\)g/L) to their hydrophilic glycosidically bound forms in order to avoid their crossing the tonoplast and reducing their toxicity [1]. Of course, further studies are needed to support this hypothesis. The evolution of \(\beta\)-damascenone--the most potent aromatic C13-norisoprenoids with fruity and floral notes--differed from that of the terpenoids, indicating a distinct biosynthetic pathway [1,40]. In contrast to terpenoids, which exist mainly in the glycosidically bound form, the free form of \(\beta\)-damascenone was much more abundant than its bound form (Figure 6). The bound form of \(\beta\)-damascenone gradually increased in the early stages of dehydration, and then slightly decreased after peaking at T3. The free form of \(\beta\)-damascenone showed fluctuating changes in the early stages of dehydration but gradually increased steadily after T3. The total content of \(\beta\)-damascenone at T5 reached 1.91 times that at T1. Combined with the concentration effect caused by dehydration, its content in juice (\(\mu\)g/L) increased sharply in the late stages of dehydration, providing the molecular foundation for the complex floral and fruity characters in the final dessert wine. As has been demonstrated in other studies, \(\beta\)-damascenone is a key contributor to the aroma of icevine [21], noble-rotted wine [32], and late-harvest wine [9]. However, [PERSON] et al. (2016) have found that free \(\beta\)-damascenone peaked in the middle of the late-harvest icing process (TM + 40 or TM + 50) in Beihghong grapes, after which the content (ug/berry) gradually decreased [19]. They suggested that this change in \(\beta\)-damascenone was related to the regulation of \(V\)\(v\)CCD1 and its biotransformation in the final stage. Before this, [PERSON] et al. (2012) have observed that the expression of \(V\)\(v\)CCD1 in off-vine drying Aleatico grapes was regulated by the dehydration degree and drying temperature [16]. A decrease in \(\beta\)-damascenone at the final stage of Vidal icewine grapes has also been reported by other authors [31]. We believe that the opposite tendency of \(\beta\)-damascenone in the last stage of late-harvest in our study, compared to that in the studies of [PERSON] [19] and [PERSON] [31], was due to the complete difference in berry structure at this stage. In their study, the grape cell structure was repeatedly damaged by freeze-thaw cycles, exposing the volatiles to an oxidative environment [19]. In contrast, the grapes in our study were not frozen during the dehydration process (Figure S1a, with the lowest temperature of \(-2.6\)\({}^{\circ}\)C on 2 December); therefore, the cell structure and biochemical function remained intact and non-destructive, keeping the volatiles away from oxygen. Furthermore, their studies were conducted in northeastern China, where temperatures were extremely low during the later stage, whereas ours was in the southwest, at a lower latitude, with abundant sunlight and heat during the daytime throughout the dehydration period. As sunlight is positively correlated with an increase in \(\beta\)-damascenone [41], the biosynthesis of this compound might have been still progressing at the later dehydration stage in our study.