## 3 Results and Discussion
### Evolution of the Volatile Compounds during Dehydration
#### 3.3.1 Volatile Compounds Evolution Expressed as \(\upmu\)g/L  
From an enological point of view, the content of grape volatiles expressed as \(\upmu\)g/L could directly correlate to the wine composition, which is why many studies have used this unit to calculate their results. The influence of dehydration degree on grape volatiles could also reflect, to some extent, its effect on the final wine aroma. The evolutions of free, bound, and total (free + bound) volatile compounds are illustrated in the heatmaps in Figure 3a. According to the hierarchical clustering results, we divided the evolution of all volatiles during dehydration into three clusters. Cluster 1 gradually increased during dehydration and reached a peak at T5. Cluster 2 fluctuated and reached its maximum value between T3 and T5. Cluster 3 was highest at the beginning of dehydration and gradually decreased during the process. The clustering categorization of the volatiles is listed in Table S1. Generally speaking, most of the volatile compounds increased during the on-vine grape dehydration process, with only a few decreasing. FADs: Eighteen FADs were classified into Cluster 1 in both the free and bound forms, six FADs were classified into Cluster 2 in either the free or bound forms, four FADs were classified as Cluster 3 in free forms, and no bound forms of FADs were classified into Cluster 3. For the total FADs (free + bound), twenty compounds were classified into Cluster 1, four compounds were classified into Cluster 2, and only two esters (butyl propionate, propyl octanoate) were classified into Cluster 3. Among the most important C6/C9 compounds of FADs, nine of them were classified as Cluster 1 in both the free and bound forms, while only four were classified into Cluster 2 or Cluster 3. The bound forms of _trans_-2-hexenol, as well as the free forms of hexanal and _trans_-2-hexenal were all classified into Cluster 2, with fluctuating levels during berry dehydration and reaching a maximum at T3 or T4. Considering the impact on sensory quality, the evolution of some critical C6 compounds during the dehydration process is shown in Figure 4a. With typical grassy notes, hexanal was present at levels above its thresholds at different levels of dehydration. Dehydration promoted an increase in _trans_-2-hexenal content and exceeded the threshold after T2, with a small decrease after peaking at T3. AADs: Twenty-two free forms and twenty-three bound forms of AADs were grouped into Cluster 1, including twelve compounds with both forms classified in this group. Seven free forms and twelve bound forms AADs were grouped into Cluster 2. Only the free form of one compound (2,4-di-_tert_-butylphenol) was grouped into Class 3. In Cluster 2, the majority (sixteen compounds) were phenylpropanoids in free or bound forms, and only four compounds belonged to the branched-chain aliphatics. The fluctuating variation in phenylpropanoids--especially the bound forms of volatile phenols--during on-vine berry dehydration indicated that water-loss enrichment was not the only affecting factor in this process. Interestingly, the content of 2-phenethyl acetate, with rose and honey notes, was dramatically increased at the late stage of dehydration (T5), reaching 2.79 times its threshold (Figure 5a). Another powerful floral composition with a low threshold was phenylethylaldehyde, the free form content of which increased gradually throughout the water-loss process. IPDs: Most of the free forms of IPDs (24 compounds) were classified into Cluster 1, except for five kinds of monoterpenes, namely, \(\upbeta\)-myrcene, \(\upbeta\)-phellandrene, \(\upbeta\)-_cis_-Oximene, \(\upbeta\)-_trans_-Oximene, and _cis_-allo-ocimene. In contrast, all the bound fractions of IPDs were grouped into Cluster 2, except for citronellol and \(\upbeta\)-damascenone.