## 3 Results and Discussion
### Evolution of the Volatile Compounds during Dehydration
#### 3.3.2 Volatile Compounds Evolution Expressed as ng/berry
The variation in C6 compounds during grape dehydration is probably due to lipoxygenase (LOX) activity changes in response to the water stress caused by dehydration and the reactions between the aldehydes and grape polyphenols [2; 26]. AADs: The majority of free AADs were classified into Cluster A, except for 2-phenethyl acetate, 2-methyl-1-propanol, 2-methyl-1-butanol, 3-methyl-1-butanol, and 1-octen-3-ol, all of which were classified as Cluster B. For the bound fractions of AADs, twenty-three compounds were assigned into Cluster A, while fifteen compounds (dominated by 11 phenylpropanoids) were assigned into Cluster B. Volatile phenols were found mainly in the bound forms, all of which were classified into Cluster B, except for 4-methylphenol and 2,4-di-_tert_-butylphenol. Taking guaiacol as an example, the free forms decreased slightly during the dehydration process, while the bound forms fluctuated in the later stages of this process (Figure 5b). 2-Phenethyl acetate, characterized by a pleasant rose and honey aroma, decreased rapidly in the early stages of dehydration, but significantly increased in the later stages of dehydration, where the berry weight loss exceeded 30% (T4). Consequently, its content in the juice (\(\mu\)g/L) rapidly exceeded its threshold at T5 under the combined effect of the dehydration concentration and biosynthesis through some unknown mechanism(s) (see Figure 5). Two other important floral compounds, phenylethylaldehyde and \(\beta\)-phenylethanol, were gradually degraded during the dehydration process, but their content in grape juice (\(\mu\)g/L) did not decline, as the concentration effect of dehydration was greater than the degradation effect (Figure 5). It should be emphasized that the levels of the mushroom-like odorant 1-octen-3-ol--a typical molecular marker of _Botrytis cinerea_ or other fungal infections [21; 32; 33; 34]--did not increase notably throughout the dehydration process (Table S2), indicating that the grapes in this study was not infected by such pathogens during dehydration. IPDs: Nineteen free IPDs were grouped into Cluster A, with a gradual decrease after T1, and another ten free IPDs were grouped into Cluster B, with fluctuating changes during the same process. The evolutions of bound and total IPDs were relatively complex, all of which were classified into Cluster B, except 6-methyl-5-hepten-2-ol. With a continuous decrease in the free fractions, the predominately glycosidically bound forms of five key monoterpenoids showed different variation trends during the dehydration process (Figure 6b). They increased in the early stages of dehydration and then decreased significantly, followed by a clear rebound at T5. Progressive decreases in free monoterpenoids during on-vine dehydration have also been observed by various authors in different grape varieties, such as Beibinghong [19], Aleatico [35], and Moscato Bianco [10; 35; 36]. Several recent studies have focused on the evolution patterns of glycosidic precursors during grape ripening, some of which found no significant changes [11] or slight increases [23] in terpen during dehydration, while others observed dramatic decreases [35]. The inconsistent results of these studies suggest that the accumulation patterns of monoterreponoids are greatly influenced by multiple factors, such as the volatiles themself [37], grape variety [38], and vintage [39], making it difficult to draw a simple conclusion. [PERSON] et al. (2013) have pointed out that the different evolution patterns of aromatic compounds during late-harvest drying in different regions and vintages suggest that the content variations of volatiles are largely determined by environmental factors [10]. Different from C6 compounds (Figure 4b), the free and bound forms of terpenoids evolved in a completely different pattern during the dehydration process in our study (Figure 6b). An increase in glycosylated terpenoids and decrease in free forms caused by water loss have also been observed by other authors [36].