## 3 Results and Discussion
### Evolution of the Volatile Compounds during Dehydration
#### 3.3.2 Volatile Compounds Evolution Expressed as ng/berry  
To further elucidate the evolution pattern of overall volatiles in each grape berry during on-vine dehydration, ng/berry expression was employed as a measurement unit for the calculation of volatile content. The resulting dataset was also processed for hierarchical clustering heatmap analysis. We expected that some useful information on the biochemical metabolism of volatiles during on-vine dehydration could be obtained in this way. The evolutions of the free, bound, and total (free + bound) volatile compounds in a single berry are visualized in the heatmaps in Figure 3b. All volatiles were grouped into two clusters through hierarchical clustering. In Cluster A, the volatiles gradually decreased throughout the dehydration process. In Cluster B, volatiles fluctuated during dehydration and peaked at some stage after T1. The cluster information of volatiles is listed in Table S2. According to the heatmaps shown in Figure 3, the tendency of volatiles during dehydration calculated in ng/berry presented a completely different pattern than that in ug/L. For a single grape berry, most of the volatiles were reduced by the biochemical process substantially at the end of grape dehydration (T5), except for a few IPDs in free  
Figure 5: The evolution of some phenylpropanoids during on-vine grape dehydration. The red dashed line marks the threshold level: (**a**) \(\mathrm{\SIUnitSymbolMicro g}\)/L; (**b**) ng/berry. T1, T2, T3, T4, and T5 represent the sampling points at different dehydration degrees. Values are the means of three replicates. The error bars represent the standard deviation. forms. During the on-vine dehydration process, the volatiles showed a complex variation trend under the combined action of two opposite effects, namely, biodegradation and water-loss concentration. This finding provides a good explanation for the content fluctuations of some volatiles during on-vine dehydration in this study, as well as in those conducted by others [13; 19; 31]. Figure 6: Evolution of some IPDs during on-vine grape dehydration. The red dashed line marks the threshold level: (**a**) \(\mathrm{\mu g}\)/L; (**b**) \(\mathrm{ng}\)/berry. T1, T2, T3, T4, and T5 represent the sampling points at different dehydration degrees. Values are the means of three replicates. The error bars represent the standard deviation. FADs: With the exception of _trans_-2-hexenal and ethyl hexanoate, most of the straight-chain aldehydes and esters derived from fatty acids (Cluster A) gradually decreased after T1. Some key C6 compounds (Figure 4b) were classified into Cluster B in both the free and bound forms, including hexanol, _trans_-3-hexenol, _cis_-3-hexenol, _trans_-2-hexenol, and _trans_-2-hexenal. Both free and bound forms of hexanal gradually decreased at almost the same rate throughout the dehydration process, and the total content (free + bound) at T5 was only 37.13% that at T1. Due to the water-loss concentration effect, the content of hexanal presented in grape juice (\(\mu\)g/L) remained relatively constant during the whole dehydration process (Figure 4a). The contents of hexanol, _trans_-2-hexenol, and _trans_-2-hexenal increased at the early stage of dehydration and reached a peak at T2 or T3, and then decreased at different levels. It is worth noting that _trans_-2-hexenol was increased considerably in the early stage of dehydration, reaching a peak at T3, being 6.83 times higher than that at T1. A remarkable increase in glycosidically bound C6 alcohols and a consecutive decrease in C6 aldehydes (ng/berry) has also being observed in the on-vine dehydration of Beibinghong grapes for icewine-making in the northeast of China [19]. A similar phenomenon during an off-vine artificial grape-drying process has also been noted by other authors, with a continuous decrease in C6 aldehydes and a previous increase followed by a subsequent decrease in C6 alcohols. [11].