## 3 Results and Discussion
### Aldehydes  
The number of aldehydes was the highest in the WDF_t\({}_{0}\) sample, with 34 out of the 102 molecules detected being aldehydes, and was the second class in terms of total absolute abundance. During storage, the number of identified aldehydes decreased (27 and 9 for WDF_t\({}_{1}\) and WDF_t\({}_{2}\), respectively). The total abundance in terms of the TIC area decreased by 45% after 30 days of storage, from 1109 \(\times\) 10\({}^{6}\) to 608.5 \(\times\) 10\({}^{6}\), and subsequently increased to a value of 940.4 \(\times\) 10\({}^{6}\). This increase is related to the degradation of more complex molecules, such as amino acids, fatty acids, and carotenoids, which release volatile aldehydes as byproducts. In contrast, the TIC area of more unstable aldehydes decreased over time, some of which eventually disappeared. They characterize the WDF_t\({}_{0}\) aroma and are typical flavor compounds of the fresh fruit, i.e., Gavina(r) watermelon. They probably underwent degradation due to oxidative, biological, and photochemical processes, thus giving rise to other species, and leading to a change in the aroma profile of the sample. Table 1 collects the results relating to this molecular class. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & & & **WDF\_t\({}_{0}\)** & **WDF\_t\({}_{1}\)** & **WDF\_t\({}_{2}\)** & \\ \hline
**Analyte** & **Aroma** & **LRI** & **Area (\(\times\)10\({}^{6}\))** & **Area (\(\times\)10\({}^{6}\))** & **Area (\(\times\)10\({}^{6}\))** & **Ref** \\ \hline Acetaldehyde & Pungent, fresh, fruity & 435 & 54.6 \(\pm\) 0.3 & 3.96 \({}^{\circ}\) & - & [2, 23, 26, 27] \\ \hline Propanal & Ethereal, pungent, fruity & 498 & 13.5 \(\pm\) 0.1 & 12.5 \(\pm\) 0.3 & - & [26, 28] \\ \hline
2-Methyl-propanal & Fresh, aldehyd, green & 559 & 1.57 \({}^{\circ}\) & 1.17 \({}^{\circ}\) & 48.1 \(\pm\) 0.3 & - \\ \hline (Z)-2-Butenal & - & 570 & 2.31 \(\pm\) 0.06 & 6.55 \(\pm\) 0.06 & - & - \\ \hline Butanal & Pungent, cocoa, green, malty & 593 & 2.06 \(\pm\) 0.07 & 2.25 \(\pm\) 0.07 & - & - \\ \hline \(\epsilon\)-2-Butenal & Green, fruity & 655 & 8.18 \(\pm\) 0.1 & 6.49 \(\pm\) 0.06 & - & [2, 6, 23, 29] \\ \hline
3-Methyl-butanal & Aldehyde, fermented, fatty & 661 & 17.2 \(\pm\) 0.2 & 22.1 \(\pm\) 0.2 & 257 \(\pm\) 0.5 & [6, 10, 30] \\ \hline
2-Methyl-butanal & Fermented, nutty, furfural & 671 & 7.19 \(\pm\) 0.07 & 8.55 \(\pm\) 0.08 & 195 \(\pm\) 0.4 & [2, 6, 10] \\ \hline Pentanal & Fermented, fruity, green & 703 & 19.0 \(\pm\) 0.4 & 40.3 \(\pm\) 0.4 & 93.5 \(\pm\) 0.2 & [6, 23, 31, 32] \\ \hline
2-Methyl-2-butenal & Pungent, green, penetrating & 748 & 2.79 \({}^{\circ}\) & 2.19 \({}^{\circ}\) & - & - \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Composition of the VOC fraction of Gavina® watermelon pomace WDF identified using HS-SPME-GC-MS analysis: aldehydes class. Figure 2: Representation of the VOC number belonging to each molecular class of the Gavina® WDF samples at different aging times (t\({}_{i}\)). C9 aldehydes characterize the flavor and aroma of different species of Cucurbitaceae [3, 6, 26, 33, 34, 35] and have been previously reported in several studies on similar samples [5, 29, 30, 33]. We identified 4-nonenal, nonanal, (Z,Z)-3,6-nonadienal, (Z)-2-nonenal, (E,E)-2,6-nonadienat(E)-2-nonenal, and 2,4-nonadienal. They are among the species that mainly define the characteristic aroma of Gavina(r) watermelon, since they were particularly abundant in WDF_t0. The abundance of these species decreased sharply during storage, and only nonanal was detected after 24 months (WDF_t2). The most unstable and rapidly degrading species (4-nonenal, (Z)-2-nonadienal, 3,6-nonadienal, and (E,E)-2,6-nonadienal) were not detected after 30 days (WDF_t1). At the biological level, C9 aliphatic aldehydes are produced by the enzymatic oxidation of polyunsaturated fatty acids (PUFA), particularly linolenic and linoleic acids, by lipoxygenase [6, 34]. Figures S1 and S2 show the degradation mechanisms of these fatty acids and the VOCs formation.