## 3 Results and Discussion
### Terpenes and Derivatives
The presence of 6-methyl-5-hepten-2-one in non-negligible abundance in the freshly prepared sample highlights the high instability of lycopene, which underwent rapid degradation. The species shown in Figure 4 are commonly related to \(\beta\)-carotene degradation [29, 48] by breaking bonds in the different structural positions of the molecule. Among the degradation products, the most representative ones are:  
* \(\beta\)-ionone, produced following the breaking of the C9-C10 bond, is present in comparable quantities in WDF_t0 and WDF_t1 (35.8 \(\times\) 10\({}^{6}\) and 31.2 \(\times\) 10\({}^{6}\), respectively), while it is much less abundant after 24 months of storage (0.93 \(\times\) 10\({}^{6}\)). It is a molecule with a floral, slightly fruity, and pleasant aroma [44]; is widely used in all sectors of perfumery; and is used industrially for the synthesis of vitamin A [1]. In our samples, \(\beta\)-ionone was also identified in its epoxidized form (5,6-ionone epoxide), which reached a higher abundance in WDF_t1 (34.4 \(\times\) 10\({}^{6}\)). In this aging step, the quantities of \(\beta\)-ionone and 5,6-ionone epoxide were comparable. As previously reported for 2,3-epoxy-geranial, the epoxidation of these terpen occurs quite easily in the presence of atmospheric oxygen. These two species were the most abundant among those involved in \(\beta\)-carotene degradation. Therefore, we can hypothesize that the C9-C10 bond is the most susceptible to breaking. * Dihydroactinidiolide, whose formation mechanism is not well understood, is hypothesized to form starting from 5,6-ionone epoxide according to a radical oxidation mechanism [44, 48] or following the oxidation of the C8-C9 bond. Like 5,6-ionone epoxide, dihydroactinidiolide also reaches its maximum quantity after 30 days of storage, and then halves after two years (44.7 \(\times\) 10\({}^{6}\) and 22.0 \(\times\) 10\({}^{6}\) for WDF_t1 and WDF_t2 samples, respectively), giving the aged fiber a fruity aroma with woody notes. Although \(\beta\)-carotene is not the most abundant carotenoid in watermelon pulp, it contributes significantly to its aroma, as its degradation products have a particularly low odor threshold. Therefore, even if present in small quantities, they are easily perceived by humans sense of smell [4, 48]. After 24 months of storage, the revealed the presence of some species considered secondary in the degradation process of \(\beta\)-carotene, such as 2,2,6-trimethylcyclohexanone and \(\beta\)-cyclodextrin. \(\zeta\)-carotene is an acyclic, faint yellow carotenoid [4], a precursor of lycopene and \(\beta\)-carotene, structurally similar to lycopene, but with a lower number of unsaturations (Figure 5). Its presence has been hypothesized in several fruits that contain carotenoids, including watermelons and tomatoes. We assume that \(\zeta\)-carotene is present in our samples because its main degradation product, geranylacetone, was detected at each aging step, albeit in progressively lower quantities over time (181 \(\times\) 10\({}^{6}\), 104 \(\times\) 10\({}^{6}\), and 2.36 \(\times\) 10\({}^{6}\) for WDF_to, WDF_t1, and WDF_t2, respectively). Geranylacetone is a volatile molecule with a floral and fruity aroma and a high odor threshold [39], lacking a distinctive effect on the overall fragrance of our samples. Furthermore, 6-methyl-5-hepten-2-one is derived from \(\zeta\)-carotene, a ketone already described above in relation to lycopene, and has a high odor threshold [28]. As  
Figure 4: Fragmentation scheme of \(\beta\)-carotene and analytes of successive orders. The red line indicates the bond where the fragmentation of the molecule occurs. Figure 5: Fragmentation scheme of the \(\zeta\)-carotene molecule and higher order analytes. The red line indicates the bond where the fragmentation of the molecule occurs. 6-methyl-5-hepten-2-one can result from the simultaneous degradation of two precursor analytes, the significant amount detected in the WDF samples could be justified.