## 3 Discussion
### Aldehydes  
Aldehydes (ALD) are known to be key compounds that define the flavor and aroma profile of fruits and vegetables. These analytes are usually present in higher concentrations in non-ripe melon cultivars, then are gradually replaced by esters and alcohols as the degree of ripeness progresses [28]. Therefore, most of them (especially C_n_ with \(n<8\)) are not detected or are only recovered in very small quantities in ripe fruits [32]. Nevertheless, [PERSON] et al. [32], and [PERSON] et al. [10] also reported that aldehydes were the most abundant compounds in some _Inodorus_ melon cultivars. The results obtained in the present study are in accordance with literature information since, in this case, aldehydes also represent a very important family of the VOC fraction of the M_AI_V_t0 fiber (\(\approx\)25%), together with alcohols (\(\approx\)34%). This is the reason that _Inodorus_ melons are attributed the lowest odor perception but the highest freshness score, generally being characterized by a \"green-like\" and herbaceous aroma [32]. Previous studies suggest that saturated and unsaturated C9 aliphatic aldehydes, together with C9 alcohols, play a key role in melon and watermelon aroma [10; 29; 39]. Figure 4: Total ion chromatogram of the VOCs from M_B_O_t24 fibrous sample, obtained using HS-SPME-GC-MS analysis (a.u. = arbitrary units). For melons belonging to the _Inodorus_ group, the main analytes playing this role are (Z)-6-nonenal, nonanal, (_E_,_Z_)-2,6-nonadien, (_E_)-2-nonenal, 6-nonen-1-ol and 1-nonanol [10]. All these molecules contribute, to some extent, to imparting waxy, fresh and green notes to the smell of winter melon. Among these aldehydes, we detected only nonanal, both in samples M_AI_V_t0 and M_B_O_t0, in the amounts of 3.16 \(\times\) 10\({}^{7}\) and 30.5 \(\times\) 10\({}^{6}\), respectively (TIC peak area); this confirms that this analyte is particularly representative of the DFs and persists even after the strong invasive treatment with H\({}_{2}\)O\({}_{2}\). The other aldehydes were not detected, probably due to many factors, including their very low quantity in our samples (below the detection limit), the selected cultivar, origin and cultivation practices, fruit ripening, handling, and post-harvest processing. All these variables can influence the concentration of VOCs in melons [30], altering their aromatic profile. Among the aldehydes, we observed that 2-butenal and 2-methyl-2-butenal were also present in rather high levels in the HS of M_AI_V_t0 (TIC = 19.8 \(\times\) 10\({}^{7}\) and 1.28 \(\times\) 10\({}^{7}\), respectively). However, only the former persisted after the H\({}_{2}\)O\({}_{2}\) bleaching process (Table 2), albeit in quantities (2.03 \(\times\) 10\({}^{6}\)) about 100 times lower than the value of the fresh raw material. Pentanal and heptanal, on the other hand, showed exactly the opposite behavior, increasing their concentration by about three to four times, respectively, immediately after the treatment of the fiber with H\({}_{2}\)O\({}_{2}\) (M_B_O_t0). During the aging of the DF, these two aldehydes were partially converted to their most oxidized form, namely pentanoic and heptanoic acid, respectively. This may be due to radical oxidation processes initialized by oxygen present in the residual atmosphere inside the vials storing the samples. The branched-chain molecules 2-methylbutanal and 3-methylbutanal, described as having a'malty-like' aroma, were detected in all three of the different samples analyzed. These compounds are suggested to be synthesized from isoleucine and leucine, respectively, even though the pathways are not well recognized yet [30]. However, which fraction of the total quantity of 2-methylbutanal and 3-methylbutanal is not directly related to the aforementioned amino acids cannot be established. It is assumed that 2-methylpropanal is also an intermediate product in the catabolism of two amino acids (leucine and valine); together with 2-methylbutanal, it is considered a key compound in several foods, including melon.