## 3 Discussion
### Aldehydes
Following the literature suggestions, it should, therefore, be converted into parental compounds--the oxidized one, 2-methylpropanoic acid, or the reduced one, 2-methylpropanol [19] (Figure 5). Figure 5: Scheme of 2-methylpropanal biosynthetic pathway and red-ox molecular evolution. Six-carbon aldehydes, such as hexanal and (E)-2-hexanal, as well as their corresponding alcohols and straight-chain C9 compounds, such as (_E,E_)-2,6-nonadienal and (_E,Z_)-2,6-nonadienal, are typically produced by lipoxygenase activity in damaged plant tissues [(26; 30)]. For this reason, these compounds increase in number and concentration after the disruption of cells and tissues. These observations perfectly match our results, as these analytes were absent in the M_AI_V_t0 sample. In addition, hexanal was detected in both bleached samples, becoming the aldehyde present in the greatest quantity compared to all the others. In fact, in the sample M_B_O_t0 (Table 2), the hexanal had a peak TIC area of 155 \(\times\) 10\({}^{6}\), corresponding to a relative area of 43% within the class of aldehydes. In the M_B_O_t24 sample, it reached a quantity of 448 \(\times\) 10\({}^{6}\) (Table 3), that is, almost 72% of all aldehydes. It conferred grassy, green-like, and fatty notes to the aroma of our bleached fibers. This behavior may have been caused by the synergistic effect of the damage to the plant tissues and the oxidative action of H\({}_{2}\)O\({}_{2}\) towards the molecules originally present in the matrix. In fact, hexanal is used as an index for the degree of oxidation of fatty acids in food chemistry [(32)]. Among the identified aromatic aldehydes, benzaldehyde, benzeneacetaldehyde and \(\alpha\)-ethylidene-benzeneacetaldehyde, were detected, albeit in small concentrations. They covered only about 2% of the entire volatile fraction of the vacuum-freeze-dried melon pulp (Table 1). In particular, benzeneacetaldehyde was the only one initially identified both in the M_AI_V_t0 fiber and after the bleaching treatment (M_B_O_t0), but none of the three molecules were preserved during the aging period. Unlike what is reported by [PERSON] et al. [(32)], in our case, the presence of acetaldehyde was not detected, probably for the same reasons previously suggested for the C9 aldehydes. Furthermore, it is important to consider that most of the studies found in the literature refer to \"fresh\" fruit pulp, or juice, and not to dehydrated material. This aspect certainly contributes to the differences between this study and previous ones. On the other hand, to the best of our knowledge, this is the first time that \(\alpha\)-ethylidene-benzacetaldehyde (2-phenyl-2-butenal) has been mentioned among the melon VOCs. For this reason, it is quite difficult to highlight its contribution in terms of perceived aroma. Finally, in all three samples, furfurral was identified. It is a natural dehydration product of five-carbon sugars, such as xylose and arabinose, which are typically found in the hemicellulose fraction of fruit and vegetables [(40)]. In particular, its amount, expressed as the TIC peak area, increased by about four times from the M_AI_V_t0 sample (0.634 \(\times\) 10\({}^{7}\), Table 1) to the M_B_O_t0 (26.6 \(\times\) 10\({}^{6}\), Table 2) one. After the long aging period, the DF presented a furfurral amount of 9.24 \(\times\) 10\({}^{6}\) (Table 3).