## 3 Discussion
### Ketones  
As can be seen from Table 1, the M_AI_V_t0 fiber is characterized by the presence of 3-hydroxy-2-butanone (acetoin) and butanedione, detected in similar amounts based on the TIC area (6.45 \(\times\) 10\({}^{7}\) and 7.88 \(\times\) 10\({}^{7}\), respectively). Among the analytes belonging to this class of molecules, these two are found in the highest quantities in the previously mentioned sample. Acetoin has a pleasant creamy-yogurt smell, generally used in the food industry to enhance the flavor of some products. It has been identified in several dietary products such as yogurt and cheese, vinegar, fruits, vegetables, and some types of flours [43]. In relation to the category of melons, it has previously been detected in some cultivars in Italy [44]. It also has good biocompatibility and solubility, which extends its usage to soaps, lotions, and cosmetics [45]. In general, acetoin has three natural origins: (i) microbial fermentation, (ii) vegetable synthesis by fruits or plants, and (iii) animal synthesis. Its biosynthesis in plant cells was confirmed in the literature. It is also reported that, along with other odorous compounds, it confers to plants rich fragrances, with the purpose of attracting insects and higher animals to help with its pollination and propagation [43]. Being a very active molecule, acetoin generates many derivatives, which can be detected through HS-SPME-GC-MS analysis.  
For example, 2,3-butanediol is a product of the acetoin pathway in many bacterial species. This diol is produced from pyruvate during fermentation processes via several intermediate metabolites, where different enzymes are involved. At present, the exact metabolic function of 2,3-butanediol is not well known. It may play a role in avoiding excessive intracellular acidification through the conversion of an acid compound (pyruvate acid) to a neutral one (acetoin) [45; 46], as represented in Figure 6.  
Butanedione is also produced at the beginning of fermentation processes by yeasts, but it is generally rapidly reduced to acetoin and 2,3-butanediol. Together with acetoin, the diol imparts a buttery aroma to the food in which it is contained [47]. In our case it is quite unlikely that fermentation processes took place markedly, given the speed of handling the melon pulp from the fresh fruit to the dried fiber. Thus, most of the acetoin could, instead, have had a natural origin. However, since all the metabolites and intermediates previously  
Figure 6: Scheme of acetoin and 2,3-butanediol biosynthetic pathway [34].  
cited have been detected, it cannot be excluded, a priori, that some microorganism initiated the metabolic processes just described.  
Among other ketones, 6,10-dimethylundeca-5,9-dien-2-one (geranylacetone), which was present only in the M_AI_V_t0 sample (Table 1), is known to be derived from the natural degradation of long-chain terpen; these are typically \(\beta\)-carotene and lycopene, which give a floral aroma to the ripe fruit [10]. The same occurs for 6-methyl-5-hepten-2-one. In this case, given the pale yellowish-greenish color of the winter melon flesh, we suppose that it may originate from some other carotenoid or xanthophyll naturally present in this fruit and not from lycopene or \(\beta\)-carotene. These two molecules may also result from the degradation of \(\zeta\)-carotene, a linear carotenoid with a weak yellow color [48; 49]. Finally, among ketones, C8 compounds were the main ones in the HS of both the bleached fibers, with 2-octanone; 3-octen-2-one; and 3,5-octaleni-2-one. The amounts of these analytes were about 35% and \(\approx\)43%, respectively, for the M_B_O_t0 and M_B_O_t24 samples (the relative area % only refers to the ketones class). In particular, 3,5-octaleni-2-one was present in greatest quantities among the three above-mentioned species, existing in the form of two conformational isomers.