## 3 Results and Discussion
### Terpenes and Derivatives  
Terpenes play a fundamental role in defining the aroma and fragrance of fruits, flowers, and spices [1, 4, 42, 43]. They probably constitute one of the largest groups of compounds known in nature, with approximately 30,000 molecules identified to date, isolated from plants, microorganisms, and animals. Lighter species are important constituents of essential oils and plant resins, whereas heavier species include carotenoids. Similar to monoterpenes, carotenoids are biosynthesized starting from geranyl diphosphate (Figure S6), which, through further coupling of isoprene units, each catalyzed by specific enzymes, progressively forms derivatives with a lipophilic chain gradually longer, up to the tetraterpenes (C40). They typically have a 40-carbon atom chain backbone, and are among the most important pigments in fruits and flowers [4, 44]. They show different colors, from the yellow of lutein, to the orange of \(\beta\)-carotene, to the red of lycopene, owing to their system of conjugated double bonds, on which their chemical, physical, and biological properties depend. The high delocalization of \(\pi\) electrons allows them to stabilize reactive intermediates, such as carbocations or radicals, by resonance. Therefore, they have efficient antioxidant properties and protect cells from oxidative stress induced by reactive species, such as free radicals. Foods containing carotenoids have a high antioxidant capacity [45, 46], and their nutritional intake is strongly recommended, since it is associated with the prevention of several diseases, including cardiovascular diseases and cancers, and with increased immune response [4, 44, 46]. However, because of their high number of double bonds, carotenoids are particularly sensitive to degradation induced by heat or atmospheric oxygen, biological oxidation processes, and exposure to electromagnetic radiation, leading to the formation of characteristic volatile compounds. For this reason, many carotenoids have considerable commercial value in the food and cosmetic industries, not only as pigments but also as substances capable of imparting pleasant aromas to products [47]. Given the increase in demand for natural and healthy products and the problems related to the synthesis of these analytes, the flavor and fragrance industry is increasingly searching for new methods to obtain natural species to replace their synthetic counterparts. Among the emerging production methods, the use and transformation of matrices containing carotenoids are becoming increasingly invasive, as a natural alternative capable of supplying precious aroma volatiles [48, 49]. The monitoring of carotenoid degradation compounds in commercial products can provide information on storage time, product quality and acceptability, nutritional value, vitamin and aroma content [48], color, and visual impact. Based on their detection, it is possible to draw conclusions regarding the presence of a given carotenoid. These observations applied to the watermelon WDF can be exploited to evaluate, in an approximate way, the variation of the carotenoid content during storage and to make considerations both on its nutritional value and on its aroma. All the results relating to the terpen and derivative class are shown in Table 7.