# Watermelon Rind and Flesh Volatile Profiles and Cultivar Difference
## 4 Discussion  
The most abundant grouped volatiles in rind and flesh were aldehydes and alcohols, predominantly volatile nine-carbon compounds. Though volatiles in rind have not yet been reported, the findings of flesh volatiles were consistent with literature in that the most abundant volatile compounds in watermelon flesh are known to be nine-carbon aldehydes and alcohols [29]. Other grouped compounds such as ketones, terpen, esters, lactones, acid, sulfur compounds, furans, aromatic, and hydrocarbons have also been reported [1; 29]. Though the identification of these compounds depending on genotypes, growing conditions, and analytical method. Though diverse groups of compounds occur in watermelon flesh, it has been established that the saturated and unsaturated nine-carbon aldehydes and alcohols are derived from fatty acids, especially polyunsaturated linoleic (C18:2) and linoleic (C18:3). C18:2  
Figure 3: Heat map of 55 selected volatiles (% peak area, filtration interquartile range threshold = 0.01) in the rinds and fleshes of Captivation, Exclamation, Excursion, and Fascination watermelons. Volatile compound names for V1–V132 could be found in Table 1. V = volatiles. and C18:3 are the main precursors of most fruit aroma volatiles via \(\alpha\)- and \(\beta\)-oxidation in intact fruits, or the LOX pathway in fresh-cut fruits and juices [29]. Nevertheless, studies on the impact of genotypes and metabolism pathway of volatile aroma compounds in watermelon are scarce, though a few studies have reported the volatile compounds derived from the degradation of carotenoid pigments. Consequently, genotypic variation in carotenoid composition underscores differences in the terpenoic and norisoprenoid volatile profiles [1]. Hundreds of volatiles have been identified in watermelon. Nine-carbon aldehydes and alcohols, such as (\(Z\),\(Z\))-3,6-nonadien-1-ol, (\(Z\))-3-nonenal, (\(Z\),\(Z\))-3,6-nonadienal, and (\(Z\))-3-nonen-1-ol, have been identified to be the most important to watermelon aroma as they imparted melon, fresh, sweet, and green notes [32] The most potent volatile compounds determined in watermelon were (\(E\),\(Z\))-2,6-nonadienal, (\(Z\))-6-nonenal, (\(E\))-2-nonenal, (\(E\))-2-nonenal, (\(E\),\(Z\))-3,6-nonadien-1-ol, (\(Z\))-2-nonenal, (\(E\),\(E\))-2,4-nonadienal, nonanal, (\(E\))-2-nonen-1-ol, and (\(E\),\(Z\))-2,6-nonadien-1-ol according to their odor-active values [3]. Another study also indicated nine-carbon aldehydes, alcohols, and their esters were identified as main compounds in watermelon; controversially, ethyl acetate, acetaldehyde, tetradecanoic acid, and methyl acetate were found the most abundant components [33]. In a consumer acceptance study of freshly cut watermelon, high ratings for liking of overall quality were associated with high concentrations of (\(E\))-2-nonen-1-ol, (\(Z\))-6-nonen-1-ol, (\(E\))-2-nonenal, and (\(Z\))-6-nonenal. A decrease in some alcohols by 50-80% and some aldehydes by \(\sim\)90% occurred over an eight day storage period, which resulted in decreased odor detection of freshness, green, cucumber, and watermelon aromas [34]. The decline in volatiles and sensory characteristics over a time was also observed by other researchers [35; 36]. In addition to nine-carbon aldehydes and alcohols, several other compounds from minor groups might be important to the aroma of watermelon flesh, though their contribution is under-explored. For example, 6-methyl-5-hepten-2-one, the most abundant ketone in watermelon flesh (77.3-83.5% of the total ketones in this study), can contribute green, musty, fruit odors [3; 28]. Increased concentration of some volatiles such as acetophenone might contribute to off odors [34]. Terpenes/ terpenoids such as \(\beta\)-ionone can contribute floral notes (violet-like) to watermelon [3], along with other terpen/ terpenoids such as linalool, citral, and geranloi; however, their contribution is not well defined. Esters are well known to contribute fruity notes to many fruits.