# Watermelon Rind and Flesh Volatile Profiles and Cultivar Difference
## 1 Introduction  
Watermelon (_Citrullus lanatus_) is globally consumed, possessing unique organoleptic properties, hydration function, and well-studied health benefits owed to its red flesh, though verities with orange and yellow flesh also produced [1]. Roughly, watermelon consists of skin, rind, flesh, and seeds. Watermelon flesh accounts for approximately 60% of the total watermelon mass [2]. Watermelon flesh contains a large amount of water, which is approximately 93% of the total weight of the flesh [3]; it also contains micronutrients such as vitamins (A, B, C, E), mineral salts (K, Mg, Ca, Fe), amino acids (citrulline and arginine), lycopene, and phenolic compounds [2].  
The edible rind makes up approximately 40% of the total watermelon mass yet is often discarded as waste [4]. Direct disposal of the rind waste is causing environmental issues, though several approaches of reusing watermelon rind have been investigated in a laboratory scale. The specialized function of the rind's polysaccharide composition (pectin and fiber) has been considered a potential reason for its reuse [2, 5]. It would be favorable to take advantage of the nutritional potential of rind and create commercial value, rather than limiting it to agricultural waste. Approaches have been introduced to reduce the accumulation of solid watermelon waste by converting the rind's polysaccharides into other products such as biosorbent [6, 7], bioremediation [8], biochar [9], and bioethanol [4]. Additionally, watermelon rind has been studied as a source of nutritional food ingredients such as antioxidants [10], amino acids [11, 12, 13, 14], and pectin [15, 16], especially citruline. In processed foods, rind has been tested in pickled form and in jam [17, 18]. Watermelon rind in powder form has been examined to apply in carbohydrate-based goods includingcakes [12; 19], cookies [20; 21], noodles [22; 23], beef patties [24], and pork patties [25]. Furthermore, a few studies have investigated watermelon rind as a possible growth medium for microbials [26; 27].  
Though the chemical composition of watermelon rind has been studied, its volatile profile has not yet been reported. Watermelon aroma is derived from volatile composition, playing a key role in flavor quality. The most abundant volatiles in watermelon flesh are six-carbon and nine-carbon volatiles. Nine-carbon saturated or unsaturated linear aldehydes and alcohols are the best representation of watermelon aroma, which include (\(Z\))-2-nonenal, (\(E\))-2-nonenal, (\(E\),\(Z\))-2,6-nonandien, (\(Z\))-3-nonenol, (\(E\))-6-nonenol, (\(E\),\(Z\))-3,6-nonadienol, (\(E\),\(E\))-3,6-nonadienol, and (\(Z\),\(Z\))-3,6-nonadienol [28]. These nine-carbon compounds are derived from fatty acids via the lipoxygenase metabolic pathway [29]. The volatile profiles indicated variety difference [29], though limited studies have focused on watermelon volatile variety difference.  
Our previous study shows watermelon variety difference based on their sensory qualities [30]; however, volatiles in these varieties have not been investigated yet. It is unknown if volatile difference between flesh and rind exists and if volatiles in watermelon rind show a variety difference. Understanding volatile composition in watermelon rind will provide evidence for potential uses of watermelon rind in food and beverages. Therefore, this study aimed to (1) identify volatile compounds in watermelon rinds and the corresponding flesh, and (2) investigate part difference (rind vs. flesh) and volatile variety difference among four watermelon varieties.