## 3 Discussion  
Watermelon is a rich source of amino acids and other nutrients [2]. Our current study indicated that watermelon rind had higher total free amino acids than flesh; however, the free amino acids in watermelon rind had less diversity. Cit and Arg were the two most abundant free amino acids in both watermelon rind and flesh, and the concentration of Cit significantly increased along with rind concentration in the samples, with the rind alone (100% rind) containing the highest amount of Cit. The finding of higher Cit content in rind than flesh was consistent with literature reports [9; 25], though there are also conflicting \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**LRI** & **Compound** & **0\% Rind** & **10\% Rind** & **20\% Rind** & **30\% Rind** & **100\% Rind** & **OT (ug/kg)** & **Odor Descriptions** \\ \hline
672 & Acetaldehyde & 4255.2 b & 4546.8 b & 4881.2 b & 4117.3 b & 2348.7 a & 15 & Fresh, green, fruits \\
794 & Propanal & 84.9 c & 80.9 b & 79.5 b & 80.7 bc & 1.4 a & 10 & Musty, early, vegetative \\
882 & 2-Methylbutanal & 34.0 c & 30.2 b & 33.3 c & 26.4 b & 6.0 a & 3 & Cocoa, nutty \\
885 & 3-Methylbutanal & 21.0 b & 22.6 b & 27.8 c & 20.7 b & 6.6 a & 0.2 & Cocoa, nutty \\
960 & Pentanal & - & 21.7 b & 21.8 b & 24.9 b & 20.5 a & 12 & chocolate \\
1075 & Hexanal & 1050.8 c & 750.1 b & 705.5 b & 856.5 b & 185.7 a & 4.5 & Green, grassy \\
1130 & (E)-3-Hexenal & 1.2 a & 1.1 a & 1.4 a & 1.6 b & 1.2 a & 160 & Green, fruit, apple-like \\
1135 & (_Z_)-3-Hexenal & 1.7 a & - & 2.3 a & 1.6 a & 17.9 b & 0.25 & Green, grassy, leafy \\
1210 & (E)-2-Hexenal & 52.7 c & 24.9 a & 36.5 b & 36.3 b & 24.7 a & 17 & Green, fruits \\
1388 & Nonanal & 91.1 c & 113.1 c & 105.4 c & 75.3 b & 7.0 a & 1 & Green, fatty \\
1422 & (E)-2-Octenal & 21.2 c & 21.2 c & 13.6 b & 12.8 b & 2.0 a & 3 & Fatty, nutty \\
1435 & (E)-6-Nonenal & 4.2 b & 7.3 c & 7.6 c & 7.8 c & 0.3 a & 0.07 & Soapy, floral \\  & (off-notes) & (off-notes) & (off-notes) & (off-notes) & (1445 & (Z)-6-Nonenal & 65.0 c & 85.0 d & 64.9 c & 47.7 b & 4.6 a & 0.02 & Green, cucumber \\
1499 & (_Z_)-2-Nonenal & 6.1 b & 6.8 b & 6.1 b & 5.1 b & 0.7 a & 0.02 & Green, cucumber \\
1531 & (_E_)-2-Nonenal & 202.9 c & 216.3 c & 191.3 b & 181.8 b & 31.8 a & 0.08 & Green, cucumber \\
1567 & (_E_)-2-6-Nonadenal & 3.0 b & 3.4 b & 2.9 b & 2.0 b & 0.7 a & 0.5 & Green, cucumber \\
1581 & (_E_)-2-3-Nonadenal & 243.4 c & 226.6 c & 185.9 b & 182.1 b & 59.9 a & 0.01 & Green, cucumber \\
1346 & Hexanol & 101.3 b & 129.1 b & 165.9 c & 106.8 b & 16.4 a & 2500 & Green, floral \\
1376 & (_Z_)-3-Hexen-1-ol & 23.2 b & 43.1 c & 73.1 d & 44.5 c & 5.7 a & 70 & Green, grassy \\
1656 & Nonanol & 4.9 b & 15.3 c & 24.9 d & 8.8 b & 1.4 a & 50 & Green, fatty \\
1680 & (_Z_)-3-Nonenen-1-ol & 134.4 b & 247.4 c & 274.6 d & 221.8 d & 5.7 a & - & Fruity, sweet \\
1709 & (_Z_)-6-Nonen-1-ol & 6.9 a & 22.4 c & 24.4 c & 12.2 b & 0.4 a & 1 & Green, cucumber \\
1712 & (_E_)-Nonen-1-ol & 4.3 b & - & - & - & 1.9 a & 130 & Green, fatty \\
1737 & (_Z_)-3-Nonaden-1-ol & 0.9 a & 3.3 c & 47 c & 1.8 b & 0.7 a & 3 & Fatty, fishy \\
1747 & (_Z_)-3-6-Nonadenal-1-ol & 165.2 b & 329.5 c & 375.1 d & 275.2 c & 7.7 a & 10 & Fatty, fishy \\
1764 & (_E_)-2-6-Nonadeni-1-ol & 12.9 b & 31.9 c & 27.7 c & 15.2 b & 1.1 a & 0.2 & Green, cucumber \\
1331 & 6-Methyl-5-hepten-2-one & 270.7 c & 310.2 c & 186.8 b & 184.1 b & 7.3 a & 50 & Musty, popcorn, rubber \\ \hline \hline \end{tabular} Different letters (a–d) within each row indicate significant differences of each volatile compound among five samples according to one-way ANOVA and [PERSON]’s HSD test (\(\alpha\leq 0.05\)). Note: LRI = linear retention index, \"-\" = not detected. Odor descriptions were mainly adopted from the Good Scents Company. OT: odor threshold in water, was based on [PERSON]’s Compilations of Odour Threshold Values in Air, Water, and Other Media. 2003 and Boelens Aroma Chemical Information Service (BACIS) database.  
\end{table} 
Table 2: Volatile compounds in watermelon rind-flesh blends (ug/kg). reports [10].