## 3 Results and Discussion
### Volatile Flavor Evaluation of Black Rice
#### 3.5.3 Pearson Correlation Cluster Analysis  
With a theoretical odor detection limit of approximately 10\({}^{-19}\) mol [62], the human nose makes sensory evaluation a valuable and highly sensitive approach for analyzing rice aroma and odor-active volatiles [45]. Therefore, the Pearson correlation coefficient was calculated to visualize the key volatiles and sensors clustering that affect the perception of cooked black rice aroma (smell score in sensory test) in Pearson correlation cluster analysis. Cluster analysis was performed in order to determine the key volatiles and sensors of electronic noses reflecting odor value (smell score in sensory test). According to the Pearson correlation coefficient, 12 volatile compounds were grouped into 7 categories and 17 sensors were grouped into 5 categories (Figure 6). On the X-axis, the odor value (smell score in sensory test) exhibited a significant positive correlation with the concentration of 2-pentyl-furan and 2-methoxy-phenol (guaiacol). Additionally, cluster analysis classified these volatile compounds and odor values (smell score in sensory test) in the same category, indicating that the trend characteristics of these substances were similar to the smell score in sensory test. Therefore, the changes in the odor value (smell score in sensory test) of black rice with different milling degrees were mainly caused by the changes in the content of 2-pentyl-furan and guaiacol. Many researchers have studied volatile compounds in cooked black rice and found that guaiacol presented in black rice bran was responsible for the characteristic component in black rice [63]. It was reported that the aroma difference between red and black rice bran was caused by the content difference of guaiacol [63]. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \multirow{2}{*}{**Compound**} & **Odor** & **Odor** & \multirow{2}{*}{**OAV (A)**} & \multirow{2}{*}{**OAV (B)**} & \multirow{2}{*}{**OAV (C)**} & \multirow{2}{*}{**OAV (D)**} & \multirow{2}{*}{**OAV (E)**} & \multirow{2}{*}{**OAV (F)**} \\  & **Description** & & & & & & & & \\  & & **(ppb)** & & & & & & \\ \hline \multirow{4}{*}{Hexanal} & Green & \multirow{4}{*}{4.5 green, grassy} & \multirow{4}{*}{19.50 \(\pm\) 1.18 \({}^{\text{bc}}\)} & \multirow{4}{*}{22.60 \(\pm\) 6.94 \({}^{\text{b}}\)} & \multirow{4}{*}{15.58 \(\pm\) 1.85 \({}^{\text{c}}\)} & \multirow{4}{*}{14.94 \(\pm\) 2.26 \({}^{\text{c}}\)} & \multirow{4}{*}{20.21 \(\pm\) 3.94 \({}^{\text{bc}}\)} \\  & tomato, green, grassy & & & & & & & & \\  & Aldehyde, & & & & & & & \\  & waxy, citrus, & 1 & 24.78 \(\pm\) 5.13 \({}^{\text{a}}\) & 19.17 \(\pm\) 3.19 \({}^{\text{b}}\) & 15.39 \(\pm\) 7.04 \({}^{\text{a}}\) & 12.95 \(\pm\) 1.27 \({}^{\text{c}}\) & 12.78 \(\pm\) 3.09 \({}^{\text{c}}\) & 24.63 \(\pm\) 2.56 \({}^{\text{a}}\) \\  & tart sweet & & & & & & & & \\  & Citrus & 0.1 & 35.00 \(\pm\) 13.00 \({}^{\text{a}}\) & 23.40 \(\pm\) 8.70 \({}^{\text{a}}\) & 45.00 \(\pm\) 19.40 \({}^{\text{a}}\) & 44.1 \(\pm\) 39.60 \({}^{\text{a}}\) & 20.60 \(\pm\) 5.50 \({}^{\text{a}}\) & 24.80 \(\pm\) 8.40 \({}^{\text{a}}\) \\  & Medicinal & 530 & 1.23 \(\pm\) 0.18 \({}^{\text{a}}\) & 0.44 \(\pm\) 0.03 \({}^{\text{b}}\) & 0.36 \(\pm\) 0.07 \({}^{\text{b}}\) & 0.17 \(\pm\) 0.02 \({}^{\text{c}}\) & 0.10 \(\pm\) 0.01 \({}^{\text{c}}\) & 0.09 \(\pm\) 0.02 \({}^{\text{c}}\) \\  & Furan & & & & & & & & \\  & 2-pentyl- & & & & & & & & \\  & Phenol, & & & & & & & & \\  2-methoxy- & & & & & & & & & \\  1-Cten-3-cl & Mushroom & 1 & 19.56 \(\pm\) 4.07 \({}^{\text{b}}\) & 16.67 \(\pm\) 2.25 \({}^{\text{b}}\) & 18.38 \(\pm\) 5.14 \({}^{\text{b}}\) & 16.01 \(\pm\) 3.03 \({}^{\text{b}}\) & 18.71 \(\pm\) 3.70 \({}^{\text{b}}\) & 24.93 \(\pm\) 2.23 \({}^{\text{a}}\) \\ \hline \end{tabular} Different lowercase letters in the lines represented statistically significant differences (\(p<0.05\)).  
\end{table} 
Table 6: Odor description, odor threshold, and OAV in cooked black rice with different milling degrees (A–F).