## 3 Results and Discussion
### Volatile Compounds in Roasted Tamarix Lamb
#### 3.4.2. Volatile Compound Markers in Roasted _Tamarix_ Lamb  
In order to clearly investigate the changes of key volatile compounds or volatile markers in charcoal and electric-roasted _tamarix_ lamb, PLSR combined with VIP procedure was performed. The model parameters R\({}^{2}\)X and R\({}^{2}\)Y represent the interpretation rate of the established model to the X and Y matrices, and R\({}^{2}\) describes how well the model fits. Q\({}^{2}\) represents the predictive ability of the model. The biplot of PLSR shows that X variable (R\({}^{2}\)X = 0.856) was used to explain the Y variable (R\({}^{2}\)Y = 0.971) (Figure 4A) in the charcoal (\(p\) < 0.05, Q\({}^{2}\) = 0.940), while X variable (R\({}^{2}\)X = 0.863) was also used to explain the Y variable (R\({}^{2}\)Y = 0.970) in the electric (\(p\) < 0.05, Q\({}^{2}\) = 0.947) (Figure 4B), which demonstrated good fitting and predictability for the changes of volatile compounds. Charcoal-roasted lamb had a total of 29 volatile markers, including 13 aldehydes, 5 alcohols, 1 ketone, 4 alkanes and 6 other types, while electric-roasted lamb had 25 volatile markers, including 12 aldehydes, 4 alcohols, 2 ketones, 2 alkanes, and 5 other types (Table 4). Most of the markers were distributed in the positive direction of the t1, close to each other, and only 3-hydroxy-2-butanone was distributed in the negative direction of the t1. With extended roasting time, the projections of charcoal and electric-roasted lamb had similar migration trend in the biplot, at 0 min, their projections were located at the negative direction of t1 and t2. However, at 2 min, their projections quickly migrated to the positive of t2, suggesting there was a significant change in volatile makers in roasted lamb, which was highly in line with the changes of E-tongue, E-nose, and free fatty acids. As roasting time extended from 4 min to 8 min, their projections were gradually migrated from the negative of t1 and positive of t2 to positive of t1 and negative of t2, where most of the volatile markers had higher contents. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Variable** & \multicolumn{2}{c}{**VIP**} \\ \cline{2-3}  & **Charcoal** & **Electric** \\ \hline Hexanal & 1.114 & 1.121 \\ Heptanal & 1.107 & 1.108 \\ Octanal & 1.088 & 1.116 \\ Nonanal & 1.085 & 1.106 \\ Decanal & 1.009 & — \\ Tridecanal & 1.107 & 1.112 \\ Tetradecanal & 1.088 & 1.030 \\ Pentadecanal & 1.069 & 1.019 \\ Benzaldehyde & 1.068 & 1.114 \\ (E)-2-Octenal & 1.104 & 1.102 \\ (E)-2-Nonenal & 1.105 & 1.106 \\ (E, E)-2-4-Decadienal & 1.078 & 1.105 \\
4-methoxy-Benzaldehyde & 1.076 & 1.097 \\
1-Pentanol & 1.115 & 1.110 \\
1-Heptanol & 1.008 & — \\
1-Octanol & 1.127 & 1.108 \\
1-Octen-3-ol & 1.108 & 1.099 \\ (E)-2-Octen-1-ol & 1.111 & 1.096 \\
2,3-Octanedione & 1.131 & 1.117 \\
3-Hydroxy-2-Butanone & — & 1.018 \\ Tetradecane & 1.104 & 1.135 \\ Pentadecane & 1.016 & — \\
1-Tetradecene & 1.134 & — \\
1-Pentadecene & 1.012 & 1.087 \\ Phenol & 1.123 & 1.144 \\ Anethole & 1.106 & 1.108 \\ Estragole & 1.005 & 1.122 \\ Naphthalene & 1.120 & 1.139 \\ dippropyl Disulfide & 1.032 & — \\
1,2,3,4-tetramethyl-Benzene & 1.104 & 1.075 \\ \hline \hline \end{tabular}  
* Variable importance in the projection, VIP; “—” means VIP < 1.  
\end{table} 
Table 4: The volatile compounds with VIP >1 (markers) of _tamarix_ lamb roasted by charcoal or electric. The 13 aldehydes markers in the charcoal-roasted lamb contained all of the 12 aldehydes markers that were detected in the electric-roasted lamb, and all the content of the aldehyde markers detected in electric-roasted lamb decreased compared with the charcoal-roasted lamb. Among all the aldehydes detected, hexanal was the highest, followed by nonanal, heptanal, and octanal. They were mainly formed by the oxidation degradation of unsaturated fatty acids [30]. It was reported that hexanal, nonanal, heptanal, and octanal presented grassy notes, floral notes, fat wax notes, and fruity notes, respectively [32].