## 3 Results and Discussion
### Specific VOC Compounds Analysis  
Orthogonal projection latent structure discriminant analysis (OPLS-DA) is an essential method for dimensionality reduction and identification of spectral features that drive group separation. It is a supervised modeling method used to discriminate among the varieties in a dataset relevant to predicting group labels and variations unrelated to predicting group labels [25]. Figure 6A shows the OPLS-DA biplot for the discrimination between Heilongjiang, Jilin, and Liaoning rice samples. In total, 33 VOCs were used as the independent variable X for OPLS-DA analysis, X variance R\({}^{2}\)x = 0.986, Y variance R\({}^{2}\)y = 0.998, and cross-validation predictive power Q\({}^{2}\) = 0.964, R\({}^{2}\)x \(-\) R\({}^{2}\)y < 0.3, indicating that the model is reliable. Thus, the variable importance for the projection (VIP) applied to determine specific VOCs for geographical discrimination of rice samples [15] and obtained from the corresponding loadings plot is shown in Figure 6B, Table 1. In total, 14 VOCs with VIP > 1 were selected as marked compounds to identify rice samples from three regions. Figure 5: Predictive class probabilities for samples and ROC plots based on the cross-validation (CV) performance. (**A,B**) Heilongjiang and rice samples, AUC = 1. (**C,D**) Heilongjiang and Liaoning rice samples, AUC = 1. (**E,F**) Jilin and Liaoning rice samples, AUC = 1. The numbers corresponding to the volatile compounds' names are as follows:  
1. Decane; 2. Undecane; 3. 2-methyl-decane; 4. Dodecane; 5. Tridecane; 6. Tetradecane; 7. Hexadecane; 8. 2,6,10,14-tetramethyl-hexadecane; 9. Styrene; 10. 2,6-dimethyl-decane; 11. 2-methyl-dodecane; 12. 2,6,10-trimethyl-dodecane; 13. Heptanal; 14. (_E,E_)-2,4-nonadienal; 15. Nonanal; 16. (_E_)-2-heptenal; 17. Decanal; 18. Pentadecanal; 19. Hexadecanal; 20. (_E_)-6,10-dimethyl-5,9 undecadien-2-one; 21. 6,10-dimethyl-2-undecanone; 22. Hexanol; 23. 1-octanol; 24. n-heptadecanol; 25. 2-hexyl-1-decanol; 26. 3,7,11-trimethyl-hexadecane; 27. Hexanol; 28. 2,6,10-trimethyl-hexadecane; 29. 3,7,11-trimethyl-hexadecane; 30. 3,7,11-trimethyl-hexadecane; 31. 3,7,11-trimethyl-hexadecane; 32. 3,7,11-trimethyl-hex  
1-dodecanol; 27. Octanoic acid; 28. n-Hexadecanoic acid; 29. Hexanoic acid; 30. Nonanoic acid; 31. Benzyl alcohol; 32. Toluene; 33. 2-butyl-1-octanol. Generally, alkane and alkenes compounds have no contribution to the rice aroma or flavor; these compounds were considered to be related to lipid decomposition [4]. In this study, eight alkane compounds (tridecane, tetradecane, hexadecane, 2,6,10,14-tetramethyl-hexadecane, styrene, 2,6-dimethyl-decane, 2-methyl-dodecane, and 2,6,10-trimethyl-dodecane) were detected as discriminative markers to identify Heilongjiang, Jilin, and Liaoning rice samples (Figure 7, Table 1). Except for 2,6,10,14-tetramethyl-hexadecane, the other seven compounds were higher in Liaoning rice samples than in the Heilongjiang and Jilin rice samples. The average concentration of tridecane, tetradecane, hexadecane, styrene, 2,6-dimethyl-decane, 2-methyl-dodecane, and 2,6,10-trimethyl-dodecane) in Liaoning rice samples were 56.73, 456.68, 14.96, 28.36, 12.15, 178.96, and 149.58 \(\upmu\)kg, respectively. [PERSON] et al. [4] and [PERSON] et al. [1] also demonstrated the occurrence of alkanes in Chinese rice. Alkanes are also highly sensitive to environmental factors such as light conditions, temperature, and humidity [4]. Thus, the concentration of alkanes in rice varies from region to region. In conclusion, although alkanes did not contribute to rice flavor, they played an important role in rice VOC identification. Aldehydes are the key volatile odorant in the formation of rice flavor, which plays an essential role in rice flavor [26]. Relevant studies show that the odor activity value of aldehydes accounts for 97% of the odor activity value of various flavor substances in rice [27].