## 3 Results and Discussion
### Screening of Key Differential Volatiles  
The results of analyzing the differential volatiles in Lapsang Souchong black tea by OPLS-DA show that during the construction of the OPLS-DA model, after 200 random simulations, the goodness-of-fit (R\({}^{2}\)Y = 0.993, \(p<\) 0.005) and predictability (Q\({}^{2}\) = 0.963, \(p<\) 0.005) of the model both reach significant levels (Figure 4a). It can be seen that the OPLS-DA model can effectively distinguish different-grade Lapsang Souchong black tea samples, and the model construction meets the requirements and can be used for further analysis. The analysis results of the scores plot of the OPLS-DA models of A, B, and C show (Figure 4b) that the three grades can be effectively distinguished in different regions. The first principal component explains 40.2% of the differences, and the second principal component explains 36.8% of the differences. This indicates that there are differences in the aromas of Lapsang Souchong black tea of the three grades. Further, the S-Plot of the constructed OPLS-DA model was analyzed to obtain the variable importance in projection values (VIP values) of different volatiles and obtain the key differential volatiles. The analysis results show (Figure 4c) that a total of 26 key differential volatiles were obtained (Table S5). According to the classification of the obtained key differential volatiles, there were three alcohols, eight phenols, four terpenes, and one each of ketones, esters, aldehydes, and arenes. A contains abundant 3-Hydroxymandelic acid, ethyl ester, 3,7-dimethyl-1,6-octaIien-3-ol, 1-Ethyl-1H-pyrrole-2-carbaldehyde, Phthalan, (+) -4-Carene, D-Limonene with sweet, floral, and fruits aromas [26; 27; 28; 29; 30]. B contains abundant 2-Furanmethanol, 5-ethenyltetrahydro-\(\alpha\),\(\alpha\),5-trimethyl-, cis-\(A\),5-di-epi-aristolochene, 1-Octen-3-ol with earthly, floral, sweet, woody, and green aromas [21; 25]. However, its aroma characteristics are not as elegant as those in A, so the aroma evaluation score of A is higher than that of B. C contains 3,7-dimethyl-1,6-octaIien-3-ol, 2,3-dimethylthiophene, Dihydroactinidiolide, Naphthalene-1-methyl-7-(1-methylethyll)- with green, herbal scent, fruits, and slightly sweet aromas [31; 32; 33], and at the same time, C contains 4-ethyl-2-methoxyphenol with a rich smoky flavor during the smoking and baking process of Lapsang Souchong black tea.  
Figure 3. Pairwise analysis of volatile metabolites in three grades of Lapsang Souchong black tea: (**a**-**c**) OPLS-DA analysis illustrating the differences between (**a**) grades A and B, (**b**) grades A and C, and (**c**) grades B and C; red dots indicate metabolites with significant differences, while green dots represent those without significant differences. (**d**-**f**) Heatmaps showing the volatile compound profiles for (**d**) A compared to B, (**e**) A compared to C, and (**f**) B compared to C. Differential metabolites were identified based on their variable importance in projection (**VI** > 1).  
There were significant differences in the key differential volatiles of the national standard samples of Lapsang Souchong black tea of the three grades. The main sweet, floral, and fruity aromas in A are mainly attributed to compounds such as D-Limonene, 3,7-dimethyl-1,6-octadien-3-ol, and 3-Hydroxymandelic acid,ethyl ester. 2-Furanmethanol, 5-ethenyltetrahydro-\(\alpha\),\(\alpha\),5-trimethyl-, cis-,4,5-di-epi-aristolochene, 1-Octen-3-ol mainly contribute to the floral, sweet, and woody aromas in B. The large amount of smoky substances, such as 4-ethyl-2-methoxyphenol in C, may lead to a decrease in its floral and fruity aromas.