## 3 Results and Discussion
### Regional Differences in Non-Volatile Compounds of SDT  
The taste properties of tea result from the comprehensive effect of water-soluble substances in the tea infusion on human taste receptors. These are mainly affected by the content and ratio of taste substances, such as polyphenols, alkaloids, amino acids, sugars, and organic acids [33]. Quantitative analysis revealed substantial variation in the concentration ranges of water extract (WE), total polyphenols (TP), free amino acids (FAA), soluble sugars (SS), caffeine (CA), gallic acid (GA), and epigallocatechin gallate (EGCG) across SDT samples from 13 distinct regions. Specifically, the measured ranges were as follows: WE (49.79-52.86%), TP (20.45-26.49%), FAA (1.52-2.87%), SS (4.11-6.10%), CA (36.94-77.12 mg/g), GA (0.48-7.98 mg/g), and EGCG (65.54-85.26 mg/g). The total polyphenol (TP) content in SDT exhibited significant regional variation across the 13 studied areas, with the MN group demonstrating the highest concentration (26.49 \(\pm\) 0.27%), followed by the MA group (26.02 \(\pm\) 0.98%). Conversely, the free amino acid (FAA) content was lowest in the MN group (1.52 \(\pm\) 0.33%), with the MA group showing the second-lowest concentration (1.67 \(\pm\) 0.14%). Consequently, the TP/FAA ratio reached its maximum in the MN group (18.09 \(\pm\) 4.35), followed by the MA group (15.84 \(\pm\) 1.29). Samples from GFZ and SH regions were characterized by pronounced bitterness and astringency, accompanied by a distinct heavy and thick mouthfeel. Notably, the TP/FAA ratios in the WLS, PS, DG, DBS, SH, GFZ, and BD groups were consistently below 10, with the GFZ group exhibiting the lowest ratio (7.51 \(\pm\) 0.90), followed by the SH group (7.82 \(\pm\) 0.75). The SH group simultaneously displayed the highest FAA content (2.87 \(\pm\) 0.28%) among all the samples. Interestingly, despite their initial bitterness, the GFZ and SH samples developed a noticeable sweet aftertaste. The WLS group exhibited unique chemical characteristics, including the lowest flavonoid (FN) content (3.69 \(\pm\) 0.31%), which correlated with reduced bitterness perception. This group also demonstrated the highest concentrations of total catechins (235.94 \(\pm\) 0.85 mg/g), gallocatedin gallate (GCG) (18.92 \(\pm\) 0.48 mg/g), and catechin (CA) (77.12 \(\pm\) 0.98 mg/g), resulting in a predominantly sweet and mellow flavor profile with a persistent sweet aftertaste. This sensory characteristic may be attributed to the formation of hydrogen bonds between alkaloids and catechins in the tea infusion, with the resulting hydrogen bond complexes potentially enhancing the mellow and fresh characteristics of the tea infusion. Cluster analysis revealed that the SDT samples from 13 distinct regions could be categorized into two primary groups based on their chemical composition profiles. The first group (designated as L1) comprised tea samples from the PS, BD, DBS, DG, SH, GFZ, and WLS regions, while the second group (L2) included samples from the XN, BKN, MA, MN, BDao, and BS regions (Figure 2A). Quantitative analysis demonstrated that the TP/FAA (total polyphenols to free amino acids) ratio in L1 group samples was consistently below 10, correlating with a predominantly sweet and mellow flavor profile. In contrast, L2 group samples exhibited TP/FAA ratios exceeding 10, corresponding to a heavier and thicker flavor characteristic. Figure 2: Analysis of regional differences in non-volatile compounds in SDT: heat map (**A**); volcano map (**B**); and VIP value map (**C**). Orthogonal partial least squares discriminant analysis (OPLS-DA) was subsequently conducted to identify key differential metabolites between the L1 and L2 groups. The model demonstrated satisfactory goodness of fit, with parameter values of R2X:0.745, R2Y:0.931, and Q2:0.892.