## 4 Discussion  
### The Selection of Non-Volatile Compounds and Their Correlation Between Different TSH Grades  
The quality characteristics of black tea, including heavy, thick, umami, and sweet attributes, are closely associated with non-volatile secondary metabolites such as catechins, amino acids, and caffeine [35]. Sensory evaluation revealed that the taste profile of TSH progresses from mellow to moderately rich and mellow, and finally to rich, mellow, and refreshing with increasing grade. This enhancement in intensity and thickness correlates strongly with ester-type catechins (ECG and EGCG), which increase synchronously with taste attributes. In contrast, non-ester catechins (C, EC, EGC) are present at lower levels and show weaker correlations. Our previous study on the innovative black tea MKH [19] revealed multiple catechin monomers closely associated with grade differentiation. However, in TSH, only ester-type catechins showed significant grade-dependent correlations--a pattern consistent with [PERSON] et al.'s (2023) findings on graded Hehong Congou tea [36]. This suggests that in traditionally processed black teas, ester-type catechins exhibit stronger associations with quality grades, likely attributable to their characteristic heavy fermentation process. Traditional methods typically use a bud with one or two leaves as the material and employ high-temperature, high-humidity fermentation to achieve signature sensory qualities like sweet mellowness, robust thickness, and \"red liquor with red leaves\" [37]. Prolonged fermentation promotes the conversion of non-esterified to ester-type catechins [38]. In contrast, innovative teas like MKH primarily adopt light fermentation techniques. Thus, the enrichment of ester-type catechins in high-grade traditional Congou black teas may serve as a critical indicator of premium quality.  
In high-grade TSH (TSH-0 and TSH-1), a distinctive \"umami\" sensory experience was observed. Quantitative analysis of 20 amino acids revealed that nine--including Thea, GABA, Phe, and His--increased with grade elevation. Correlation analysis identified Thea, GABA, and Arg as showing the strongest associations with umami. L-Thea (the most abundant amino acid in tea [39]) is a Glu derivative that contributes significantly to freshness and is closely linked to tea grade. Higher-quality black teas under varied processing conditions consistently exhibited elevated L-Thea levels [19]. Arg, the second most abundant free amino acid, imparts a mildly sweet taste [40]. [PERSON] et al. [41] identified Arg as a key differential component in graded Dianhong teas, while [PERSON] et al. [42] noted that traditional Congou processing (with prolonged heat transfer) promotes Arg accumulation, enhancing sweet-mellow notes. GABA--a glutamate decarboxylation product--also showed strong umami correlation. Contrary to studies linking GABA enrichment solely to anaerobic conditions (e.g., heavy fermentation), our MKH data revealed grade-dependent GABA increases, suggesting that withering-induced dehydration is the primary driver: heavier withering correlates with higher GABA accumulation and concurrent aroma compound formation [43], explaining the floral prominence in premium traditional black teas. Thus, Thea, Arg, and GABA collectively shape TSH's fresh-sweet profile while serving as critical biomarkers for premium-grade products.