## 3 Results and Discussion
### Stuck Fermentation Altered the Profile of Non-Volatile Metabolites in Summer Black Tea  
We examined the metabolomics of YH and FHYH using UPLC-MS/MS in order to ascertain the impact of stuck fermentation on the non-volatile metabolites of summer black tea. Signal stability was shown by the substantial curve overlap of total ion flow in metabolites identification in both positive and negative ion modes (Figure S2A-D). Additionally, more than 75% of the quality control (QC) samples included components with a coefficient of variation (CV) less than 0.3, indicating robust experimental results (Figure S2E).  
A total of 1248 non-volatile metabolites were detected, including 106 amino acids and derivatives, 85 alkaloids, 305 flavonoids, 42 lignans and coumarins, 162 lipids, 68 nucleotides and derivatives, 95 organic acids, 228 phenolic acids, 28 tannins, 14 terpenoids, and 115 other metabolites (Figure 2A and Table S2). The YH and FHYH samples were split into two different clusters based on these metabolites, and the three repetitions of every group were grouped collectively and recognizable from one another. (Figure 2B). This indicated considerable homogeneity among the replicates, along with high data reliability. [PERSON] et al. also reported that the metabolites of tea samples with different levels of microbial fermentation showed significant separation on PCA analysis [17]. Furthermore, HCA (Figure 2C) and OPLS-DA (Figure 2D) also confirmed the significant differences between YH and FHYH, demonstrating the dependability of the model and lack of overfitting (Figure S3). Significant alterations in these metabolites after stuck fermentation were also demonstrated in a study tracking the dynamic changes in nonvolatile metabolism during stuck fermentation of green tea, which is consistent with our research [18]. Thus, stuck fermentation significantly altered the metabolic profile of summer black tea.  
We identified 551 differential non-volatile metabolites between the two groups (Figure 2E, Table S3, VIP \(\geq\) 1, \(p\)\(\leq\) 0.05, \(\mid\) Log\({}_{2}\)FC\(\mid\)\(\geq\) 1), and KEGG enrichment analysis showed that these metabolites were enriched in 14 pathways, including ABC transporters (ko02010), aminoacyl-tRNA biosynthesis (ko00970) and metabolic pathways (ko01100) (Figure 2F, \(p\)\(\leq\) 0.05). Notably, 6 of the 14 enriched pathways are related to amino acids, including biosynthesis of amino acids (ko01230), histidine metabolism (ko00340), valine, leucine and isoleucine degradation (ko00280), alanine, aspartate and glutamate metabolism (ko00250), tryptophan metabolism (ko00380), phenylalanine, tyrosine and tryptophan biosynthesis (ko00400). These results suggest that stuck fermentation of summer black tea may affect the relative content of metabolites and amino acid metabolism through multiple pathways. The variations in amino acids and their derivatives, phenolic acids, flavonoids, organic  
Figure 1: Sensory evaluation and electronic tongue analysis. (**A**) Dry tea of YH. (**B**) Tea soup of YH. (**C**) Tea base of YH. (**D**) Quantitative descriptive analysis of taste (*: \(p\)\(<\) 0.05, **: \(p\)\(<\) 0.01). (**E**) Dry tea of FHYH. (**F**) Tea soup of FHYH. (**G**) Tea base of FHYH. (**H**) Quantitative descriptive analysis of aroma (*: \(p\)\(<\) 0.05, **: \(p\)\(<\) 0.01).  
acids, alkaloids, saccharides, and lipids--all of which are directly connected to the flavor of summer black tea--are investigated more thoroughly in the sections that follow.