# Improving the Sensory Quality of Black Tea by Blending Varieties During Processing
## 3 Results and Discussion
### Analysis of the Main Taste Compounds
#### 3.2.2 Analysis of Amino Acids  
Amino acids play a significant role in the umami, sweetness, and mellow taste of tea [31]. Amino acids can be categorized based on their contributions to taste profiles, which include umami, sweetness, and bitterness [32]. Research has indicated that aspartic acid, asparagine, glutamic acid, glutamine, and theanine are primarily responsible for the umami taste in tea infusion [33]. Therein, theanine is the most abundant amino acid in tea, constituting about 50-60% of the total amino acids present [29]. As shown in Supplementary Materials Figure S2B, the content of theanine was highest in the JX samples, while the theanine content in Fud was significantly higher (\(p<0.05\)) than in JGY. The content of umami amino acids in FJX and FJG was significantly higher (\(p<0.05\)) than that in Fud (Figure 2B), indicating that blending fresh leaves positively influences the umami taste profile. JX presented a higher content of theanine compared to Fud [34], suggesting mixing fresh _Fudingdabai_ leaves with _Jinxuan_ could enhance the levels of black tea after processing.  
Sweet amino acids, including serine, glycine, alanine, and threonine, proline [33], were significantly increased in FJX and FJG compared with Fud (Figure 2B). This may be one of the reasons why FJG and FJX have a sweeter taste. Research shows that approximately 1.2% of the protein in tea leaves is broken down by peptidases, resulting in an increase in the amino acid content in the tea [35]. Meanwhile, blending fresh leaves of two different varieties during processing may cause changes in the expression of peptidase genes, promoting the breakdown of more proteins into amino acids. JX and JGY did not show consistent advantages in all sweet amino acids (Supplementary Materials Figure S2B). When blending finished tea, the addition of JX to Fud resulted in higher sweet amino acid content than the addition of JGY (Supplementary Materials Figure S3B), likely due to the higher amino acid content in JX (Supplementary Materials Figure S2B). Bitter amino acids include valine, leucine, isoleucine, tyrosine, phenylalanine, histidine, lysine, and arginine [33]. As shown in Supplementary Materials Figure S2B, JX exhibited the highest content of total bitter amino acids, while JGY had the lowest. The content of bitter amino acids was also elevated in FJX and FJG, consistent with the changes observed in sweet amino acids (Figure 2B). Although the levels of these bitter amino acids were relatively lower and their taste activity values were not high [36], they did not significantly influence the bitterness contribution to the teas. Moreover, valine, leucine, isoleucine, phenylalanine, lysine, and arginine can contribute to important aroma components via the Strecker degradation process in black tea [37]. GABA, cystine, and tryptophan are classified as undefined amino acids in terms of taste; these amino acids may not directly influence the taste of the infusion, but they can induce synergistic effects that enhance umami or inhibit bitterness and astringency. Blending fresh leaves during processing can adjust the contents and components of amino acids in black tea, meaning it could be a potential method to promote or regulate the taste flavor of black tea during processing.