# Effect of Leaf Grade on Taste and Aroma of Shaken Hunan Black Tea
## 3 Results
### Analysis of Main Taste Compounds  
Tea polyphenols, catechins, gallic acid, alkaloids, and amino acids are the primary taste components found in tea. In this study, PCA analysis was performed on Hunan black tea samples of various grades, focusing on the content of these taste components. The R\({}^{2}\)X value of 75.3% suggests that the model has strong predictive ability (Figure 2). The PCA diagram revealed distinct clustering and separation patterns among the different samples,indicating variations in the main taste substances of Hunan black tea derived from fresh leaves of varying grades. The objective of this study was to examine the variations in taste substances among different grades of shaken Hunan black tea. The results (Table 2) showed a gradual decrease in the levels of tea polyphenols, catechins, gallic acid, and alkaloids as the grade of fresh leaves decreased. In contrast, there was an increase in amino acid content as the fresh leaf grade decreased, which contradicted the trend observed in conventional leaves [12]. One possible explanation for this phenomenon could be attributed to two factors. Firstly, the selection of raw materials played a role. In this study, raw materials of different grades were obtained from the same fluffy tops with the same growth potential. As a result, the lower-grade samples had a higher stem content, which is known to be rich in free amino acids [14]. Secondly, the shaking process used in this study may have also contributed to this outcome. Previous research has shown that shaking effectively reduces the levels of polyphenols, leading to a decrease in the bitterness and astringency of black tea [8]. \begin{table}
\begin{tabular}{l l l l l} \hline \hline
**Grades** & **Taste Description** & **Score** & **Aroma Descriptions** & **Score** \\ \hline First grade & Slightly bitter & 90.5 & Tender sweet & 90.5 \\ Second grade & Sweet, mellow, slightly & 92 & Floral, honey, sweet & 93.5 \\ Third grade & Sweet, mellow, floral & 92.5 & Sweet with floral & 93 \\ Fourth grade & Mellow & 90.5 & Pure sweet & 91 \\ \hline \hline \end{tabular} First grade: one bud and one leaf; Second grade: one bud and two leaves; Third grade: one bud and three leaves; Fourth grade: one bud and four leaves.  
\end{table} 
Table 1: Sensory evaluation results of Hunan black tea with different leaf grades. Figure 2: Analysis of the main taste components of shaken Hunan black tea. (**A**): PCA analysis; (**B**): thermogram analysis; (**C**): analysis of main taste components in different categories. Different low case letters above columns indicate statistical differences at \(p<0.05\). Additional heat map analysis was conducted on different taste components, including theobromine and caffeine, various catechins, phenylalanine, leucine, and a few other amino acids. The results indicated a decrease in the content of these compounds as the fresh leaf grade decreased. This can be attributed to the fact that alkaloids such as caffeine and theobromine are synthesized in the stem tips of tea plants, and their content gradually diminishes as the leaves and stems mature [15]. This result ties well with previous studies; secondary metabolites tend to accumulate in higher quantities in the stems or leaves of lower grades [16]. The contribution of major taste substances to the taste of tea is influenced not only by their content but also by their taste active value (TAV) (Table 3) [17]. Caffeine, ranging from 30.77 to 46.74, is a significant source of bitterness in shaken Hunan black tea (Table 3). EGCG, with a TAV ranging from 1.09 to 1.82, is an important astringent substance in shaken Hunan black tea. Gallic acid, glutamic acid, and aspartic acid, with TAVs greater than 1, have a notable impact on acidity. Catechin, gallic acid, and alkaloids contribute to the bitter and astringent taste attributes, which play a crucial role in determining the intensity of bitter and astringent tastes in tea. These attributes decrease in magnitude as the grade of the fresh leaf decreases [18].