# Effect of Leaf Grade on Taste and Aroma of Shaken Hunan Black Tea
## 3 Results
### Analysis of Aroma Compounds
#### 3.3.2 Analysis of Differential Aroma Compounds  
In order to examine the variations in volatile components among various grades of Hunan black tea, a Principal Component Analysis (PCA) was conducted on a total of 144 volatile components (Table 4). The obtained R\({}^{2}\)X value of 68.2% suggests that the model exhibits a satisfactory level of predictability. Notably, each sample of Hunan black tea, when subjected to shaking, displayed distinct separation and clustering patterns in the PCA plots, indicating the presence of disparities in the volatile components across different grades of Hunan black tea. Subsequently, the 144 volatile components were assessed for differential composition using criteria such as a fold change value \(\leq\)0.5 or \(\geq\)2 and a \(p\) value \(<\) 0.05. A total of 63 differential volatile components were screened, including  
Figure 3: Analysis of volatile components of Hunan black tea. (**A**): PCA analysis; (**B**): heat map analysis of differential volatile components; (**C**): content of total volatile components and content of various volatile components. Different low case letters above columns indicate statistical differences at \(p\)\(<\) 0.05.  
29 FADV components, 24 VT components, 3 CDV components, 2 AADV components, and 2 Others components.  
The concentrations of alcohols, aldehydes, ketones, and CDVs in FADVs exhibited a gradual increase as the fresh leaf grade decreased, while the concentration of esters in FAVDs initially increased and then decreased. Previous research has demonstrated that the levels of fatty acids and carotenoids increase progressively with the maturity of fresh leaves. Consequently, lower grade fresh leaves contain higher amounts of precursors for FADVs and CDVs, such as linoleic acid, linoleic acid, \(\beta\)-carotene, and zeaxanthin, leading to an enhanced production of FADVs and CDVs during processing [21; 22; 23]. Notably, the ester content of FADVs in the Fourth grade exhibited a lower level compared to that of third grade and second grade Hunan black tea, which could potentially be attributed to the diminished activity of acyltransferase in the fresh leaves of Fourth grade tea [24].  
Except for trans-4,8-dimethyl-1,3,7-nonatriene, trans-\(\beta\)-farnesene, and \(\alpha\)-farnesene, the content of VTs decreased with decreasing fresh leaf grade, which is consistent with the results of Yin [2]. The synthesis of volatile terpenoids, specifically monoterpenes and sesquiterpenes, primarily occurs through the mevalonate metabolic pathway and the 2-methyl-D-erythritol-4-phosphate metabolic pathway. The higher levels of VTs observed in Hunan black tea with higher grades, which undergo vigorous leaf metabolism during shaking, may be attributed to this metabolic activity [25]. The levels of trans-4,8-dimethyl-1,3,7-nonatriene, trans-\(\beta\)-farnesene, and \(\alpha\)-farnesene exhibited a progressive increase as the fresh leaf grade decreased. It is plausible that these three terpene volatiles stem from the degradation of carotenoids, such as phytoflulene, present in tea. This degradation process can yield \(\alpha\)-farnesene [26].  
The levels of benzyl nitrile and indole exhibited a pattern of initial increase followed by a decrease as the grade decreased, with the highest concentration observed in the third grade. Benzyl nitrile is a byproduct of phenylalanine metabolism, while Indole is produced through the catalysis of 3-glycerol phosphate by tryptophan synthase [20; 27]. Further investigation is required to elucidate the underlying factors contributing to the variations in benzyl nitrile and indole contents among Hunan black tea products processed using different grades of fresh leaves, despite the observed preference for increased levels of these compounds through the shaking of fresh leaves with appropriate grade leaves in the study of oolong tea.