# Characterization of the Key Aroma Compounds in Different Aroma Types of Chinese Yellow Tea
## 3 Results and Discussion
### Volatile Profile in Different Aroma Types of Yellow Tea  
The volatile profiles of different aroma types of yellow tea were analyzed by HS-SPME/GC-MS with external standard method (Table S3). A total of 103 volatile compounds were identified and quantified in the different aroma types of yellow tea (Table 1), and 101, 91, 90, and 97 volatile compounds were identified in the floral fragrance, high-fired fragrance, fresh fragrance, and corn-like fragrance types of yellow tea, respectively. These identified volatile compounds belong to seven categories, including 16 alcohols, 2 phenols, 13 nitrogenous compounds, 10 aldehydes, 33 hydrocarbons, 13 ketones, and 16 esters. As shown in Figure S1A, alcohols (33.48%) and aldehydes (21.90%) were the predominant categories in the floral fragrance type of yellow tea, and (\(Z\))-linalo oxide (furanoid), (\(E\))-linalo oxide (furanoid), linalool oxide (pyranoid), hotrienol, (\(Z\))-citral, and citral were the most abundant volatiles compared with the other aroma types of yellow tea (Figure S1B). Indeed, the alcohols and aldehydes have been reported to provide 'floral' and 'fruity' attributes, which have a good coordinating effect on the aroma profiles of tea products [6; 22]. Nitrogenous compounds (60.10%) and hydrocarbons (17.27%) were the main categories in the high-fired fragrance type of yellow tea (Figure S1A). Among them, 1-ethyl-1H-pyrrole, 2,4-dimethyl-3-ethylpyrrole, 2,5-dimethyl-3-ethylpyrazine, 2-methyl-5-ethylpyrazine, methyl anthranilate, and p-xylene were the most abundant volatile compounds compared with the other aroma types of yellow tea (Figure S1B). These pyrroles, pyrazines, and their derivatives are usually generated from the Maillard reaction during roasting and baking treatments, which are thought to contribute to the formation of the 'roasted' aroma [1; 7]. Esters (45.06%) and alcohols (28.03%) were the dominant categories in the fresh fragrance type of yellow tea (Figure S1A). Among them, ethyl hexanoate, ethyl nonanoate, and (E)-3-hexen-1-ol were the most abundant chemical structures, compared with the other aroma types of yellow tea (Figure S1B). Esters have been reported to play an important role in the fresh aroma characteristics formation of yellow tea [6]. Figure 1: Flavor wheel of yellow tea (**A**) and radar of sensory aroma notes profile among the four aroma types of yellow tea: (**B**) ‘floral fragrance’ type. (**C**): ‘high-fired fragrance’ type. (**D**): ‘fresh fragrance’ type. (**E**): ‘corn-like fragrance’ type. Hydrocarbons (21.68%) and ketones (11.62%) were the dominant categories in the corn-like fragrance type of yellow tea (Figure S1A). Compared with the other aroma types, styrene, (_E_)-\(\beta\)-ocimene, \(\beta\)-cedrene, toluene, mestilp oxide, 2,3-octanedione, and \(\beta\)-ionone were the most abundant in the corn-like fragrance type (Figure S1B). Hydrocarbons and ketones with'sweet', 'floral', and 'fruity' aroma notes, which are found to dominate the volatile compounds of black tea and are considered to provide a unique aroma for black tea [23,24]. The results showed that there were significant differences in the composition of volatile compounds of yellow teas with different aroma types. To investigate the similarities and differences of volatile compounds among the four aroma types of yellow teas, PCA and HCA were performed based on the quantitative results of the identified volatile compounds. The results of PCA (the total variance of model was 89%) (Figure 2A) and HCA (Figure 2B) showed that the yellow teas could be divided into four groups according to their aroma types, which indicated that there were significant differences in the volatile profile among the four aroma types of yellow teas. Based on these results, six OPLS-DA models were established to investigate the discriminatory volatile compounds (Figure S2).