# Characterization of the Key Aroma Compounds in Different Aroma Types of Chinese Yellow Tea
## 1 Introduction  
Tea is a kind of widely consumed beverage ranking second only to water around the world [1]. Yellow tea is one of the six major tea categories in China, which is famous for its unique appearance and flavors, such as yellow dry tea, yellow infusion, and yellow brewed leaves that have an elegant fragrance and smooth taste. Meanwhile, yellow tea also possesses several healthy benefits for humans, such as anti-cancer, anti-bacterial, antioxidant, and gastrointestinal protecting effects [2, 3, 4]. The main manufacturing process of yellow tea are fixing, rolling, sealed yellowing, and drying. Among them, sealed yellowing is the key processing stage that contributes to the typical flavor characteristic formation of yellow tea [2]. Although yellow tea is increasingly deeply loved by consumers, its key aroma compounds that responsible for the special aroma formation are still unknown. It is difficult to establish flavor directed processing technology for yellow tea. Therefore, the study of yellow tea, particularly, research on its key aroma compounds for this special aroma, has important value for improving the flavor quality of yellow tea during the manufacturing process. Tea aroma is a critical criterion to evaluate the overall quality of yellow tea [4]. Many aroma components were identified in yellow tea by previous studies, but different yellow tea possesses a wide range of key aroma compounds. This might result from the differentraw materials, processes, and parameters of different yellow teas. The detailed information is summarized in Table S1. Our previous study indicated that different levels of tenderness of tea leaves will form the different characteristic aromas and key aroma compounds in yellow tea. Phenylacetaldehyde was identified as the key aroma compound in the bud-type yellow tea, due to its special fresh aroma; Ethyl hexanoate, benzyl alcohol, geraniol, phenethyl alcohol, citral, neral, and myrcene were identified as major contributors in bud-leaf yellow tea due to their flowers, fruity and sweet aroma notes; 2-pyrrole formaldehyde, 3-ethyl-2,5-dimethylpyrazine, 2-ethyl-5-methylpyrazine, and 2,3-diethyl-5-methylpyrazine are the key aroma compounds in multi-leaf yellow tea due to their roasted aroma notes [5]. Another study also revealed that key aroma compounds, including (_E,E_)-3,5-octalen-2-one, (_Z_)-linalo oxide (furanoid), (_E_)-2-heptenal, naphthalene, geraniol, (_E_)-linalo oxide (furanoid), styrene, linalool, \(\alpha\)-ionone, 1-octen-3-ol, (_E_)-\(\beta\)-ionone, and (_E_,_Z_)-3,5-octalen-2-one, presented great differences in the different tenderness levels of yellow tea [6]. In addition, the different manufacturing processes can significantly affect the aroma profile of yellow tea and lead to the different characteristics of aroma in yellow tea. A previous study revealed that the high temperature roasting process was essential for special flavor formation of yellow teas with strong 'roasted', 'nutty', and 'wo woody' odors, as well as weak 'fatty' and 'fruity' odors [7]. This was also the key process for unique 'crispy-rice-like' odor formation in yellow tea [8]. In addition, different temperatures of roasting processes could result in different aroma characteristics of yellow tea, causing odors such as 'fresh', 'tender corn', 'ripe corn', and so on [9]. The degree of yellowing process significantly affects the composition of the aroma profile in yellow tea [10]. In summary, it is reasonable to hypothesize that different raw materials and processing technologies will lead to different compositions of the aroma profile, as well as cause different types of characteristic aromas in yellow tea. However, up to now, there are scarcely any systematic comparative study focused on the composition of key aroma compounds in different aroma types of yellow tea.