## 4 Discussion  
In the present study, the levels of most metabolites first increased and then decreased. One possible reason for this observation is that during the fermentation by _A. niger_ PW-2, enzymes such as cellulases, \(\alpha/\beta\)-glucosidases and pectin lyase, which could degrade plant polysaccharides, were secreted, resulting in the lysis of the cell walls of tea leaves, causing the release of more metabolites. Oligosaccharide produced from the degradation of polysaccharides might be responsible for the mellow taste of fermented tea [10]. Additionally, monosaccharides might enter into glycolysis, acting as a flux of the carbon supply for secondary metabolisms, such as purine and amino acid metabolisms [24]. Compared to pan-fired green tea, steam-treated green tea has lower levels of geraniol, and linalool and its derivatives [13]. Among all differential volatiles, linalool, geraniol, _L-\(\alpha\)_-terpineol and (_E_)-linaloid oxide (furanoid), which give a floral aroma, were generated from the aroma precursors by microbial GHs (Table S6) [6]. 1-Octen-3-ol with an intense, persistent mushroom-like odor was derived from the oxidative degradation of linoleic acid. Nonanal (fatty, citrus-like, green), decanal (soap, orange peel, tallow) and hexadecanoic acid ethyl ester (cheese-like) originated from the oxidative degradation or esterification of fatty acids, while benzaldehyde (fragrant, sweet and almond aroma) originated from the oxidation of amino acids (Figure 6A) [25; 26]. Alcohol dehydrogenase and aldehyde dehydrogenases identified in tea samples fermented by _A. niger_ PW-2 (Table S4) might be involved in the metabolism of aldehydes. The detected differential volatile hydrocarbons included dodecane, tetradecane and 2,2,4,6,6-pentamethyl-heptane. According to their numbers, hydrocarbons were the most abundant compounds; however, they contribute little to the aroma because they provide only a few types of odors. Generally, saturated hydrocarbons do not contribute to the aroma of tea, while unsaturated hydrocarbons play an important role in contributing to the flavor of tea [27]. Phenols, including phenolic glycosides, catechins and gallic acid, are the main source of astringency in teas [28]. As a previous study reported [4], the stringent scores of theabrownins at different concentrations were the same as the tasteless scores. The decrease in catechin and gallic acid contents and the increase in theabrowninin contents might lead to the reduction in astringency and asrtingent aftertaste (Figure 4C,D) [28]. As illustrated in the proposed metabolic pathway of tea phenols (Figure 5A), the hydrolysis of the glycosidic bonds in phenolic glycosides by GHs produces phenolic aglycone [29]. Some phenols such as gallates can be hydrolyzed by tannases, by which the ester linkages are broken down to release small molecular phenols (e.g., catechol) and phenolic acids (e.g., gallic acid) (Figure 5A) [30]. As shown in Figure 3E, galloyl catechins EGCG and ECG were steadily degraded; their contents were reduced to nearly zero on Day 9. The contents of gallic acid and non-galloyl catechins (EGC and EC) increased during the first three or six days and then decreased until the end of fermentation. It is possible that galloyl catechins were degraded to non-galloyl catechins and gallic acid by the hydrolysis led by extracellular enzymes during the early fermentation stage, and non-galloyl catechins and gallic acid were further degraded to other simpler compounds such as pyrogallol, catechol and phenolic acids [1]. The level of metallo-dependent hydrolase (Figure 5C, Table S7) showed a decreasing trend in a similar fashion to that of gallbladder catechins (EGCG and ECG), which may be related to the increase in gallic acids and non-galloylated catechins (EGC and EC) during the first three days of fermentation. The levels of most CODs (Figure 5C) obviously increased during the first 3 days of fermentation and then decreased, similar to the same trends of gallic acid, EGC and EC contents.