## 3 Results  
### Changes in Volatile Compounds during Fermentation by A. niger PW-2  
After screening based on their MS data and retention indices, 57 volatile compounds, including 5 alcohols, 28 hydrocarbons, 4 aldehydes, 10 esters, 6 ketones, 2 phenols and 2 acids, were identified in all samples during the whole fermentation process (Figure 2A, Table S1). As shown in Figure 2B, during the fermentation process, the total contents of volatile compounds initially increased from 5599.97 mg/kg (Day 0) to 11,218.83 mg/kg (Day 3) and declined to 4182.77 mg/kg at the end of fermentation (Day 21). The change trends of alcohol, hydrocarbon, aldehyde, ester, ketone and acid contents were consistent with the trends of total volatile contents, except for volatile phenol contents, which declined during  
Figure 2: Information of the volatile compounds for the tea samples fermented by _Aspergillus niger_ PW-2. (**A**) Heatmap representing the semiquantitative results (a), chemical classes (b), VIP values (c) and \(p\) values (d) of all volatile compounds (NS, \(p>0.05\); *, \(0.01<p<0.05\); **, \(0.001<p<0.01\); ***, \(p<0.001\)). (**B**) Volatile contents of different classes in samples during fermentation. (**C**) Score scatter plot of PLS-DA analysis based on volatile compounds of all samples. (**D**) Loading scatter plot of OPLS-DA analysis based on volatile compounds of samples on Day 0 and Day 21.  
the whole fermentation process. Alcohols, which accounted for 37.17-66.28% of the total volatile compounds, were the most abundant class of volatile compounds in teas during all fermentation stages, followed by hydrocarbons, which accounted for 9.77-33.88% of the total volatile compounds. Linalool was the most abundant volatile compound. PLS-DA score plots (Figure 2C) showed that samples on Day 12, Day 15 and Day 18 were clustered together and were distinguishable from samples at other fermentation stages. Principal components 1 and 2 explained 68% and 12.9% of the total variance, respectively. The loading plots (Figure 2D) and VIP values (Figure 2A, Table S1) obtained from OPLS-DA analysis of samples on Day 0 and Day 21 were used to identify the differential volatiles. The heatmap representing the volatile data and information of classes, VIP values and \(p\) values are illustrated in Figure 2A, and the detailed data are shown in Table S1. Volatile compounds with VIP \(>\) 1 and \(p\) < 0.05 (Figure 2A, Table S1) were considered differential compounds, and these compounds included 1-octen-3-ol, linalool, geraniol, \(L\)-\(\alpha\)-terpineol, (\(E\))-linalool oxide (furanoid), 2,2,4,6,6-pentamethyl-heptane, decanal, tetradecane, benzaldehyde, nonanal, ethyl palmitate, (\(Z\))-geranylacetone, 2,4-di-tert-butylphenol and dodecane. Among these differential volatile compounds, the contents of 1-octen-3-ol, (\(E\))-linalool oxide, 2,2,4,6,6-pentamethyl-heptane, nonanal, decanal and ethyl palmitate increased (\(p\) < 0.05) after the 21-day fermentation (Figure 2A). By contrast, the contents of linalool, geraniol, \(L\)-\(\alpha\)-terpineol, tetradecane, benzaldehyde and (\(Z\))-geranylacetone significantly decreased (\(p\) < 0.05) after the same fermentation period. and umami amino acids. During fermentation, the contents of threonine, glycine and alanine increased by 42.29%, 3.3-fold and 62.30%, respectively, whereas those of serine and proline decreased by 20.61% and 62.15%, respectively. Among all bitter amino acids, including valine, isoleucine, leucine, tyrosine, phenylalanine, lysine and arginine, lysine was the most abundant bitter amino acid in the finished sample; its content significantly increased by 4.36-fold during fermentation. In addition, the contents of isoleucine and leucine slightly increased by 11.09% and 14.26%, respectively, whereas those of valine, tyrosine, phenylalanine and arginine decreased by 26.96%, 22.18%, 37.58% and 50.02%, respectively.