## 3 Results
### Effect of Withering Time on Nonvolatile Compounds  
A total of 67 various metabolites were identified (Table S2) using MS\({}^{2}\) spectra, accurate mass, and standard compounds. Identified metabolites included 21 flavonoid/flavonoid glycosides, 4 theaflavins, 7 catechins, 8 glycosidically bound volatiles, 17 amino acids, 7 organic acids, and 3 alkanoids. A heatmap was used to visualize the fluctuations in relative content among treatments (Figure 1). The catechins content declining with the increase in withering time and overall content demonstrated a significant (\(p\) < 0.05) decrease in the following order: WT4 (13.47 mg/g) \(>\) WT6 (12.98 mg/g) \(>\) WT8 (12.73 mg/g) \(>\) WT10 (12.51 mg/g) \(>\) WT12 (11.69 mg/g) (Table S2). During withering, the relative content of amino acids significantly (\(p\) < 0.05) increased with withering time (Figure 2e). Figure 1 and Table S2 indicate the changes in the composition of free amino acids. A total of 17 amino acids were detected, and 13 of them showed a significant (\(p\) < 0.05) increase, including L-tryptophan, L-valine, L-leucine, L-proline, L-phenylalanine, L-tyrosine, L-isoleucine, L-glutamic acid, and GABA, while three of them (L-glutamine, L-theanine, and L-arginine) decreased significantly (\(p\) < 0.05) as the withering time increased. All detected amino acids were proteinogenic amino acids, except L-theanine and GABA. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Treatments** & & **Aroma** & **Liquor Color** & **Taste** \\ \hline \multirow{3}{*}{WT4} & Parameter & 0.686 & 0.181 & 1.070 \\  & odds ratio & 3.216 & 0.243 & 7.272 \\  & Pr \(>\) X\({}^{2}\) & 0.072 & 0.621 & 0.008 \\  & Parameter & 0.683 & 0.599 & 0.744 \\ WT6 & odds ratio & 3.319 & 1.908 & 3.135 \\  & Pr \(>\) X\({}^{2}\) & 0.068 & 0.167 & 0.076 \\  & Parameter & 1.00 & 1.203 & 1.682 \\ WT8 & odds ratio & 3.696 & 5.486 & 5.420 \\  & Pr \(>\) X\({}^{2}\) & 0.054 & 0.019 & 0.019 \\  & Parameter & 1.097 & 1.610 & 1.153 \\ WT10 & odds ratio & 3.359 & 5.022 & 7.155 \\  & Pr \(>\) X\({}^{2}\) & 0.066 & 0.025 & 0.007 \\  & Parameter & 2.223 & 0.682 & 1.471 \\ WT12 & odds ratio & 6.431 & 1.663 & 6.554 \\  & Pr \(>\) X\({}^{2}\) & 0.011 & 0.197 & 0.010 \\ \hline \hline \end{tabular} Note: Based on logistic regression analysis, with three sensory attributes. Parameter estimates were obtained by using analysis of maximum likelihood estimates. The significance of parameter estimates was determined with reference to the Wald X\({}^{2}\) value at (\(p\) < 0.05).  
\end{table} 
Table 2: Parameter estimates, probability, and odds ratio estimates. All individual flavonoid glycosides/flavonoid compounds detected decreased with withering time, except quercetin-7-O-\(\beta\)-D-glucopyranoside, myricetin 3-O-galactosidase, iso-vitexin 2\"-O-arabinoside, glucosyl-vitexin, and quercetin-3-o-rutinose, which increased with withering time. Four compounds (kaempfer, quercetin, quercetin-7-O-\(\alpha\)-L-rhammoside, and procyanidin B2) were the main contributors to the total relative flavonoid glycosides content detected. The relative content of total flavonoid glycosides significantly (\(p<0.05\)) reduced with withering time: WT4 (4.27 mg/g) > WT6 (4.18 mg/g) > WT8 (4.10 mg/g) > WT10 (4.03 mg/g) > WT12 (3.89 mg/g) (Table S2). After withering, the seven organic acids detected included p-cumaric acid, salicylic acid, gallic acid, D- (-)-quinic acid, and chlorogenic acid, all of which were increased, while caffeic acid and shikimic acid decreased with an increase in withering time (Figure 1 and Table S2). The cumulative organic acid content decreased significantly (\(p<0.05\)) up to WT6 and then kept reducing insignificantly (\(p>0.05\)), knowing that organic acid imparts the sour taste in the balance of the complex characteristics of black tea. The relative total flavonoid content in black tea decreased from WT4 (0.67%) to WT12 (0.39%) and the highest decrease was observed between the first and the second treatments, by comparing two successive treatments (Figure 2b).