## 4 Determination of the Chemical Composition of Theabrownins
### Determination of the Antioxidant Activity of Theabrownins  
4.4.1. Determination of the Reducing Power of Potassium Ferricyanide  
Phosphate buffer solution (2.5 mL, pH 6.6, 0.2 M) and 2.5 mL of 1% (\(w/v\)) potassium ferricyanide solution were added to 1 mL of solid-state fermented theabrownins, liquid-ferremented theabrownins, and VC, respectively, and the reaction was carried out in a water bath at 50 \({}^{\circ}\)C for 20 min [35]. Then, it was quickly cooled, 1 mL of 10% trichloroacetic acid was added, and the mixture was centrifuged at 3000\(\times\)\(g\) r/min for 10 min. Distilled water (2.5 mL) and 0.5 mL of 0.1% ferric chloride were added to 2.5 mL of the supernatant, mixed well, and then allowed to stand for 10 min [36]. The absorbance was measured at 700 nm.  
4.4.2. Hydroxyl Radical (OH) Scavenging Ability of Theabrownins  
In total, 2 mL of FeSO\({}_{4}\) solution, 2 mL of H\({}_{2}\)O\({}_{2}\) solution (6 mM), and 2 mL of salicylic acid solution (6 mM) were added to 1 mL of solid-state fermented theabrownins solution, 1 mL of liquid-fermented theabrownins solution, and 1 mL of VC solution, respectively. The mixtures were centrifuged at 5000\(\times\)\(g\) r/min for 5 min after standing for 30 min. Then, the absorbance \(A_{1}\) at 510 nm was measured. A blank control was set up, and the absorbance \(A_{0}\) was measured. The same volume of distilled water was used to replace the salicylic acid solution, and the absorbance \(A_{2}\) of the sample solution was measured [37; 38]. This was calculated as follows:  
\[\text{Hydroxyl radical scavenging activity (\%)}=\frac{A_{0}-(A_{1}-A_{2})}{A_{0}} \times 100\%\]  
where \(A_{0}\) is the absorbance of the blank, \(A_{1}\) is the absorbance of the sample, and \(A_{2}\) is the absorbance of the sample in the system.  
4.4.3. Superoxide Anion (O\({}_{2}\)\({}^{-}\)) Scavenging Ability of Theabrownins  
Tris-HCl buffer solution (4.5 mL, pH 8.2, 50 mM) was preheated in a water bath at 25 \({}^{\circ}\)C for 20 min. Then, the preheated solution was added to 1 mL of solid-state fermented theabrownins solution, 1 mL of liquid-fermented theabrownins solution, and 1 mL of VC solution, respectively. Then, 0.4 mL of pyrogallol solution (25 mM) was added, mixed well, and reacted in a 25 \({}^{\circ}\)C water bath for 5 min. Then, 1 mL of HCl (8 M) solution was added to stop the reaction, and the absorbance \(B_{1}\) at 299 nm was measured. The same volume of distilled water was used to replace the pyrogallol solution, and the absorbance \(B_{2}\) of the sample solution was measured [39; 40]. This was calculated as follows:  
\[\text{Superoxide anion clearance (\%)}=\frac{B_{0}-(B_{1}-B_{2})}{B_{0}} \times 100\%\]  
where \(B_{0}\) is the absorbance of the blank, \(B_{1}\) is the absorbance of the sample, and \(B_{2}\) is the absorbance of the sample in the system.  
4.4.4. Theabrownins Scavenging DPPH Free-Radical Ability  
A DPPH free-radical solution (2 mL; 2 \(\times\) 10\({}^{-4}\) M) prepared with anhydrous ethanol was added to 2 mL of the solid-state fermented theabrownins solution, liquid-fermented theabrownins solution, and VC solution. The mixtures were reacted for 30 min in the dark, and their absorbance at 518 nm \(C_{1}\) was measured. To eliminate the sample's influence, 2 mL of the sample and 2 mL of absolute ethanol mixture were used to measure the absorbance, \(C_{2}\), [41; 42]. The calculation formula is as follows:  
\[\text{DPPH free radical scavenging rate}\left(\%\right)=\left(1-\frac{C_{1}-C_{2}}{C_{0}} \right)\times 100\]  
where \(C_{0}\) is the absorbance of the blank, \(C_{1}\) is the absorbance of the sample, and \(C_{2}\) is the absorbance of the sample in the system.