Optimization of the Theabrownins Process by Liquid Fermentation of Aspergillus niger and Their Antioxidant Activity

Theabrownins are macromolecular compounds with many hydroxyl, carboxyl, and phenolic functional groups. They are usually extracted from dark tea, in which they are present at low concentrations. In this paper, a low-cost microbial liquid fermentation method was established by inoculating Aspergillus niger with minced tea extract as the raw material. After applying the Box–Behnken and response surface approach design, the optimum fermentation conditions in the fermentor were determined to be an 8% (80 g/L) sucrose concentration, 1:31 (0.03226 g/mL) solid–liquid ratio, 4.141 × 106 CFU/mL bacterial liquid concentration, 5 d fermentation time, 28 °C fermentation temperature, 187 r/min (rpm) rotation speed, and an oxygenation of 0.5 V/V·min (V). After fermenting about 168 h, the theabrownins content reached the maximum of 28.34%. The total phenolic content of the liquid-fermented theabrownins was 25.74% higher than that of solid fermentation. Acidic functional groups were determined, indicating that the phenolic hydroxyl groups were the main acidic groups of the theabrownins. The antioxidant activity of theabrownins was verified by measuring the potassium ferricyanide reducing power, hydroxyl radical scavenging rate, superoxide anion radical scavenging rate, and 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging rate of solid- and liquid-fermented chabein. The results of this study show that the production of theabrownins by the liquid fermentation of Aspergillus niger is fast, high in yield, and has antioxidant activity, which provides a basis for industrial production of theabrownins.