Preparation of instant dark tea by liquid-state fermentation using sequential inoculation with Eurotium cristatum and Aspergillus niger: Processes optimization, physiochemical characteristics and antioxidant activity

Preparation of instant dark tea (IDT) by liquid-state fermentation (LF) is an effective procedure, of which the process is more easily controlled than solid-state fermentation (SSF). The present study focused on developing a sequential inoculation liquid-state fermentation (SILF) procedure using the Eurotium cristatum and Aspergillus niger to prepare IDT. The key parameters (turbidity, redness, and tea pigments) were optimized by response surface methodology (RSM). Optimum conditions were modeled to be a water-tea ratio of 24.0 mL/g, inoculum size of 8.7% (v/v), and time of 9 d. The predicted and experimental values of redness, turbidity and tea pigments content were within a 5% difference using the optimized conditions. Bench scale was achieved in a 5-L fermenter, which generated IDT with a redness value of 43.50, a turbidity of 5.67 NTU and tea pigments content of 23.53%, w/w. The evaluation by E-nose and E-tongue showed that IDT had a similar aroma and mouthfeel with a commercial instant Fu-brick tea (IFT), and the antioxidant activities in vitro, color and clarity of tea infusion of IDT were higher than those of the IFT, respectively. These results indicate that preparation of IDT by SILF is feasible, this work expect to set up a new aspect for preparation of IDT.