## Materials and Methods
### Antioxidant Activity and the Contents of Total Phenolic, Total Flavonoid, and Dietary Fibers  
The total antioxidant capacity was determined using the ferric reducing ability of plasma (FRAP) assay in accordance with the instructions of the T-AOC Kit (SolarBio, Beijing, China). Before the test, the R-B mixed juice before and after fermentation was centrifuged at 2500\(\times\)\(g\) for 10 min to collect the supernatant for FRAP assay. The absorbance was determined at 593 nm, and the results were expressed as umol Fe\({}^{2+}\) equivalents/100 mL [23].  
The iron ion-reducing ability was determined by the Prussian blue method using the Texting Kit (Congyi Biology, Shanghai, China) [24]. The reduction ability of iron ions was expressed by the absorbance value at 700 nm, whereby a larger absorbance value denoted a stronger reduction ability of the R-B mixed juice to iron ions. The supernatant was ninefold diluted with ultrapure water for Prussian blue analysis, and the antioxidant Trolox (0.04 mM) was used as a positive control.  
The hydroxyl radical-scavenging activity was determined using the Micro Hydroxyl Free-Radical Scavenging Capacity Assay Kit (SolarBio, Beijing, China). The inhibition degree of the 536 nm absorbance decline rate (%) of the R-B mixed juice was used to reflect its ability to scavenge hydroxyl radicals [25]. Ascorbic acid (0.075 mg/mL) was used as a positive control.  
The superoxide anion-scavenging ability was determined using the Inhibition and Produce Superoxide Anion Assay Kit (Jiancheng Bioengineering Institute, Nanjing, China). The scavenging ability was expressed by the absorbance value at 550 nm, whereby a lower absorbance value denoted a stronger scavenging ability of the R-B mixed juice to superoxide anion. The superoxide anion scavenging ability of the R-B mixed juice was calculated with ascorbic acid as the standard (U/100 mL).  
To determine total phenolic content, the R-B mixed juice before and after fermentation was ninefold diluted with ultrapure water, followed by placing 2 mL of the diluent and 1 mL of Folin-phenol solution in a volumetric flask. After incubation for 6 min, the mixture was supplemented with 1 mL of 20% sodium carbonate solution and incubated at 30 \({}^{\circ}\)C for 1 h. The absorbance was determined at 760 nm using a UV/visible spectrophotometer, with distilled water as the blank control. Finally, the total phenolic content was calculated on the basis of a gallic acid calibration curve (\(R^{2}\) = 0.9996) [26; 27; 28].  
To determine total flavonoid content, the R-B mixed juice before and after fermentation (1 mL) was supplemented with 0.5 mL of sodium nitrite solution (5 g/100 mL) in a volumetric flask, followed by incubation for 6 min and the addition of 0.5 mL of aluminum nitrate solution (10 g/100 mL). After incubation for 6 min, the mixture was supplemented with 4 mL of sodium hydroxide solution (4 g/100 mL) and then thoroughly mixed for 1 min. The absorbance was determined at 510 nm using a UV/visible spectrophotometer, with distilled water as the blank control. The total flavonoid content was calculated on the basis of a rutin calibration curve (\(R^{2}\) = 0.9989) [29].  
The contents of total, insoluble, and soluble dietary fibers in the R-B mixed juice before and after fermentation were determined by the AOAC methods [30].