## 2 Results and Discussion
### Phenolic Antioxidants and Structure-Activity Relationships in Goji Juice
#### 2.3.3 Free Radical Scavenging Activity and Structure-Activity Relationships  
The antioxidant capacities of the goji juice samples were determined using two methods based on different principles comprising the 2,2-diphenyl-l-picrylhydrazyl radical (DPPH) and the hydroxyl radical assay. The free radical scavenging rates in the juice samples ranged from \(58.52\pm 3.28\%\) to \(80.21\pm 1.17\%\) and \(70.48\pm 5.65\%\) to \(82.39\pm 5.94\%\) according to the DPPH and hydroxyl radical assays, respectively (Figure 3). The DPPH free radical scavenging rate increased with fermentation (Figure 3a). There were no obvious differences in the DPPH free radical scavenging rates among the three fermentation mixtures (\(p>0.05\)), suggesting that fermentation promoted the release of antioxidant compounds. In addition, some bacterial strains have antioxidant capacities. For example, [PERSON] et al. [36] found that \(L\). _nhamnosus_ had a high DPPH free radical scavenging ability. In the present study, each fermentation mixture contained \(L\). _nhamnosus_ and the presence of this strain may have improved the DPPH free radical scavenging activity of the fermented juice. [PERSON] et al. [37] reported that the extracellular polysaccharide produced by _L. plantarum_ exhibited an antioxidant activity, where the DPPH free radical scavenging rate of the purified exopolysaccharide was more than 80%. The hydroxyl radical scavenging rate of the raw goji juice was 82.39% (Figure 3b). There were no major differences in the hydroxyl radical scavenging rates among the fermented and raw juice samples. [PERSON] et al. [PERSON] _et al._ (2018) also obtained similar results after fermentation using a combination of lactic acid bacteria containing _L. plantarum_ and _L. rhamnus_, which did not increase the hydroxyl radical scavenging rate in yam juice. These strains might not have altered the substances in the goji juice that act on hydroxyl radicals. Thus, fermentation did not enhance the hydroxyl radical scavenging rate of the goji juice, but the fermented juice samples still had high rates. The fermentation of the goji juice with each mixture clearly increased the DPPH free radical scavenging rate, and high hydroxyl radical scavenging rates were maintained, possibly due to the presence of different phenolic compounds that act on different free radicals. Different antioxidant indexes were correlated with the concentrations of the free and bound forms of phenolic compounds (Table 3). Among all of the phenolic compounds detected in the present study, the free _p_-hydroxybenzoic acid, chlorogenic acid, ferulic acid, rutin, and naringenin had high positive correlations with the DPPH free radical scavenging rate. The concentrations of these free fractions in the fermented goji juices were higher than those in the unfermented goji juice, which may explain why the DPPH free radical scavenging rates were higher in the fermented juices. However, among all of the bound forms of the phenols detected, only that of _p_-coumaric acid was positively correlated with the DPPH free radical scavenging rate. Our results agree with those reported by [PERSON] et al. [PERSON] _et al._ (2018), who demonstrated the major contribution of the bound form of _p_-coumaric acid to the antioxidant capacity. None of the phenolic compounds detected in the present study had high positive correlations with the hydroxyl radical scavenging rate, except for the bound form of chlorogenic acid. The poor correlations between the phenolic substances and hydroxyl radicals may be related to the lack of obvious differences in the hydroxyl radical scavenging rate among the four samples. The antioxidant activities were affected by the molecular structure of phenols [PERSON] _et al._ (2018) and phenolic compounds have different sites that are active with diverse free radicals. Thus, the structures of the phenolic compounds that act on hydroxyl radicals might not have changed during the fermentation process.