## 2 Results and Discussion
### Phenolic Antioxidants and Structure-Activity Relationships in Goji Juice
#### 2.3.2 Extractable and Bound Individual Phenolics  
Phenolics are among the most frequently studied bioactive compounds in fermented juices. In the present study, we identified 10 bioactive phenolics comprising five phenolic acids and five flavonoids by high-performance liquid chromatography in the free and bound extracts from the goji juice samples (Table 2). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multicolumn{2}{c}{**Phenolics**} & \multicolumn{1}{c}{**CK**} & \multicolumn{1}{c}{**SLV**} & \multicolumn{1}{c}{**SZP**} & \multicolumn{1}{c}{**SZVP**} \\ \hline \multicolumn{2}{c}{**Phenolic Acids**} & & & & \\ \hline \multirow{3}{*}{\(p\)-Hydroxybenzoic acid} & Free & \(4.24\pm 2.24\)\({}^{\text{c}}\) & 9.09 \(\pm\) 0.52\({}^{\text{b}}\) & 6.44 \(\pm\) 2.97\({}^{\text{b}}\) & 20.04 \(\pm\) 0.54\({}^{\text{a}}\) \\  & \multirow{2}{*}{Bound} & 1.81 \(\pm\) 0.55\({}^{\text{a}}\) & -\({}^{\text{b}}\) & -\({}^{\text{b}}\) & -\({}^{\text{b}}\) \\  & & Total & 6.05 \(\pm\) 2.79\({}^{\text{b}}\) & 9.09 \(\pm\) 0.52\({}^{\text{b}}\) & 6.44 \(\pm\) 2.97\({}^{\text{b}}\) & 20.04 \(\pm\) 0.54\({}^{\text{a}}\) \\ \hline \multirow{3}{*}{Chlorogenic acid} & Free & 0.99 \(\pm\) 0.02\({}^{\text{b}}\) & 1.10 \(\pm\) 0.06\({}^{\text{b}}\) & 1.32 \(\pm\) 0.67\({}^{\text{b}}\) & 2.41 \(\pm\) 0.10\({}^{\text{a}}\) \\  & Bound & 0.18 \(\pm\) 0.04\({}^{\text{b}}\) & -\({}^{\text{c}}\) & 0.21 \(\pm\) 0.00\({}^{\text{a}}\) & -\({}^{\text{c}}\) \\  & Total & 1.17 \(\pm\) 0.06\({}^{\text{b}}\) & 1.10 \(\pm\) 0.06\({}^{\text{b}}\) & 1.53 \(\pm\) 0.67\({}^{\text{b}}\) & 2.41 \(\pm\) 0.10\({}^{\text{a}}\) \\ \hline \multirow{3}{*}{Caffeic acid} & Free & 0.62 \(\pm\) 0.23\({}^{\text{a}}\) & 0.95 \(\pm\) 0.27\({}^{\text{a}}\) & 0.91 \(\pm\) 0.32\({}^{\text{a}}\) & 0.48 \(\pm\) 0.01\({}^{\text{b}}\) \\  & Bound & 0.72 \(\pm\) 0.24\({}^{\text{b}}\) & 0.51 \(\pm\) 0.01\({}^{\text{c}}\) & 0.98 \(\pm\) 0.02\({}^{\text{a}}\) & 0.94 \(\pm\) 0.02\({}^{\text{aAll of the goji juice samples contained five types of phenolic acids, comprising \(p\)-hydroxybenzoic acid, chlorogenic acid, caffeic acid, \(p\)-coumaric acid, and ferulic acid. In the raw goji juice and fermented goji juice samples, the contents of the free forms of \(p\)-hydroxybenzoic acid, chlorogenic acid, and \(p\)-coumaric acid were higher than those of the bound forms. Compared with the raw goji juice, the concentrations of the free forms of \(p\)-hydroxybenzoic acid and chlorogenic acid were increased by 11.1-372.64% in the fermented juice samples. The bound forms of \(p\)-hydroxybenzoic acid and chlorogenic acid were not detected in the juices fermented with SLV and SZVP. Fermentation can promote the hydrolysis of chlorogenic acid to yield caffeic acid [5]. The concentration of the free form of \(p\)-coumaric acid was increased to 1.81 times with SLV, whereas almost none of the free form of \(p\)-coumaric acid was detected in the other two fermented samples, possibly due to the presence of \(L\). _plantarum_ in SZP and SZVP. It was found that phenolic acid decarboxylase metabolized \(p\)-coumaric acid into phloretic acid or \(p\)-hydroxybenzoic acid. _Lactobacillus plantarum_ contains the \(p\)-coumarate decarboxylase gene [33]. In addition, the concentrations of the bound forms of caffeic acid and ferulic acid were higher in the raw goji juice compared with the free forms. The concentration of the free form of caffeic acid increased after fermentation with SLV and SZP, possibly due to the hydrolysis of chlorogenic acid. However, the concentration of the bound form of caffeic acid decreased by 1.29 times with SLV, suggesting the further bacterial metabolization of caffeic acid [5]. Compared with the raw goji juice, the concentrations of the free from of ferulic acid increased by 13.23-14.77 times in the fermented juice samples. Fermentation may have promoted the structural breakdown of plant cell walls, thereby leading to the increases in the concentration of free phenolic compounds [34].