## 4 Discussion
### Analysis of Response Surface Plot  
3D response surface and 2D contour plots established by the Design-Expert software were observed to explore the effects of parameters (the extraction time, the extraction temperature, and the ethanol concentration) and their interactions on the TPC. Figure 2A,B and Figure 3A,B suggest the significant interactions existed between the extraction temperature and the extraction time or the ethanol concentration on TPC extraction due to an elliptical contour plot indicated the significant interaction between these parameters [9]. However, there were some abnormal points existed in Figures 2C and 3C that stunted the analysis of the relationship between the ethanol concentration and extraction time on the TPC extraction. The likely causes of this condition were; the inaccuracy of some instruments (balance or cylinder) that could distort the calculation of results. Also, during the sample preparation, cut black goji berry into pieces by shear or hand cannot produce exactly same sample and there existed some hollow berries, which were not abandoned, that would influence the results of experiments. Normally, if the requirements for sample homogeneity and sensitivity are met, the small sample size is preferred [19]. Besides, the amount of drying agent (diatomite) used in the extraction pool should be controlled at a basically same level. Otherwise, an insufficient amount of diatomite could not dry the sample completely and improve the dosage of extraction solvent in the whole process. Thereby, the volume of extract would be higher causing wrong results. Moreover, there was quite a high content of bound-water in black goji berry. In this situation, no matter what dry agent is used, the procedure that sodium sulfate should be added in a sample bottle and then passed through a dry column or drying silo to dry sample thoroughly. Although little problems existed in these 3D response surface plots, some conclusions could be inferred based on the results gathered from  
\begin{table}
\begin{tabular}{l c c c c c c} \hline \hline
**Antioxidants** & \begin{tabular}{c} **TPC** \\ **(mg GAE/g)** \\ \end{tabular} & \begin{tabular}{c} **TFC** \\ **(mg CAE/g)** \\ \end{tabular} & \begin{tabular}{c} **MAC** \\ **(CyE/g)** \\ \end{tabular} & \begin{tabular}{c} **DPPH** \\ **(mmol TE/g)** \\ \end{tabular} & \begin{tabular}{c} **ABTS** \\ **(mmol TE/g)** \\ \end{tabular} & \
\begin{tabular}{c} **FRAP** \\ **(mmol of Fe\({}^{2+}\)E/100 g)** \\ \end{tabular} \\ \hline \hline
\begin{tabular}{l} Mean \(\pm\) STD \\ (n = 3) \\ \end{tabular} & 17.92 \(\pm\) 0.593 & 9.62 \(\pm\) 0.133 & 6.22 \(\pm\) 0.354 & 6.44 \(\pm\) 0.066 & 19.74 \(\pm\) 2.63 & 270.94 \(\pm\) 3.45 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Phenolic substances content and antioxidant capacities of black goji berry under the optimal extraction condition (70%, 13 min, 89 \({}^{\circ}\)C).  
experiments. From these three figures, the optimal extraction conditions of TPC from black goji berry were the ethanol concentration of 70%, the extraction time of 13 min and the extraction temperature of 89.36 \({}^{\circ}\)C. Obviously, the TPC extraction was heavily affected by the extraction temperature and time.