## 3 Results and Discussion
### Comparison of SWE with Other Extraction Methods  
The chromatogram of RES from grape seeds sample is displayed in Figure 5a. As we can see, there was a very obvious peak. However, there was difference in retention times of RES between the chromatogram of sample extract and standard. This difference was the result of two aspects. First, sample and standard were not measured at the same time: the concentration and pH of the mobile phase had a slightly difference. In addition, the sample contained lots of impurities and the influence of these impurities may have led to the difference of the retention time. In the spike recovery test, there was one peak rather than two overlapping peaks. Therefore, the method of subcritical water extraction of resveratrol from grape seeds proved to be feasible. As shown in Figure 5b, subcritical water extraction only once proved to gain highest RES yield, followed by microwave extraction, reflux extraction, and ultrasonic extraction. The results affirmed that extraction by subcritical water at high temperature modified the dielectric constant of water to allow the increased solubility of weakly polar  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & **Added** & **Detection Value** & **Rate of Recovery** & **Average Absolute Recovery** & **RSD** \\  & **(ug/g)** & **(ug/g)** & **(\%)** & **(\%)** & **(\%)** & **RSD** \\ \hline \(1\) & & 9.90 & 87.00 & & & \\ \(2\) & 3.45 & 9.78 & 83.60 & 86.47 & 0.03 \\ \(3\) & & 9.96 & 88.80 & & & \\ \hline \(1\) & & 13.10 & 89.90 & & & \\ \(2\) & 6.90 & 12.94 & 87.54 & 87.78 & 0.02 \\ \(3\) & & 12.83 & 85.90 & & & \\ \hline \(1\) & & 19.73 & 93.00 & & & \\ \(2\) & 13.80 & 19.61 & 92.07 & 91.98 & 0.01 \\ \(3\) & & 19.44 & 90.87 & & & \\ \hline \hline \end{tabular}
\end{table}
Table 4: The results of average absolute recovery.  
Figure 4: The normal probability plot of residuals (**a**); plot of internally residuals versus experimental runs (**b**); and plot of predicted and actual values (**c**).  
compounds. Therefore, subcritical water extraction of RES is an efficient method and this method is suitable for extraction on active natural product ingredients.  
A general definition of green chemistry is the invention, design, and application of chemical products and processes to reduce or to eliminate the use and generation of hazardous substances [34]. In the case of the green extraction of natural products, the process also should reduce energy consumption and extraction time, allow for the use of alternative solvents, obtain useful co-products, and ensure a safe and high quality of product [35]. In our research, the SWE process of RES from grape seeds was consistent with the above principles. Apart from the higher extraction rate, the high added value of extraction RES by SWE was proven by the data of different methods in Table 5. First, compared to other methods, water was used instead of organic solvent. Second, extraction time was significantly reduced. Additionally, the raw materials (grape seeds) are a by-product of grape processing; while extracting resveratrol, procyanidin was also extracted and extraction yield up to 87.12 mg/g. This method was also easy to operate. Although high temperature and high pressure operating conditions were required during subcritical water extraction, a shorter period of time meant less energy consumption, which could be further reduced by energy recovery. For example, a prototype of eco-evaporator able to recover up to 55% of the energy was developed in the south of France for steam distillation of laverader. The overall saving in thermal energy was around 80%, with no impact on the product's quality [34]. To sum up, compared to other methods, extraction RES from grape seeds by SWE has a high added value.