## 3 Different Extraction Methods of Polyphenols
### Cold Plasma (CP) Pretreatment  
After solid, liquid, and gas, plasma is the fourth state of matter. In general, plasma is classified as thermal and nonthermal. The thermal plasma is produced when a gas is heated and ionized at a high temperature (up to 20,000 K), while nonthermal plasma is generated as a result of an elastic collision of the gas particles, atoms, and electrons by the applied energy. This causes the transfer of kinetic energy to other particles, resulting in the cooling of the uncharged particles and neutral ions, which is faster than the energy transfer to the electrons. As a result, the electrons stay at a higher temperature, while the neutrons, ions, and radicals reach ambient temperature. This enables the gas bulk to stay at a low temperature. Thus, this type of plasma is called nonthermal. This condition makes it possible to treat thermolabile food components. Nonthermal CP is generated by ionization of some process gases (e.g., N\({}_{2}\), O\({}_{2}\), CO\({}_{2}\), or noble gases (He, Ar, or Ne) or their combinations) with a strong electric field under room temperature. The CP comprises reactive chemical species, such as ions, electrons, UV photons, atoms, molecules, and free  
Figure 8: Simplified schematic of extraction of polyphenols using OH pretreatment. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Food By-Product**} & \multirow{2}{*}{**Solvent**} & \multirow{2}{*}{**Voltage (V)**} & \multicolumn{2}{c}{**Electrical**} & \multirow{2}{*}{**Temperature**} & \multicolumn{2}{c}{**Time**} & \multicolumn{2}{c}{**TPC 1 in Final**} \\  & & & & (V/cm) & \(\mathrm{\SIUnitSymbolMicro C}\) & **(min)** & **Extract** & **(mg GAE 2/g)** & **Reference** \\ \hline Yacon leaves & NaCl solution & & & & & & & \\ (red, fresh) & (0.3\%) & 150 & NR 3 & NR & 10 & 76.67 \(\pm\) 21.67 & [116] \\ \hline Grape skins & Water & NR & 16 & 100 & 1 s & 3.2 & [118] \\ \hline Tomato peels & Ethanol (\(\mathcal{O}\%\)) & NR & NR & 70 & 15 & 2.550 \(\pm\) 0.072 & [119] \\ \hline Vine pruning residue & Ethanol (45\%) & NR & 840 & 80 & 60 & 3.1 \(\pm\) 0.2 & [120] \\ \hline Stevia rebaudiana & & & & & & & \\ leaves & Water & NR & 150 & NR & 1 & 84.36 \(\pm\) 4.61 & [121] \\ \hline \hline \end{tabular}  
* Total phenolic content; 2 gallic acid equivalent; 3 not reported.  
\end{table} 
Table 5: Recovery of polyphenols from various food by-products using OH pretreatment. radicals. In general, these active agents can decompose covalent bonds and produce many chemical reactions [122]. Figure 9 shows the usage of a dielectric barrier discharge cold plasma apparatus as a pretreatment in the extraction of polyphenols from food by-products. Indeed, CP is another novel method that is used as a pretreatment before the extraction of polyphenols using conventional or novel methods [123]. It is stated in the literature that CP can significantly affect the amount of phenolic compounds. [PERSON] et al. (2016) reported that after the treatment of CP, the phenolic content of green tea leaves was slightly increased [124]. Moreover, [PERSON] et al. (2019) reported that CP increased TPC in blueberry juice as it breaks the covalent bonds and cell membrane [125]. In CP treatment, the source of plasma, time, various active species, power of treatment, and type of food matrix are prominent factors affecting the TPC. As an example, air CP generates many reactive oxygen species, causing oxidation of phenolic compounds [123; 125]. Therefore, using a CP source that reduces the amount of reactive oxygen species and increases the extraction rate of phenolic compounds from plants is crucial [123]. Overall, CP has many applications in the food industry, including microbiological decontamination [126] and enzyme inactivation [127]. However, there is only limited research applying CP as a pretreatment before the extraction of polyphenols. [PERSON] et al.