## 3 Different Extraction Methods of Polyphenols
### Pulsed Electric Field (PEF) Pretreatment  
PEF is an emerging technology based on the usage of an external electric field that yields reversible or irreversible electroporation in cell membranes. The electroporation phenomenon induces cell transformation or rupture under the utilization of a few to several hundred short pulses with a period ranging from microseconds to milliseconds [29; 98]. In other words, electroporation refers to the exposure of cells to transmembrane electrical pulses [99]. Involving an external electric field in cells gives rise to the formation of pores in the membrane. Since pore formation is a dynamic process according to the intensity of the PEF, electroporation can be reversible or irreversible. When the generated pores are smaller than the membrane area and are produced with a low-intensity PEF, the electric breakdown is reversible [100]. The rise in the electric field strength and treatment duration induces an increase in the intensity of the treatment, which results in the transformation of reversible permeabilization to the irreversible disruption of the cell membrane. The irreversible disruption of the cell membrane facilitates the usage of treated compounds in various  
Figure 6: Simplified schematic of extraction of polyphenols using PLE. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Food By-Product** & **Solvent** & **Pressure (MPa)** & **Temperature (°C)** & **Time (min)** & **TPC \({}^{1}\) in Final Extract (mg GAE \({}^{2}\)/g)** & **Reference** \\ \hline Burdock roots & Ethanol (70\%) & NR \({}^{3}\) & NR & \(105\) & \(12.13\pm 0.34\) & [56] \\ \hline Cocoa bean hulls & Ethanol (70\%) & 10 & 70 & 20 & \(9.6\pm 0.3\) & [92] \\ \hline Pomegranate peel & Ethanol (77\%) & 10.34 & 200 & 20 & \(17\pm 3.6\) & [93] \\ \hline Mulberry pulp & Methanol (74.6\%) & 10.13 & 99.4 & 10 & 2.18 & [94] \\ \hline Jabuticaba skin & Ethanol (99.5\%) & 5 & 120 & 15 & \(18.7\pm 0.4\) & [95] \\ \hline Parsley flakes & Ethanol (50\%) & 6.9 & 40 & 5 & 22.9 & [96] \\ \hline Grape marc & Ethanol (50\%) & 10 & 100 & 40 & \(65.68\pm 2.24\) & [97] \\ \hline \hline \end{tabular}  
\({}^{1}\) Total phenolic content; \({}^{2}\) gallic acid equivalent; \({}^{3}\) not reported.  
\end{table} 
Table 3: Recovery of polyphenols from various food by-products using PLE. areas, including the extraction of bioactive compounds from different sources [98,101]. When PEF is used as a pretreatment in the extraction, diffusion of polyphenols from the cell matrix into aqueous media occurs under mild conditions, and it does not need additional solvents [29]. The efficiency of PEF pretreatment depends on some factors, including extraction time, treatment temperature, pulse frequency, pulse shape, specific energy input, electric field strength, pH, pulse width, food matrix density and size, and chemical properties of extracting by-product [102]. PEF has some key advantages, such as the maintenance of the quality of the extracted products and the possibility of its application on a continuous industrial scale [103,104]. Figure 7 displays the simplified schematic of extraction of polyphenols using PEF pretreatment. [PERSON] et al. (2019) evaluated the influence of PEF pretreatment in the recovery of polyphenols from lemon residues using pressing extraction technique, and they reported that the electric field intensity of 7 kV/cm and applying 30 pulses in the duration of 3 us increased the efficiency of polyphenol extraction by 300% compared to the untreated extracts [105]. Moreover, [PERSON] et al. (2019) used different nonthermal treatments, including PEF, in the extraction process from pomeranate peels, and they concluded that PEF with the electric field of 10 kV/cm selectively improved the recovery of ellagic acid compared to other methods [106]. According to the research conducted by [PERSON] et al. (2016), the application of a two-step extraction procedure can allow a noticeable enhancement of the extraction yields of phenolic compounds (+400%) from mango peels.