## 3 Different Extraction Methods of Polyphenols
### Olmic Heating (OH) Pretreatment  
OH is a thermal-electrical method developed in the past few decades [112]. In the extraction process, OH reduces the treatment time, which this feature causes the least thermal damage to the polyphenols. However, this technology has some disadvantages, including its inefficiency in non-conductive and non-homogeneous food matrices. Moreover, the application of OH in foods with a high quantity of proteins may lead to deposit formation on the surface of electric-supplying electrodes, resulting in an electrical arcing [113]. This technique works on the basis of the contact of an electrode with a food matrix flowing an alternating electrical current (AC) with a frequency of 50 Hz to 100 kHz through it. This AC generates heat inside the food due to its natural electrical resistance. The food is heated rapidly in several seconds to a few minutes, and the amount of this heat depends on the voltage difference and electrical conductivity of the food [114]. The food matrix acts as the element of the electric circuit allowing the AC to flow. The yielded energy in this method is directly proportionate to the square of the electric field strength and the electrical conductivity of the food matrix. The most prominent factors in OH technology are the electrical field strength, the electrical conductivity of the food matrix, frequency, the type of waveform (Sine, Square, Triangle, Pulsed), concentration, electrodes, and particle size. This rapid heating approach has been used for many food products (e.g., dairy products, fruits, vegetables, and meat products) and is practical in the food industry for blanching, sterilization, pasteurization, evaporation, cooking, thawing, starch gelatinization, fermentation, and by-product utilization (i.e., extraction of bioactive compounds) [115; 112; 113]. OH can induce an electro and thermal-permeabilization of cell membranes, causing disturbances on their permeability and structural alterations, which contributes to the release of higher amounts of phenolic compounds [116]. Although OH has been applied for some time, there are few studies on its influence as an extraction method of polyphenols [87]. A simplified schematic of extraction of polyphenols using OH pretreatment is shown in Figure 8.  
[PERSON] et al. (2019) reported that when OH is used as a pretreatment (70 \({}^{\circ}\)C for 15 min using 70% ethanol as a solvent) in the extraction of polyphenols from tomato by-products, rutin is recovered 77% higher than with conventional methods. The application of OH pretreatments was reported to induce the permeabilization of cell membranes and to facilitate the extraction of polyphenols with ethanol addition [31]. [PERSON] et al. (2021) used OH as a pretreatment for extracting polyphenols from cornelian cherry before using UAE, and they reported that the highest extracted TPC was 7.52 mg GAE/g compared to the maceration and UAE methods [117]. It is also reported that the usage of OH as a pretreatment in the extraction of anthocyanins from grape skins increases the efficiency of extraction significantly [118]. Table 5 provides more information about the research conducted in this area.