# Recovery of Polyphenols from Agri-Food By-Products: The Olive Oil and Winery Industries Cases
## 5 From Agri-Food Wastes to Polyphenols: Processing Techniques
### Extraction Processes
Some enzymes, such as cellulases, hemicellulases, pectinases, or amylases, can break down or weaken cell walls, releasing cytoplasmic content (e.g., phenolic compounds) into the extraction solvent, and thus improving extraction recoveries. Factors such as enzyme type and concentration, pH, or temperature have an influence on extraction recovery. EAE can also be combined with other extraction techniques such as UAE, MAE, PLE, or SFE [87]. Ohmic heating (OH). In OH the material is in direct contact with the electrodes and an alternating electrical current, with a frequency between 50 and 100 kHz, passes throughout the material therefore rapid and uniform heating occurs. OH has found several applications in food technology, and it also has been applied to the extraction of phenolic compounds from agri-food wastes [85]. Pulsed electrical fields (PEF). PEF uses high voltage, between 10-40 kV, for an extremely short time, in the range of a few \(\upmu\)s, producing a strong electric field. It is considered a non-thermal technology because in PEF the temperature rise is usually low--a few \({}^{\circ}\)C. PEF produces electroporation of the cell membrane, and consequently the permeation throughout the membrane increases, and thus extraction improves. Parameters such as the strength of the electric field, the amplitude of the pulses, the duration of the pulses, etc., have an effect on the efficiency of the extraction [88]. Examples of the application of the different techniques and solvents to the extraction of polyphenols from the olive mill and winery wastes are presented in Table 3. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Technique** & **Sample** & **Solvent** & \
\begin{tabular}{c} **Experimental** \\ **Conditions** \\ \end{tabular} & **Polyphenols Concentration** & **Reference** \\ \hline \multicolumn{5}{c}{Olive Mill Residues} \\ \hline Liquid–liquid & Olive mill & \multirow{2}{*}{Ethyl acetate} & 25 \({}^{\circ}\)C, four & 6.490 \(\pm\) 0.063 g GAE L\({}^{-1}\) & [50] \\ extraction & wastewater & & extraction cycles & 1407 mg GAE L\({}^{-1}\) & [89] \\ Liquid–liquid & Olive mill & \multirow{2}{*}{Ethyl acetate} & 1:1 \(\
u\)/c, 25 \({}^{\circ}\)C, two & 8.90 \(\pm\) 0.728 mg GAE L\({}^{-1}\) & [90] \\ extraction & wastewater & & extraction cycles & 9.8 g tyrosol equivalents L\({}^{-1}\) & [21] \\ Liquid–liquid & Olive mill & \multirow{2}{*}{Ethyl acetate} & 1:2 \(\
u\)/c, 27 \({}^{\circ}\)C, 30 & 3440 mg GAE L\({}^{-1}\) & [91] \\ extraction & wastewater & & min & & \\ SLE & Olive pomace & Methanol & 1:25 \(w\)/c, 70 \({}^{\circ}\)C, 12 & 4.37 mg GAE g\({}^{-1}\) & [45] \\ SLE & Olive pomace & Ethanol & 1:5 \(w\)/c, 25 \({}^{\circ}\)C, 180 & 1.23 \(\pm\) 0.21 caffe acid equivalents & [92] \\ SLE & Olive pomace & Ethanol &
\begin{tabular}{c} min, pH 2 \\ 1:10, \(w\)/c, 180 \({}^{\circ}\)C, \\ 90 min \\ \end{tabular} & 45.2 mg CAE g\({}^{-1}\) & [93] \\ SLE & Dry olive mill & \multirow{2}{*}{Water} & 1:15 \(w\)/c, 25 \({}^{\circ}\)C, 40 & 25 mg GAE g\({}^{-1}\) & [94] \\ SLE & Olive pomace & Dimethyl & 1:3 \(w\)/c, 25 \({}^{\circ}\)C, 30 & 1.3 g kg\({}^{-1}\) & [48] \\ SLE & Olive pomace & sulfoxide & min & 171 \(\pm\) 4 mg of gallic acid 100 g\({}^{-1}\) & [95] \\ SLE & Olive leaves & Dimethyl & 1:15 \(w\)/c, 25 \({}^{\circ}\)C, 30 & 50 g kg\({}^{-1}\) & [48] \\ MAE & Olive leaves & \multirow{2}{*}{Water} & 86 \({}^{\circ}\)C, 3 min & 104.22 \(\pm\) 0.61 mg GAE g\({}^{-1}\) & [96] \\ UAE & Olive pomace & Isopropanol:water & 1:5 \(w\)/c, 25 \({}^{\circ}\)C, 40 & 69.66 mg GAE g\({}^{-1}\) & [47] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Extraction techniques for polyphenols recovery from the olive mill and winery wastes. For instance, [PERSON] et al. [97] compared UAE, MAE, and SLE for the extraction of 3-hydroxytyrosol, maslinic acid, and oleanolic acid from olive pomace. As a result, UAE yielded the maximum extraction of 3-hydroxytyrosol (55.1 mg g\({}^{-1}\)), maslinic acid (381.2 mg g\({}^{-1}\)), and oleanolic acid (29.8 mg g\({}^{-1}\)), followed by MAE and SLE. The optimal