## 3 Bioactive Compounds in Wine By-Products
### Extraction Methods of Bioactive Compounds
#### 3.1.2 Emerging Extraction Methods  
Promising new 'green' and advanced extraction techniques are being developed and applied to bioactive extraction from wine by-products (Table 1) to overcome conventional extraction limitations by increasing the overall yield and the selectivity of bioactive components from plant materials without fragmenting the treated tissue, reducing the subsequent purification steps and achieving a cleaner and safer extraction [88; 89; 90]. Some of the most promising new methods are microwave-assisted extraction (MAE), high-pressure extraction (HPE), pulsed electric fields extraction (PEFE), ohmic heating (OH), pressurized hot water extraction (PHWE), supercritical fluid extraction (SFE), ultrasound extraction (UAE), and the use of natural deep eutectic solvents (NADES) [91; 92; 93; 94; 95; 34; 96; 97]. Recent studies have proven the success of using these methods for the extraction of bioactive compounds from wine by-products [91; 96; 97; 87]. Microwave-assisted extraction (MAE): This is a process that utilizes temperature to extract polar compounds. Essentially, electromagnetic energy within the frequency range of 300 MHz and 300 GHz is converted into heat through ionic conduction and dipole rotation [98; 34]. The advantage of this method is heating for the extraction of the compounds. However, this may also be a disadvantage for the extraction of thermosensitive compounds. Other advantages include reduced thermal gradients, reduced equipment size, and increased extract yield [99; 100]. [PERSON] and [PERSON] [94] used an acidic aqueous solution with 2% citric acid as a solvent to separate the phenolic compounds from grape pomace, using both UAE and MAE. The results show that for both extraction methods, the content of the total phenolic compounds and the antioxidant activity measured by ABTS and DPPH increased with time, highlighting that MAE provided the best condition for extraction. [PERSON] et al. [97] produced bioactive-enriched extracts from grape juice by-products using three different methods (UAE, MAE, and liquid-liquid). Among the methods studied, the MAE of grape juice by-products using polar solvents, such as ethanol and water, provided the best yield and chemical composition, obtaining flavonoid-rich extracts. High-pressure extraction (HPE): This method is based on two principles: (1) the isostatic principle, which states that, regardless of the size and geometry of the material, the pressure acts uniformly, instantaneously, and homogeneously in all directions and (2) the principle of [PERSON], in which, for each phenomenon, when the volume decreases, the pressure is increased/decreased, bringing the system to a new state of equilibrium [101; 102]. In HPE, heat is not used in order to avoid its deleterious effects on bioactive compounds and, consequently, preserve their biological activities [103]. When subjected to high pressures ranging from 100 to 800 MPa or even up to 1000 MPa, the matrices may undergo structural changes. These changes include the destruction of plant tissues, protein denaturation, cellular deformation, damage to cellular membranes and organelles, and an increase in solvent transfer into the materials and the soluble components into the solvents [103; 104; 105; 106]. [PERSON] et al. [107] performed the extraction of anthocyanins from grape skin pomace using HPE. They tested the use of different concentrations of water/ethanol and water/methanol solutions but concluded that the type of solvent used did not have a significant influence on the extraction yield. However, increasing the percentage of the solvent in the water favored this process and increased the temperature from 22 to 30 \({}^{\circ}\)C. On the other hand, increasing the pressure up to 500 MPa had no significant effect on the process yield. Theseauthors verified that 55.7% of the anthocyanins present in the extracts were malvidin and the optimal conditions for their extraction were 268 MPa at 30 \({}^{\circ}\)C for 3.3 min.