*3.2.1. The microwave-assisted extraction (MAE)*

Based on the literature, there is no consensus about the best solvent to extract polyphenols. However, it has been widely accepted that higher polarity usually means better solubility of polyphenols into extraction solvents; however, differences in the structure of phenolic compounds may be critical for their solubility. Thus, the extraction of polyphenols and other phytochemicals must be prior tested and adapted to the solvent, because the diverse structures of polyphenols, such as multiple hydroxyl groups, conjugated or not with sugars, acids or alkyl groups, interfere in the extraction process. Therefore, it is very difficult to say what type of solvent is better to develop a standard method for all type of polyphenols, but the majority of the authors seem to agree that a good solvent system is the one that allows the maximization of polyphenols extracted without any modifi-

**Figure 2.** Miscibility of organic solvents used in the extraction of phytochemicals from natural sources. Adapted with

permission from Refs. [56–58].

66 Phenolic Compounds - Natural Sources, Importance and Applications

The MAE is a method that uses energy of microwave radiation to heat solvent in contact with the sample [74–77]. The heat produced increases the diffusivity of the solvent towards the powdered sample, extracting and diffusing the phytochemicals out of the matrix [77]. The disruption of hydrogen bonds, as a result of microwave-induced dipole rotation of molecules, enhances the penetration of the solvent into the matrix, allowing the dissolution of the components into the liquid matrices [78]. This method has the advantage of less consuming time and solvent volumes than the classical approach [77–79]. This method has been largely used for the extraction of monomeric polyphenols (short chains) such as phenolic acids and flavonoids [78–80], but it has been less used to extract polymeric polyphenols such tannins and anthocyanins, because polyphenols with a higher number of hydroxyl-type substituents (long-chains) and those sensitive to higher temperatures (e.g.,


**Table 3.** Application of advanced methods in extraction of polyphenols in different foods and agro-waste matrices [63–73].

anthocyanins) may be degraded under MAE extraction conditions [79, 80]. The temperature used for extraction is proportional to the power (watts) and time and inversely proportional to the heat capacity of the solvent and the mass of sample [80]. Higher temperatures and small amounts of sample increase the rate of solvent diffusion and promote faster extraction kinetics [80].

Numerous phytochemical compounds, including polyphenols, have been extracted MAE system as shown in **Table 4**. It seems that MAE system provides higher polyphenols yield in less consuming time and solvents. However, there are some concerns when the MAE is used to extract polyphenols. Factors such as type of matrix, type and purity of solvent, the microwave application time, power, contact sample surface area and temperature can affect their efficiency. One of the most critical factors is the nature of solvent, which affects not only the solubility of the target components but also the efficiency of all physical process. The choice of solvent must take into account not only the affinity to the target phytochemicals but also the ability to absorb microwave energy [81]. For example, solvents like hexane or dichloromethane, which are transparent to microwaves, do not heat up under microwave [82, 83]; thus, they should not be used in this system. Others, such as ethanol, methanol, or even water, have good microwave absorbing capacity [83], and they get heated up faster; thus, the length of the time and microwave power must be adapted to the solvent to enhance the extraction process without any deleterious effect on thermolabile components.
