**3.1. Classical solvent extraction**

The classical solvent extraction of polyphenols usually includes extraction by maceration and percolation and by successive Soxhlet extraction [41–45].

The maceration, widely used in the past, is nowadays in underuse since other methods are more feasible. It is a simple procedure in which the powdered sample is soaked in an appropriate solvent in a closed container, normally under room temperature with constant or sporadic agitation [41, 46]. In the end of the extraction, the solid parts need to be separated from the solvent, which can be done by filtration, clarification and/or decantation [47]. This method is quite simple to hand but has the main disadvantage of time-consuming, requires a large volume of solvent [41, 42, 48, 49].

Similar to the maceration, the percolation method is characterized by placing the powdered sample in a closed container (normally cylindrical) in which the solvent is discharged from the top towards the bottom in a slow movement (drop wise). [41, 42, 50]. In this case, the filtration is not necessary because the percolator device has itself a filter which placed at the bottom, and we can only collect the final liquid. This method faces the same issues of maceration, which are time-consuming, large volumes of solvent, solubility of polyphenols, particle size of sample and contact time between solvent and sample.

In Soxhlet extraction [41, 42], the powdered samples are sealed in cellulose bags and placed in an extraction chamber located on top of a collecting flask beneath a reflux condenser, and after the addition of the solvent, the system is heated and the solvent condenses after reaching certain level of temperature [51–53]. A reflux occurs continuously. At the end, the liquid extract is collected to the flask positioned beneath the system [51–53]. The Soxhlet extraction is a continuous process with the advantage of being less time and less solvent consuming than the maceration or percolation methods [54]. However, some authors have stated that Soxhlet extraction must be handled carefully because the excess of temperature, always near to the boiling point, can destroy or modify some thermolabile polyphenols [44]. Others reported that Soxhlet extraction is used widely because of its convenience [41, 42, 44, 54]. Although these have variations, all these three methods have the common usage of organic solvents in a solid/liquid ratio. Solvents such as water, methanol, ethanol, acetone, n-hexane, chloroform, propanol and ethyl acetate have been most commonly used for the extraction of polyphenols (**Table 2**). The difference between solvents resides in their polarity (**Figure 1**) which affects their capacity in extract phytochemicals. The miscibility of organic solvents (**Figure 2**) with each other's or even other types of solvents is another fact to be considered in order to improve the polyphenol extraction yield as shown by several studies [59–63].


1 Data collected from http://macro.lsu.edu/HowTo/solvents.htm [55].

2 Data collected from Speight (2005) [57].

and enzyme-assisted extraction (EAE) are even better to enhance the content of polyphenols in the extracts [33–39]. Despite this diversity, all have the common fact that the extraction must be conducted carefully but exhaustively with simple, rapid and feasible procedures, and if possible open to automation [40]. In the next paragraphs, we present a summarized information of the most commonly used methods for the extraction of polyphenols in several

The classical solvent extraction of polyphenols usually includes extraction by maceration and

The maceration, widely used in the past, is nowadays in underuse since other methods are more feasible. It is a simple procedure in which the powdered sample is soaked in an appropriate solvent in a closed container, normally under room temperature with constant or sporadic agitation [41, 46]. In the end of the extraction, the solid parts need to be separated from the solvent, which can be done by filtration, clarification and/or decantation [47]. This method is quite simple to hand but has the main disadvantage of time-consuming, requires a large

Similar to the maceration, the percolation method is characterized by placing the powdered sample in a closed container (normally cylindrical) in which the solvent is discharged from the top towards the bottom in a slow movement (drop wise). [41, 42, 50]. In this case, the filtration is not necessary because the percolator device has itself a filter which placed at the bottom, and we can only collect the final liquid. This method faces the same issues of maceration, which are time-consuming, large volumes of solvent, solubility of polyphenols, particle

In Soxhlet extraction [41, 42], the powdered samples are sealed in cellulose bags and placed in an extraction chamber located on top of a collecting flask beneath a reflux condenser, and after the addition of the solvent, the system is heated and the solvent condenses after reaching certain level of temperature [51–53]. A reflux occurs continuously. At the end, the liquid extract is collected to the flask positioned beneath the system [51–53]. The Soxhlet extraction is a continuous process with the advantage of being less time and less solvent consuming than the maceration or percolation methods [54]. However, some authors have stated that Soxhlet extraction must be handled carefully because the excess of temperature, always near to the boiling point, can destroy or modify some thermolabile polyphenols [44]. Others reported that Soxhlet extraction is used widely because of its convenience [41, 42, 44, 54]. Although these have variations, all these three methods have the common usage of organic solvents in a solid/liquid ratio. Solvents such as water, methanol, ethanol, acetone, n-hexane, chloroform, propanol and ethyl acetate have been most commonly used for the extraction of polyphenols (**Table 2**). The difference between solvents resides in their polarity (**Figure 1**) which affects their capacity in extract phytochemicals. The miscibility of organic solvents (**Figure 2**) with each other's or even other types of solvents is another fact to be considered in order to improve the polyphenol extraction yield as shown by several

pant and food matrices.

**3.1. Classical solvent extraction**

volume of solvent [41, 42, 48, 49].

studies [59–63].

percolation and by successive Soxhlet extraction [41–45].

64 Phenolic Compounds - Natural Sources, Importance and Applications

size of sample and contact time between solvent and sample.

3 Data collected from Singh et al. (2014) [57].

4 Data collected from Hakansson et al. (2016) [58].

**Table 2.** Important properties of some solvents commonly used in the extraction of polyphenols1–4 .

**Figure 1.** Polarity of the organic solvents most commonly used in phytochemicals extraction from natural sources. Adapted with permission from Refs. [55–56].

In general, organic solvents and their aqueous formulations are mostly used in the extraction of phytochemicals, but it is still no clear which solvent is most adequate for the extraction of polyphenols. For example, acetone showed to be very efficient in the extraction of polyphenols [59] from lychee (*Litchi chinenesis* Sonn.) flowers in comparison with methanol, ethanol or water. While in walnut (*Juglans regia* L.) green husks, the highest extraction yield of polyphenols (44.1%) was obtained when water was used as extraction solvent [60]. By other hand, in a recent study [61], it was found that aqueous and organic solvent have a higher extraction efficiency than absolute organic solvents. Similar situation was observed in *Phoradendron californicum* oak extracts [62], in which aqueous methanol was the solvent most efficient for the extraction of polyphenols.


**Figure 2.** Miscibility of organic solvents used in the extraction of phytochemicals from natural sources. Adapted with permission from Refs. [56–58].

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 modifications of their chemical nature. In this context, several factors must be considered when a specific solvent is selected, including (i) solvent power (selectivity); (ii) polarity; (iii) boiling temperature (should be low in order to facilitate removal of the solvent from the product); (iv) reactivity (the solvent should not react chemically with the extract neither should be decomposed quickly); (v) viscosity (low); (vi) stability (should be stable to heat, oxygen and light); (vii) safe in use (should be nonflammable and nontoxic for consumers and environment); (viii) if possible, suitability for reuse; and (ix) compatible with legislation for food applications.
