284
techniques. It has been proven that ultra-sonication has no effect on natural deep
eutectic solvent structure and may be an energy-effective combination (Singh et al.
2013). UAE in solid/liquid extraction generally exhibits faster kinetics and very
good extraction yield. This technique seems particularly suitable for the extraction
of polyphenol in deep eutectic solvents. Bakirtzi et al. studied the efficiency of
ultrasound-assisted extraction of antioxidant polyphenols from common native
Greek medicinal plants (Bakirtzi et al. 2016). Deionized water and 60% (v/v) aqueous ethanol were chosen as control solvents to compare with selected deep eutectic
solvents. The amount of total polyphenols obtained by ultrasound with deep eutectic solvents is on average twice as high as those obtained in water or in ethanol
(Bakirtzi et al. 2016). Actually, the technique combines moderate heating that is
compatible with the thermal stabilities of polyphenols with extraction optimization
due to ultrasound effect. Ultrasounds generally promote diffusion of dissolved substances from the inner part of the plant matrix to the extractant medium and penetration of the latter into the plant matrix. This is due to a mechanical effect on the plant,
thanks to cavitation effects (Tiwari 2015). On the whole, cavitation results in an
increase of the polarity of the system, including extractants, analytes, and matrix.
Unfortunately, the viscosity of deep eutectic solvent is systematically regarded as a
major limitation in this technology because the diffusion of ultrasound appears less
effective in viscous media (Goula et al. 2017; Huang et al. 2019a). To overcome this
issue, addition of water or a slight increase of temperature is often applied.
Extractions under microwave irradiation have also been studied. Although several publications refer to the use of activation by microwave irradiation, the overall
number of related studies remains quite low compared to what is generally found in
the field of extraction of natural products. Indeed, the effects of microwaves on
kinetics and extraction performance have been demonstrated for a long time (Du
et al. 2007; Casazza et al. 2010; Li et al. 2011; Dahmoune et al. 2015). It should be
noted that it is not the combination of microwaves and deep eutectic solvent that is
underrepresented but the specific extraction of polyphenols. This was already the
case for polyphenol extractions with traditional solvents because microwave activation is hardly compatible with the range of thermal stability of the thermosensitive
polyphenol compounds. However, worth is to mention that deep eutectic solvent is
fully compatible with microwave activation, as shown by the number of recent publications combining microwave and eutectic solvents (Li et al. 2015; Wang et al.
2017; Chanioti and Tzia 2018; Ivanović et al. 2018; Panić et al. 2019a) (Fig. 7.7).
A few publications compare microwave and ultrasound technologies and point to
greater efficiency of microwave compared to ultrasound (Yao et al. 2015; Cui et al.
2015, 2018; Peng et al. 2016; Chen et al. 2016; Wang et al. 2018a, 2019; Panić et al.
2019a). Only one publication reports a more positive performance in favor of ultrasounds for anthocyanin extraction (Cvjetko Bubalo et al. 2016). Generally, the
microwave irradiation leads to an increase of temperature, which in turn allows an
increase of the extraction rate, except in the case of anthocyanins. These latter are
indeed much more thermosensitive, and a microwave irradiation typically induces
temperature above 65 °C, sufficient for degrading these compounds.
L. Percevault et al.
techniques. It has been proven that ultra-sonication has no effect on natural deep
eutectic solvent structure and may be an energy-effective combination (Singh et al.
2013). UAE in solid/liquid extraction generally exhibits faster kinetics and very
good extraction yield. This technique seems particularly suitable for the extraction
of polyphenol in deep eutectic solvents. Bakirtzi et al. studied the efficiency of
ultrasound-assisted extraction of antioxidant polyphenols from common native
Greek medicinal plants (Bakirtzi et al. 2016). Deionized water and 60% (v/v) aqueous ethanol were chosen as control solvents to compare with selected deep eutectic
solvents. The amount of total polyphenols obtained by ultrasound with deep eutectic solvents is on average twice as high as those obtained in water or in ethanol
(Bakirtzi et al. 2016). Actually, the technique combines moderate heating that is
compatible with the thermal stabilities of polyphenols with extraction optimization
due to ultrasound effect. Ultrasounds generally promote diffusion of dissolved substances from the inner part of the plant matrix to the extractant medium and penetration of the latter into the plant matrix. This is due to a mechanical effect on the plant,
thanks to cavitation effects (Tiwari 2015). On the whole, cavitation results in an
increase of the polarity of the system, including extractants, analytes, and matrix.
Unfortunately, the viscosity of deep eutectic solvent is systematically regarded as a
major limitation in this technology because the diffusion of ultrasound appears less
effective in viscous media (Goula et al. 2017; Huang et al. 2019a). To overcome this
issue, addition of water or a slight increase of temperature is often applied.
Extractions under microwave irradiation have also been studied. Although several publications refer to the use of activation by microwave irradiation, the overall
number of related studies remains quite low compared to what is generally found in
the field of extraction of natural products. Indeed, the effects of microwaves on
kinetics and extraction performance have been demonstrated for a long time (Du
et al. 2007; Casazza et al. 2010; Li et al. 2011; Dahmoune et al. 2015). It should be
noted that it is not the combination of microwaves and deep eutectic solvent that is
underrepresented but the specific extraction of polyphenols. This was already the
case for polyphenol extractions with traditional solvents because microwave activation is hardly compatible with the range of thermal stability of the thermosensitive
polyphenol compounds. However, worth is to mention that deep eutectic solvent is
fully compatible with microwave activation, as shown by the number of recent publications combining microwave and eutectic solvents (Li et al. 2015; Wang et al.
2017; Chanioti and Tzia 2018; Ivanović et al. 2018; Panić et al. 2019a) (Fig. 7.7).
A few publications compare microwave and ultrasound technologies and point to
greater efficiency of microwave compared to ultrasound (Yao et al. 2015; Cui et al.
2015, 2018; Peng et al. 2016; Chen et al. 2016; Wang et al. 2018a, 2019; Panić et al.
2019a). Only one publication reports a more positive performance in favor of ultrasounds for anthocyanin extraction (Cvjetko Bubalo et al. 2016). Generally, the
microwave irradiation leads to an increase of temperature, which in turn allows an
increase of the extraction rate, except in the case of anthocyanins. These latter are
indeed much more thermosensitive, and a microwave irradiation typically induces
temperature above 65 °C, sufficient for degrading these compounds.
L. Percevault et al.