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Plant/Solvent Ratio and Water Content in Deep Eutectic Solvents
The plant material-to-solvent ratio is classically set between 2.5 and 6% in conventional extraction. The high solubility of polyphenols in deep eutectic solvent yet
suggests the possibility of using higher ratio than in traditional solvents (Dai et al.
2016). However, the largest used ratio is ca. 10%, which is not significantly more.
Cao and coworkers used a RSM study (response surface methodology; see below)
for the optimization of the plant/solvent ratio on the extraction of proanthocyanidins
from the leaves of Ginkgo biloba (Cao et al. 2018a). It is shown that beyond 10%,
the rate of extraction strongly decreases. Tang and coworkers managed to obtain a
solid/solvent ratio of 80% in the case of Chamaecyparis obtusa for flavonoid extraction (Tang et al. 2015). To our knowledge, to date, it is the only reference with such
a large ratio. The high viscosity of deep eutectic solvent is usually invoked for
explaining modest ratio. Indeed, a sufficient amount of solvent is necessary to fully
impregnate the matrix and successfully perform the extraction. It could be noted
that high plant/solvent ratio (but still 3 to 6% at the maximum) is often used with
deep eutectic solvent containing rather large proportions of water (around 25% w/w).
The use of water in eutectic mixtures is reported in almost all works. The dilution
of deep eutectic solvent with water allows a significant lowering of their viscosity.
However, it must be emphasized that it strongly changes the structure of deep eutectic solvent mixtures, altering their composition from a binary mixture to a ternary
one. Significant changes have been observed in both the nature of extracted molecules and the extraction rates according to the percentage of water (Ruesgas-Ramón
et al. 2017; Zainal-Abidin et al. 2017). A very recent study from Panic et al. focuses
on the impact of plant/solvent ratio and water dilution on the extraction of anthocyanins from grape pomace (Panić et al. 2019b). Several technologies combining with
deep eutectic solvent were evaluated. They finally demonstrated that the maximum
ratio was 3%. Likewise, they showed that the dilution of deep eutectic solvent in
water had a significant impact on this ratio. In addition, the type of studied solids
strongly influenced this ratio value. The ideal conditions emerging from this study
are a plant/solvent ratio of 3% and an amount of water fixed at 25% (w/w). These
optimal conditions are valid regardless of the techniques applied (microwave, ultrasound, or both) (Panić et al. 2019b). Surprisingly, the use of water is only considered through a pragmatic approach by comparing the extraction rates. However, a
key issue is the water content limit able to preserve the properties of the deep eutectic solvent mixtures. This limit is obviously dependent on the type of solvents.
However, most authors seem to agree on a maximum limit of 30% w/w of water
(Gutiérrez et al. 2009; Huang et al. 2017; Liu et al. 2018; Benvenutti et al. 2019).
Thus, Cui and coworkers carried out the extraction of genistin, genistein, and apigenin from pigeon pea roots by employing a deep eutectic microwave-assisted
extraction (deep eutectic solvent-MAE) procedure (Cui et al. 2015). Optimal conditions for deep eutectic solvent-MAE were established through the single factor and
the Box-Behnken design tests (BBD). They consisted of 30% water in 1,6-hexanediol/ChCl (7:1) as extraction solvent, a microwave power of 600 W, and a solid/
solvent ratio of about 6% (Cui et al. 2015). Since water is known to play a key role
L. Percevault et al.
Plant/Solvent Ratio and Water Content in Deep Eutectic Solvents
The plant material-to-solvent ratio is classically set between 2.5 and 6% in conventional extraction. The high solubility of polyphenols in deep eutectic solvent yet
suggests the possibility of using higher ratio than in traditional solvents (Dai et al.
2016). However, the largest used ratio is ca. 10%, which is not significantly more.
Cao and coworkers used a RSM study (response surface methodology; see below)
for the optimization of the plant/solvent ratio on the extraction of proanthocyanidins
from the leaves of Ginkgo biloba (Cao et al. 2018a). It is shown that beyond 10%,
the rate of extraction strongly decreases. Tang and coworkers managed to obtain a
solid/solvent ratio of 80% in the case of Chamaecyparis obtusa for flavonoid extraction (Tang et al. 2015). To our knowledge, to date, it is the only reference with such
a large ratio. The high viscosity of deep eutectic solvent is usually invoked for
explaining modest ratio. Indeed, a sufficient amount of solvent is necessary to fully
impregnate the matrix and successfully perform the extraction. It could be noted
that high plant/solvent ratio (but still 3 to 6% at the maximum) is often used with
deep eutectic solvent containing rather large proportions of water (around 25% w/w).
The use of water in eutectic mixtures is reported in almost all works. The dilution
of deep eutectic solvent with water allows a significant lowering of their viscosity.
However, it must be emphasized that it strongly changes the structure of deep eutectic solvent mixtures, altering their composition from a binary mixture to a ternary
one. Significant changes have been observed in both the nature of extracted molecules and the extraction rates according to the percentage of water (Ruesgas-Ramón
et al. 2017; Zainal-Abidin et al. 2017). A very recent study from Panic et al. focuses
on the impact of plant/solvent ratio and water dilution on the extraction of anthocyanins from grape pomace (Panić et al. 2019b). Several technologies combining with
deep eutectic solvent were evaluated. They finally demonstrated that the maximum
ratio was 3%. Likewise, they showed that the dilution of deep eutectic solvent in
water had a significant impact on this ratio. In addition, the type of studied solids
strongly influenced this ratio value. The ideal conditions emerging from this study
are a plant/solvent ratio of 3% and an amount of water fixed at 25% (w/w). These
optimal conditions are valid regardless of the techniques applied (microwave, ultrasound, or both) (Panić et al. 2019b). Surprisingly, the use of water is only considered through a pragmatic approach by comparing the extraction rates. However, a
key issue is the water content limit able to preserve the properties of the deep eutectic solvent mixtures. This limit is obviously dependent on the type of solvents.
However, most authors seem to agree on a maximum limit of 30% w/w of water
(Gutiérrez et al. 2009; Huang et al. 2017; Liu et al. 2018; Benvenutti et al. 2019).
Thus, Cui and coworkers carried out the extraction of genistin, genistein, and apigenin from pigeon pea roots by employing a deep eutectic microwave-assisted
extraction (deep eutectic solvent-MAE) procedure (Cui et al. 2015). Optimal conditions for deep eutectic solvent-MAE were established through the single factor and
the Box-Behnken design tests (BBD). They consisted of 30% water in 1,6-hexanediol/ChCl (7:1) as extraction solvent, a microwave power of 600 W, and a solid/
solvent ratio of about 6% (Cui et al. 2015). Since water is known to play a key role
L. Percevault et al.