293
using the ferric reducing-antioxidant power and ABTS assays, but not when using
the DPPH assay (Jeong et al. 2018).
These few examples show that a direct comparison between studies is difficult,
namely, if we consider that many combinations are possible for synthesizing deep
eutectic solvent, and that their properties stem from this composition variation.
Hence, caution is necessary when comparing the profitability between the extraction methods. It is reasonable to expect that enhanced extraction yields of polyphenols by utilizing (natural) deep eutectic solvent will increase the antioxidant activity
of the extracts. Like in Folin-Ciocalteu assay, it is worth noting that deep eutectic
solvent could directly affect the antioxidant activity when they are still present in
the extract itself. Natural deep eutectic solvent alone has been reported to display
antioxidant activity in the DPPH assay and may lead to biased results (Skulcova
et al. 2018). As for Folin-Ciocalteu, best practice asks for prior evaluation of natural
deep eutectic solvent alone for antioxidant activity assays.
Finally, hydrogen bond governs the formation of deep eutectic solvent, and polyphenols are prone to hydrogen bonding as well. A synergic effect of the deep eutectic solvent and polyphenols could be then envisioned. We may anticipate that this
phenomenon could definitively affect antioxidant properties beyond the extraction
yield. This effect could be expected to be positive as reported by Durand et al. who
described that formulation of antioxidants (α-tocopherol, hydroxytyrosol, CR6,
ascorbic acid) in natural deep eutectic solvent could greatly improve their activity
(Durand et al. 2017). But we cannot neither exclude prooxidant activity since plant
polyphenols bearing catechol and/or pyrogallol moieties have been demonstrated to
exert prooxidant properties on specific occasions (Quideau et al. 2011).
7.6 Conclusions
Due to their specific structure, and in particular their ability to form hydrogen bonds,
deep eutectic solvents are good solvents for the extraction of polyphenols. In addition, due to their low volatility, they can be advantageously combined with advanced
extraction technologies like microwave- or ultrasound-assisted extraction, provided
that the temperature remains controlled for avoiding degradation of the temperaturesensitive polyphenols.
Despite the large number of possible combinations for preparing deep eutectic
solvents with tunable properties, research about deep eutectic solvents is still in its
infancy, and only a few structures have been exploited for polyphenol extraction,
mainly based on choline chloride. One of the main limitations of the use of deep
eutectic solvents, other than choline chloride, is their viscosity at room temperature.
However, a few deep eutectic solvents show low viscosity, and some could be specifically designed for lowering viscosity. Thus, plenty of deep eutectic solvents
dedicated to efficient polyphenol extraction remain to be discovered.
A consequence of this huge potentiality is the need for (i) physicochemical characterization of these new media and (ii) rationalization of their impact along with a
7 Extraction of Plant and Algal Polyphenols Using Eutectic Solvents
using the ferric reducing-antioxidant power and ABTS assays, but not when using
the DPPH assay (Jeong et al. 2018).
These few examples show that a direct comparison between studies is difficult,
namely, if we consider that many combinations are possible for synthesizing deep
eutectic solvent, and that their properties stem from this composition variation.
Hence, caution is necessary when comparing the profitability between the extraction methods. It is reasonable to expect that enhanced extraction yields of polyphenols by utilizing (natural) deep eutectic solvent will increase the antioxidant activity
of the extracts. Like in Folin-Ciocalteu assay, it is worth noting that deep eutectic
solvent could directly affect the antioxidant activity when they are still present in
the extract itself. Natural deep eutectic solvent alone has been reported to display
antioxidant activity in the DPPH assay and may lead to biased results (Skulcova
et al. 2018). As for Folin-Ciocalteu, best practice asks for prior evaluation of natural
deep eutectic solvent alone for antioxidant activity assays.
Finally, hydrogen bond governs the formation of deep eutectic solvent, and polyphenols are prone to hydrogen bonding as well. A synergic effect of the deep eutectic solvent and polyphenols could be then envisioned. We may anticipate that this
phenomenon could definitively affect antioxidant properties beyond the extraction
yield. This effect could be expected to be positive as reported by Durand et al. who
described that formulation of antioxidants (α-tocopherol, hydroxytyrosol, CR6,
ascorbic acid) in natural deep eutectic solvent could greatly improve their activity
(Durand et al. 2017). But we cannot neither exclude prooxidant activity since plant
polyphenols bearing catechol and/or pyrogallol moieties have been demonstrated to
exert prooxidant properties on specific occasions (Quideau et al. 2011).
7.6 Conclusions
Due to their specific structure, and in particular their ability to form hydrogen bonds,
deep eutectic solvents are good solvents for the extraction of polyphenols. In addition, due to their low volatility, they can be advantageously combined with advanced
extraction technologies like microwave- or ultrasound-assisted extraction, provided
that the temperature remains controlled for avoiding degradation of the temperaturesensitive polyphenols.
Despite the large number of possible combinations for preparing deep eutectic
solvents with tunable properties, research about deep eutectic solvents is still in its
infancy, and only a few structures have been exploited for polyphenol extraction,
mainly based on choline chloride. One of the main limitations of the use of deep
eutectic solvents, other than choline chloride, is their viscosity at room temperature.
However, a few deep eutectic solvents show low viscosity, and some could be specifically designed for lowering viscosity. Thus, plenty of deep eutectic solvents
dedicated to efficient polyphenol extraction remain to be discovered.
A consequence of this huge potentiality is the need for (i) physicochemical characterization of these new media and (ii) rationalization of their impact along with a
7 Extraction of Plant and Algal Polyphenols Using Eutectic Solvents