257
7.4 Extraction Methodologies of Polyphenols in (Natural)
Deep Eutectic Solvent
7.4.1 Commonly Used Deep Eutectic Solvent
for Polyphenol Extraction
As mentioned previously, the (natural) deep eutectic solvents have very interesting
solubilizing properties for polyphenols. For example, the solubility of rutin, a wellknown flavonoid, is 50–100 times higher in various natural deep eutectic solvents
than in water (Choi et al. 2011; Huang et al. 2017). Dai et al. (2013c) have investigated the extraction of polyphenols from safflower with seven natural deep eutectic
solvents of various polarities and obtained higher extraction yields with the deep
eutectic solvents than with conventional solvents. The same authors have reported
the effect of water on the solubility of quercetin and carthamin in natural deep
eutectic solvents (Dai et al. 2015). These two polyphenols are poorly soluble in
water. In both cases, the solubility of the polyphenols in a weakly polar deep eutectic solvent decreases dramatically with the amount of water from 25 to 50% w/w,
which correlates with the rupture of the network of hydrogen bonds as observed by
measuring the viscosity. As expected, more polar polyphenols require more water
for an optimal solubilization. Interestingly, the optimal water content to solubilize a
given polyphenol in different natural deep eutectic solvents also seems to correlate
with the viscosity, the more viscous being less prone to accommodate the solute.
By scrutinizing the literature data, it turns out that more than 60% of the publications dealing with the extraction of polyphenols concern choline chloride-based
(natural) deep eutectic solvents (Fig. 7.5). Actually, choline chloride was also
employed for designing “cheap” ionic liquids (Abbott et al. 2001), and we could
consider that the choline chloride is the keystone from ionic liquids to deep eutectic
solvents. Originally, choline chloride was mixed with conventional solvents like
ethylene glycol in order to reduce the viscosity of the latter. The resulting mixtures
were cheap and not very viscous, therefore attracting attention toward deep eutectic
solvents (Zainal-Abidin et al. 2017).
Choline chloride is then frequently found, associated with organic acids, e.g.,
citric or acetic acid, or with alcohols, e.g., 1,4-butanediol (Table 7.2). The equimolar mixture of sodium acetate and glycerol is probably the only deep eutectic solvent
without choline chloride that is used in the extraction of polyphenols. Note that this
mixture is one of the less viscous deep eutectic solvents. As mentioned in Sect. 7.3,
the properties of deep eutectic solvent can be adjusted through the variation of the
HBA (hydrogen bond acceptor) or HBD (hydrogen bond acceptor), the molar ratio
between components, and the water content. Changing the hydrogen bond donors
allows the tuning of physical properties (viscosity, pH, color, melting point, etc.) of
the solvents but also the modification of the interactions between solvent and polyphenol compounds (Zainal-Abidin et al. 2017). Often, solvents are empirically
selected through researchers’ know-how and knowledge. Interestingly, the very
large majority of deep eutectic solvents used for the extraction of polyphenols are
7 Extraction of Plant and Algal Polyphenols Using Eutectic Solvents
7.4 Extraction Methodologies of Polyphenols in (Natural)
Deep Eutectic Solvent
7.4.1 Commonly Used Deep Eutectic Solvent
for Polyphenol Extraction
As mentioned previously, the (natural) deep eutectic solvents have very interesting
solubilizing properties for polyphenols. For example, the solubility of rutin, a wellknown flavonoid, is 50–100 times higher in various natural deep eutectic solvents
than in water (Choi et al. 2011; Huang et al. 2017). Dai et al. (2013c) have investigated the extraction of polyphenols from safflower with seven natural deep eutectic
solvents of various polarities and obtained higher extraction yields with the deep
eutectic solvents than with conventional solvents. The same authors have reported
the effect of water on the solubility of quercetin and carthamin in natural deep
eutectic solvents (Dai et al. 2015). These two polyphenols are poorly soluble in
water. In both cases, the solubility of the polyphenols in a weakly polar deep eutectic solvent decreases dramatically with the amount of water from 25 to 50% w/w,
which correlates with the rupture of the network of hydrogen bonds as observed by
measuring the viscosity. As expected, more polar polyphenols require more water
for an optimal solubilization. Interestingly, the optimal water content to solubilize a
given polyphenol in different natural deep eutectic solvents also seems to correlate
with the viscosity, the more viscous being less prone to accommodate the solute.
By scrutinizing the literature data, it turns out that more than 60% of the publications dealing with the extraction of polyphenols concern choline chloride-based
(natural) deep eutectic solvents (Fig. 7.5). Actually, choline chloride was also
employed for designing “cheap” ionic liquids (Abbott et al. 2001), and we could
consider that the choline chloride is the keystone from ionic liquids to deep eutectic
solvents. Originally, choline chloride was mixed with conventional solvents like
ethylene glycol in order to reduce the viscosity of the latter. The resulting mixtures
were cheap and not very viscous, therefore attracting attention toward deep eutectic
solvents (Zainal-Abidin et al. 2017).
Choline chloride is then frequently found, associated with organic acids, e.g.,
citric or acetic acid, or with alcohols, e.g., 1,4-butanediol (Table 7.2). The equimolar mixture of sodium acetate and glycerol is probably the only deep eutectic solvent
without choline chloride that is used in the extraction of polyphenols. Note that this
mixture is one of the less viscous deep eutectic solvents. As mentioned in Sect. 7.3,
the properties of deep eutectic solvent can be adjusted through the variation of the
HBA (hydrogen bond acceptor) or HBD (hydrogen bond acceptor), the molar ratio
between components, and the water content. Changing the hydrogen bond donors
allows the tuning of physical properties (viscosity, pH, color, melting point, etc.) of
the solvents but also the modification of the interactions between solvent and polyphenol compounds (Zainal-Abidin et al. 2017). Often, solvents are empirically
selected through researchers’ know-how and knowledge. Interestingly, the very
large majority of deep eutectic solvents used for the extraction of polyphenols are
7 Extraction of Plant and Algal Polyphenols Using Eutectic Solvents