282
between efficiency and selectivity. However, in the case of compounds which are
not very stable like most of polyphenols (Larrauri et al. 1997; Manach et al. 2004;
Quideau et al. 2011; Volf et al. 2014; Deng et al. 2018), long extraction or maceration duration may lead to the degradation of compounds.
Temperature
Temperature is an important parameter in the extraction processes by both increasing the solubility of the polyphenols to be extracted and decreasing the viscosity of
deep eutectic solvents. Choice of temperature is mainly ruled by thermal stability of
polyphenols that is generally well below that of deep eutectic solvents (generally
stable up to 120 °C). The applied temperatures usually range from 20 to 100 °C (Ali
et al. 2019; Yılmaz et al. 2019; Bakirtzi et al. 2016; Fernández et al. 2018b; Panić
et al. 2019b). Our meta-analysis of more than 100 publications shows that more
than 70% of the extractions are carried out with temperatures between 45 °C and
55 °C. These temperatures are in the range of the thermal stability of polyphenols
(Yan et al. 2010; Volf et al. 2014). They can be regarded as optimal classical
conditions.
Extraction conditions at room temperature or even at 5 °C have been also reported
(Nam et al. 2015; Qi et al. 2015; Jeong et al. 2017; Hamany Djande et al. 2018; Cao
et al. 2018a; Meng et al. 2018; Ali et al. 2019). While such conditions prevent the
thermal decomposition of polyphenols, they could drastically reduce the extraction
performances, notably its kinetics. However, in some cases according to the polyphenols and/or deep eutectic solvent structure, temperature has been increased
beyond the generally applicable temperatures in conventional solvents, i.e., up to
90 °C (Zhang et al. 2014; Yao et al. 2015; Cui et al. 2015; Vieira et al. 2018). For
instance, a microwave-assisted extraction of baicalin from Scutellaria baicalensis
Georgi was optimized (RMS method), and the authors have shown that good performance is obtained providing that the temperature reaches 85 °C (Wang et al. 2018a).
The possibility to use such a high temperature was explained by the potential stabilization of polyphenols by deep eutectic solvent (Dai et al. 2016). Actually, the
challenge is therefore to find the best compromise between the amount of extracted
polyphenols and the amount of thermally degraded polyphenols.
7.4.3 Impact of Extraction Technologies
All the parameters above could be tuned for optimizing the extraction performance
of a process. Nevertheless, apart from the choice of solvent, the selection of extracting technologies is of primary importance. Among all the possible extraction technologies, 6 extraction technologies have been mainly used in deep eutectic solvents
(Fig. 7.7), and they mostly correspond to extraction methods that were developed in
ionic liquids.
L. Percevault et al.
between efficiency and selectivity. However, in the case of compounds which are
not very stable like most of polyphenols (Larrauri et al. 1997; Manach et al. 2004;
Quideau et al. 2011; Volf et al. 2014; Deng et al. 2018), long extraction or maceration duration may lead to the degradation of compounds.
Temperature
Temperature is an important parameter in the extraction processes by both increasing the solubility of the polyphenols to be extracted and decreasing the viscosity of
deep eutectic solvents. Choice of temperature is mainly ruled by thermal stability of
polyphenols that is generally well below that of deep eutectic solvents (generally
stable up to 120 °C). The applied temperatures usually range from 20 to 100 °C (Ali
et al. 2019; Yılmaz et al. 2019; Bakirtzi et al. 2016; Fernández et al. 2018b; Panić
et al. 2019b). Our meta-analysis of more than 100 publications shows that more
than 70% of the extractions are carried out with temperatures between 45 °C and
55 °C. These temperatures are in the range of the thermal stability of polyphenols
(Yan et al. 2010; Volf et al. 2014). They can be regarded as optimal classical
conditions.
Extraction conditions at room temperature or even at 5 °C have been also reported
(Nam et al. 2015; Qi et al. 2015; Jeong et al. 2017; Hamany Djande et al. 2018; Cao
et al. 2018a; Meng et al. 2018; Ali et al. 2019). While such conditions prevent the
thermal decomposition of polyphenols, they could drastically reduce the extraction
performances, notably its kinetics. However, in some cases according to the polyphenols and/or deep eutectic solvent structure, temperature has been increased
beyond the generally applicable temperatures in conventional solvents, i.e., up to
90 °C (Zhang et al. 2014; Yao et al. 2015; Cui et al. 2015; Vieira et al. 2018). For
instance, a microwave-assisted extraction of baicalin from Scutellaria baicalensis
Georgi was optimized (RMS method), and the authors have shown that good performance is obtained providing that the temperature reaches 85 °C (Wang et al. 2018a).
The possibility to use such a high temperature was explained by the potential stabilization of polyphenols by deep eutectic solvent (Dai et al. 2016). Actually, the
challenge is therefore to find the best compromise between the amount of extracted
polyphenols and the amount of thermally degraded polyphenols.
7.4.3 Impact of Extraction Technologies
All the parameters above could be tuned for optimizing the extraction performance
of a process. Nevertheless, apart from the choice of solvent, the selection of extracting technologies is of primary importance. Among all the possible extraction technologies, 6 extraction technologies have been mainly used in deep eutectic solvents
(Fig. 7.7), and they mostly correspond to extraction methods that were developed in
ionic liquids.
L. Percevault et al.