281
in the molecular structure of the deep eutectic solvent, and therefore on extraction
performances (Passos et al. 2016), fundamental studies of the effect of water in the
extraction process should be taken further.
Extraction Duration
The extraction durations of polyphenols in deep eutectic solvent media vary from a
few minutes (Li et al. 2015; Wei et al. 2015a; Khezeli et al. 2016; Wang et al. 2018b)
to several hours (Fu et al. 2017c). The optimal extraction time will depend on various parameters according to the type of polyphenol, the type of matrix, and all the
abovementioned parameters. With liquid matrices like oils, extraction times at room
temperature are, for example, much shorter (Paradiso et al. 2016a, b) than in the
case of grape skins (Jeong et al. 2015). Thus, Paradiso et al. carried out an effective
extraction of polyphenols from olive oil at room temperature in less than 15 minutes
(Paradiso et al. 2016a, b). However, it is important to note that this extraction is a
liquid/liquid extraction, hence probably requiring less contact time. Maceration is
generally much longer. For example, Cvjetko Bubalo and coworkers, in order to
compare assisted extraction under microwave irradiation with more traditional conditions, carried out 12 h-tests on the extraction of phenolic compounds from the
grape skin (Cvjetko Bubalo et al. 2016). In the case of particularly resistant matrices
(typically grape skin), the extraction time of 1 h in maceration at 65 °C is generally
found (Radošević et al. 2016).
Importantly, this parameter is also very dependent on the technologies employed.
For example, the use of microwave irradiation drastically reduces duration, allowing effective extractions in less than 20 min. Chen et al. have performed parameter
optimization on Radix Salviae miltiorrhizae by combining RSM and BBD (BoxBehnken design) methodologies. A maximum extraction rate was determined after
11 min under microwave irradiation at 70 °C (800 W max) (Chen et al. 2016).
However, extractions without activation, at room temperature, are very rarely performed in less than 60 min (Dai et al. 2013c; Fu et al. 2017c; Xu et al. 2019). A
direct correlation between temperature and extraction time has been established by
Bajkacz et al. for the extraction of isoflavones from soy products (Bajkacz and
Adamek 2017). In this study, the central composite design (CCD) was employed to
find the optimal values between several variables, notably temperature and extraction time. Not surprisingly, the higher the temperature, the shorter is the extraction
time. By using ultrasound, it is however possible to reduce the extraction time even
at room temperature (25 °C). Meng and colleagues have shown that the optimal
extraction time for 4 flavonoids from Pollen Typhae was only 35 min (Meng
et al. 2018).
Extraction duration influences both the efficiency and selectivity of extraction.
Depending on the affinity of the metabolites for a given solvent but also depending
on their location in the plant, they are more or less rapidly extracted. Thus, if a longer extraction time may improve the extraction efficiency, it could also increase the
range of extracted metabolites. It is therefore essential to find the right balance
7 Extraction of Plant and Algal Polyphenols Using Eutectic Solvents
in the molecular structure of the deep eutectic solvent, and therefore on extraction
performances (Passos et al. 2016), fundamental studies of the effect of water in the
extraction process should be taken further.
Extraction Duration
The extraction durations of polyphenols in deep eutectic solvent media vary from a
few minutes (Li et al. 2015; Wei et al. 2015a; Khezeli et al. 2016; Wang et al. 2018b)
to several hours (Fu et al. 2017c). The optimal extraction time will depend on various parameters according to the type of polyphenol, the type of matrix, and all the
abovementioned parameters. With liquid matrices like oils, extraction times at room
temperature are, for example, much shorter (Paradiso et al. 2016a, b) than in the
case of grape skins (Jeong et al. 2015). Thus, Paradiso et al. carried out an effective
extraction of polyphenols from olive oil at room temperature in less than 15 minutes
(Paradiso et al. 2016a, b). However, it is important to note that this extraction is a
liquid/liquid extraction, hence probably requiring less contact time. Maceration is
generally much longer. For example, Cvjetko Bubalo and coworkers, in order to
compare assisted extraction under microwave irradiation with more traditional conditions, carried out 12 h-tests on the extraction of phenolic compounds from the
grape skin (Cvjetko Bubalo et al. 2016). In the case of particularly resistant matrices
(typically grape skin), the extraction time of 1 h in maceration at 65 °C is generally
found (Radošević et al. 2016).
Importantly, this parameter is also very dependent on the technologies employed.
For example, the use of microwave irradiation drastically reduces duration, allowing effective extractions in less than 20 min. Chen et al. have performed parameter
optimization on Radix Salviae miltiorrhizae by combining RSM and BBD (BoxBehnken design) methodologies. A maximum extraction rate was determined after
11 min under microwave irradiation at 70 °C (800 W max) (Chen et al. 2016).
However, extractions without activation, at room temperature, are very rarely performed in less than 60 min (Dai et al. 2013c; Fu et al. 2017c; Xu et al. 2019). A
direct correlation between temperature and extraction time has been established by
Bajkacz et al. for the extraction of isoflavones from soy products (Bajkacz and
Adamek 2017). In this study, the central composite design (CCD) was employed to
find the optimal values between several variables, notably temperature and extraction time. Not surprisingly, the higher the temperature, the shorter is the extraction
time. By using ultrasound, it is however possible to reduce the extraction time even
at room temperature (25 °C). Meng and colleagues have shown that the optimal
extraction time for 4 flavonoids from Pollen Typhae was only 35 min (Meng
et al. 2018).
Extraction duration influences both the efficiency and selectivity of extraction.
Depending on the affinity of the metabolites for a given solvent but also depending
on their location in the plant, they are more or less rapidly extracted. Thus, if a longer extraction time may improve the extraction efficiency, it could also increase the
range of extracted metabolites. It is therefore essential to find the right balance
7 Extraction of Plant and Algal Polyphenols Using Eutectic Solvents