291
citric acid, lactic acid, maltose, or glycerol by using different assisted extraction
techniques (Chanioti and Tzia 2018). Total phenolic content obtained from FolinCiocalteu was compared to HPLC analyses. Generally, the two values were hardly
correlated (Chanioti and Tzia 2018).
Paradiso et al. have also proposed a direct spectrophotometric analysis of deep
eutectic solvent extracts of olive oil (Paradiso et al. 2016a). Deep eutectic solvent
based on glucose and lactic acid was used to extract phenolic compounds in extra
virgin olive oil. By simply measuring the absorption of the deep eutectic solvent
extracts at few wavelengths, a screening of the total phenolic content of the oils
could be performed, reducing significantly the use of hazardous solvents and
reagents (Paradiso et al. 2016a).
Electrochemical techniques have been also used to quantify the amount of polyphenols, mainly flavonoids, in complex matrices such as wine (Makhotkina and
Kilmartin 2010, 2012; Šeruga et al. 2011), tea or coffee (Kilmartin and Hsu 2003;
Piljac-Žegarac et al. 2010), beer (Oliveira Neto et al. 2017a, b), or human urine
sample (Adam et al. 2007). Following these works, detection of quercetin has been
studied in highly diluted natural deep eutectic solvent (concentration of deep eutectic solvent lower than 10% in phosphate buffer) using screen-printed electrodes
(Gomez et al. 2016). The authors showed that an increase of deep eutectic solvent
concentration had a deleterious effect on the quercetin electrochemical signal. The
method was then applied to onion extracts and compared with HPLC, showing good
correlation (Gomez et al. 2016).
7.5.2 Antioxidant Activity
Along with the total phenolic content, antioxidant activity is also systematically
examined in the deep eutectic solvent extracts using classical assays (Galili and
Hovav 2014). The reducing power is generally determined by ferric reducingantioxidant power assay (FRAP), based on the reduction of ferric tripyridyltriazine
complex to the intensively blue ferrous tripyridyltriazine complex at low pH. The
complex formation is monitored by visible electronic spectrum at ca. 620 nm. The
ferric reducing-antioxidant power values in deep eutectic solvent extracts are
expressed as μmol of ascorbic acid or trolox per g of dry matter (Bakirtzi et al. 2016;
Georgantzi et al. 2017; Athanasiadis et al. 2018a, b, c; Jeong et al. 2018; Yoo et al.
2018; Pal and Jadeja 2019). Importantly, ferric reducing-antioxidant power (FRAP)
assay cannot detect antioxidant activity of compounds acting by radical quenching.
Radical scavenging activity is generally conducted with DPPH assay or alternatively by using ABTS or ORAC assays although less often employed with deep
eutectic solvent extracts. DPPH (2,2-diphenyl-1-picrylhydrazyl hydrate) and
ABTS
•+
(2,2’-azino-bis(3-ethyl-benzothiazoline-6-sulfonic) acid) are stable radical
molecules, which display intense violet and green-blue color, respectively. The
DPPH and ABTS
•+
assays are both based on the quenching of the colored radicals
because of their reduction with the polyphenols. From the decrease of the
7 Extraction of Plant and Algal Polyphenols Using Eutectic Solvents
citric acid, lactic acid, maltose, or glycerol by using different assisted extraction
techniques (Chanioti and Tzia 2018). Total phenolic content obtained from FolinCiocalteu was compared to HPLC analyses. Generally, the two values were hardly
correlated (Chanioti and Tzia 2018).
Paradiso et al. have also proposed a direct spectrophotometric analysis of deep
eutectic solvent extracts of olive oil (Paradiso et al. 2016a). Deep eutectic solvent
based on glucose and lactic acid was used to extract phenolic compounds in extra
virgin olive oil. By simply measuring the absorption of the deep eutectic solvent
extracts at few wavelengths, a screening of the total phenolic content of the oils
could be performed, reducing significantly the use of hazardous solvents and
reagents (Paradiso et al. 2016a).
Electrochemical techniques have been also used to quantify the amount of polyphenols, mainly flavonoids, in complex matrices such as wine (Makhotkina and
Kilmartin 2010, 2012; Šeruga et al. 2011), tea or coffee (Kilmartin and Hsu 2003;
Piljac-Žegarac et al. 2010), beer (Oliveira Neto et al. 2017a, b), or human urine
sample (Adam et al. 2007). Following these works, detection of quercetin has been
studied in highly diluted natural deep eutectic solvent (concentration of deep eutectic solvent lower than 10% in phosphate buffer) using screen-printed electrodes
(Gomez et al. 2016). The authors showed that an increase of deep eutectic solvent
concentration had a deleterious effect on the quercetin electrochemical signal. The
method was then applied to onion extracts and compared with HPLC, showing good
correlation (Gomez et al. 2016).
7.5.2 Antioxidant Activity
Along with the total phenolic content, antioxidant activity is also systematically
examined in the deep eutectic solvent extracts using classical assays (Galili and
Hovav 2014). The reducing power is generally determined by ferric reducingantioxidant power assay (FRAP), based on the reduction of ferric tripyridyltriazine
complex to the intensively blue ferrous tripyridyltriazine complex at low pH. The
complex formation is monitored by visible electronic spectrum at ca. 620 nm. The
ferric reducing-antioxidant power values in deep eutectic solvent extracts are
expressed as μmol of ascorbic acid or trolox per g of dry matter (Bakirtzi et al. 2016;
Georgantzi et al. 2017; Athanasiadis et al. 2018a, b, c; Jeong et al. 2018; Yoo et al.
2018; Pal and Jadeja 2019). Importantly, ferric reducing-antioxidant power (FRAP)
assay cannot detect antioxidant activity of compounds acting by radical quenching.
Radical scavenging activity is generally conducted with DPPH assay or alternatively by using ABTS or ORAC assays although less often employed with deep
eutectic solvent extracts. DPPH (2,2-diphenyl-1-picrylhydrazyl hydrate) and
ABTS
•+
(2,2’-azino-bis(3-ethyl-benzothiazoline-6-sulfonic) acid) are stable radical
molecules, which display intense violet and green-blue color, respectively. The
DPPH and ABTS
•+
assays are both based on the quenching of the colored radicals
because of their reduction with the polyphenols. From the decrease of the
7 Extraction of Plant and Algal Polyphenols Using Eutectic Solvents