# Modern Techniques for Flavonoid Extraction--To Optimize or Not to Optimize?
## 4 Selection of Variables for the Optimization of Modern Techniques for Flavonoids Extraction
### Application of Environmentally Friendly Extraction Solvents  
Although intensive work is being performed to develop new approaches to the extraction process, work continues to improve conventional methods in which extraction is based on solvent extraction. The development of new solvents for extraction has taken a step forward. To make the extraction process environmentally friendly, there is a specific focus on the selection of green solvents. Aqueous two-phase systems (ATPS, Figure 8) are recognized as an efficient alternative to the traditionally used organic solvents [144] due to their moderate process conditions, environmentally friendly qualities, capacity for continuous operation, easy scaling-up, rapid phase separation, low cost, and great throughput [58; 145; 146]. They can be formed by mixing of different components in water. The most common combination is two polymers and a polymer-salt (i.e., phosphate salt). [PERSON] et al. [147] efficiently coupled UAE and ATPS for the extraction of flavonoids from juibube peels, [PERSON] et al. [148] used ammonium sulfate-ethanol ATPS for the extraction of bioactive compounds from _Aronia melanocarpa_ berries, [PERSON] et al. [149] selected a dipotassium hydrogen phosphate-ethanol system for extracting four flavonoids (lysioside C, nevadensin-7-sambubioside, ikarisoside B, and nevadensin) from _Lysionotus pauciformis_, [PERSON] et al. [150] used polyethylene glycol-ammonium sulfate ATPS as the extraction medium for extraction of flavonoids from hawthorn leaves, and [PERSON] et al. [151] applied ethanol-ammonium sulfate for flavonoids extraction from Tibetan sea-buckthorn (_Hippophae tubletana_) fruit. Deep eutectic solvents (DESs) are becoming increasingly popular as an alternative to the traditionally used organic solvents for the extraction process [57; 152; 153; 154; 155; 156; 157]. DESs are often produced using safe, nontoxic, and affordable and biodegradable components [158; 159; 160; 161; 162]. A quaternary ammonium salt serves as both a hydrogen bond acceptor (HBA) and a hydrogen bond donor (HBD), which associate with one another through hydrogen bonding, in a DES, which is a combination of two or more compounds having a melting point lower than the individual components [160]. There are numerous examples of the use of DESs as efficient media for the extraction of bioactive molecules from various sources [163]. Figure 8: Schematic representation of target molecule separation using ATPS. Today, DESs are often used in combination with modern extraction techniques. There are also examples of the use of DESs coupled with UAE for the extraction of flavonoids. For example: (i) [PERSON] et al. [164] examined the effectiveness of nine various choline chloride-based DES systems with various compositions (contained glucose, citric acid, glycerol, urea, citric acid, glycerol, 1,4-butanediol, lactic acid, malonic acid, malic acid, or xylose alcohol as hydrogen bond donor) for the extraction of flavonoids from _C. paliurus_ leaves and the choline chloride/1,4-butanediol system (1:5 molar ratio) was selected as the optimal system for maximizing the flavonoid extraction yield; (ii) [PERSON] et al. [45] analyzed the efficiency of eight DESs (hydrogen bond acceptors (HBA) were choline chloride, levulinic acid, lactic acid, malic acid, and citric acid; hydrogen bond donors (HBD) were ethylene glycol, L-proline-levulinic acid, 1,4-butanediol, and glycerol) for flavonoids extraction from _Acanthopanax semicosus_ and DES composed of glycerol and levulinic acid (1:1) was chosen as the most appropriate extraction solvent; (iii) [PERSON] et al. [58] evaluated 18 different choline chloride-based DESs as solvent for the flavonoids extraction from common buckwheat (_Fagopyrum esculentum_) sprouts and selected choline chloride-triethylene glycol DES with 20% water as the most efficient; (iv) [PERSON] et al.