# Modern Techniques for Flavonoid Extraction--To Optimize or Not to Optimize?
## 4 Selection of Variables for the Optimization of Modern Techniques for Flavonoids Extraction
### Microwave-Assisted Extraction  
In microwave-assisted extraction (MAE) (Figure 5a), microwaves are used to heat the matrix [77]. Their frequency ranges from 300 MHz to 300 GHz. Magnetic and electric fields in microwaves are perpendicular to one another. The electric field causes heating by ionic conduction and dipole rotation [78]. Depending on their dielectric characteristics, the components absorb microwave radiation. The radiation promotes cell rupture, allowing liquid to penetrate through the plant matrix. On the other hand, plant material enters the solution from outside the cells [79]. According to [PERSON] et al. [80], the efficiency of microwave-assisted extraction is affected by: (i) solvent nature and volume, (ii) extraction time, (iii) microwave power, (iv) operating temperature and (v) properties of the matrix, and all listed factors must be taken into account during process optimization (Figure 5b).  
[PERSON] et al. [81] compared the efficiency of microwave-assisted extraction, ultrasound-assisted extraction and Soxhlet extraction using ethyl acetate as solvent for the extraction of bioactive compounds from _Lagenaria siceraria_ and showed that MAE was the most efficient method for the extraction of flavonoids (24.28 mg/g), while [PERSON] et al. [82] showed that MEA was more efficient than UAE for flavonoids from peach waste. Similarly, [PERSON] et al. [64] compared maceration, ultrasonic- and microwave-assisted extraction for total phenolic content and total flavonoid content of Cassia alata. Their results showed that microwave MAE has the highest TPC and TFC yield (37.92 mg/g DW, 135.18 mg/g DW), followed by ultrasonic-assisted extraction and maceration extraction. Moreover, [PERSON] et al. [83] compared the conventional maceration and MAE extraction yield of polyphenols and flavonides from _Pistacia vera_ L. leaves (male and female), [PERSON] et al. [84] compared the conventional maceration and MAE extraction yield for extraction of flavonoids from the _Vermonia amygdalina_ leaf, and [PERSON] et al. [85] studied UAE and MAE for extraction of flavonoids from _Myrtus communis_ L. In all the experiments described, MAE was the preferred method for obtaining the highest extraction yield of flavonoids. Similarly, [PERSON] and [PERSON] [86] presented MAE as the best method for the extraction of three flavonoids with therapeutic properties including isorhamnetin, luteolin and rutin from the leaves and flowers of the Russian olive. [PERSON] et al. [87] analyzed the most important factor affecting flavonoids extraction yields from fenugreek seeds and showed that ethanol concentration was the most significant and contributing factor for the MAE process. [PERSON] et al. [88] analyzed the effects of processing time, extraction temperature, ethanol concentration and solid-to-liquid ratio on the flavonoids recovery from tomato using MAE and their results showed that water was preferable as a solvent.  
Furthermore, [PERSON] et al. [89] coupled MAE and aqueous two-phase systems (ATPS) for the simultaneous extraction and determination of vitexin, isovitexin, orientin and isovorientin from _Crotalaria sessiliflora_ L. They described that MAE with ethanol/ammonium sulfate ATPS ensures from two to five fold higher extraction yield of specific flavonoid when compared to a mono-phase solvent such as water and ethanol. On the other hand, [PERSON] et al. [90] evaluated the applicability of the ten ionic liquids for MAE of flavonoids hesperidin, hyperoside and rutin from the leaves of _Sorbus tianschanica_ and selected 1-hexyl-3-methylimidazolium tetrafluorolate as the extracting solvent, while [PERSON] et al. [91] enhanced flavonoids extraction from _Helichrysum arenarium_ by using 1.0 M [C 8 mim]Bras as the solvent.