# Modern Techniques for Flavonoid Extraction--To Optimize or Not to Optimize?
## 4 Selection of Variables for the Optimization of Modern Techniques for Flavonoids Extraction
### Pressurized Liquid Extraction  
Since the dissolution rate, speed of mass transport, and extraction rate of bioactive molecules, as well as specimen wettability and permeability, enhance with temperature, accelerated solvent extraction (ASE), also known as pressurized liquid extraction (PLE) (Figure 6a), is undertaken statically by utilizing heat and pressure to the solvent system and specimens [92]. According to [PERSON] et al. [93] PLE is based on conventional heating at high temperatures and up to 200 bar of pressure, where high pressure is applied to ensure  
Figure 5: (**a**) Scheme of microwave assisted extraction. (**b**) Specific variables that should be optimized for UAE extraction. the solvent is in a liquid condition while high temperature is used to make the analytes more soluble and enhance their extraction from the material (Figure 6b). PLE technology is considered \"green\" extraction technology because of the decreased solvent usage (approximately 100-200 mL of solvent) and speeding up (extraction time approximately 20 min) of the extraction process [94; 95] compared to traditional solid-liquid extraction techniques such as Soxhlet extraction [96]. Another practical advantage over the classic extraction method is that the extract does not need to be filtered after processing because the starting material remains in the extraction cell [97]. Quantitative extraction of various samples of environmental organic chemicals from soils and lipids, as well as the examination of food and biological samples, have all been conducted using PLE technology [98; 99; 100; 101]. Regarding the flavonoids extraction, [PERSON] et al. [103] compared the extraction efficiency of percolation extraction PLE from _Dracoephalum kotschy_, [PERSON] et al. [104] compared the extraction efficiency of percolation extraction PLE from the roots of _Scutellaria pinmatifida_, while [PERSON] et al. [105] and [PERSON] et al. [106] compared extraction with Soxhlet method and PLE for analysis of olive leaves bioactive molecules and extraction of bioactive compounds from _Monteverdia aquifolia_ leaves, respectively. All the described studies showed extraction yield improvement when using PLE over classic extraction methods. Moreover, [PERSON] et al. [107] evaluated PLE extraction conditions (pressure 100-200 bar, flow rate 1-3 mL/min and time 0-60 min) using goldenberry as the extraction matrix and ethanol as the extraction solvent. They showed that the highest yield of bioactive compounds was obtained for 10 min of extraction and that the compounds were extracted at 100 bar and 200 bar regardless of the flow rate. On the other hand, [PERSON] et al. [108] compared PLE and UAE for the extraction of bioactive compounds from industrial Tahiti limes (_Citrus latifolia_ Tan.). While UAE was conducted with variable ultrasonic power (160-792 W), time (2-10 min), and solvent-to-feed mass ratio (20-40 kg solvent/kg dry pomace), PLE was carried out at various temperatures (60-110 \({}^{\circ}\)C) and time periods (5-40 min). In the case of PLE, the highest temperature (110 \({}^{\circ}\)C) produced the best results for the overall yield and total phenolic, with the exception of flavonoids (hesperidin and nariruttin), which were not significantly affected by temperature. Both methods used ethanol and water (3:1, wt.) as the solvent. Recently, researchers have focused on combining the UAE and PLE into ultrasound-assisted  
Figure 6: (**a**) Scheme of pressurized liquid extraction. (**b**) Specific variables that should be optimized for PLE extraction. pressurized liquid extraction (UAPLE) to improve the mass transfer rate and thus increase the extraction yield [100; 109; 110]. As described by [PERSON] et al. [97], PLE is unable to discriminate between compounds from related phenolic classes. The resulting extracts consist of a variety of compounds and therefore it is necessary to couple extraction and separation methods to obtain the target molecule. For this reason, [PERSON] et al.