Journal of Tea Science Research, 2024, Vol.14, No.1, 19-43 http://hortherbpublisher.com/index.php/jtsr 28 ethanol solutions than in water and pure ethanol, which may contribute to the higher extraction efficiency in the mixed water-ethanol solvent system (Hu et al., 2016). Under ultrasound assisted extraction, methanol was found to be the optimal solution for the extraction of white tea polyphenols with the highest radical scavenging activities (Ahmadi et al., 2022). Compared with ethanol (50% and 100%), and 50% methanol, pure methanol possessed the highest extraction yield with more than twice the amount of volatile compounds than the other solvents (Sereshti et al., 2013). Different from the conventional brewing method, solvent extraction usually has higher extraction efficiency and lower energy consumption required by heating. However, utilization of organic solvent may introduce concerns in operation safety, presence of toxic residues, and requirement for additional purification process. 3.2 Assisted extraction Conventional extraction methods are a simple process of soaking a pulverized white tea sample in the appropriate solvent in a closed system, followed by constant or sporadic agitation. However, these techniques have some specific drawbacks, such as time consuming, solvent requirement, low extraction selectivity, and decomposition of thermolabile compounds (Manousi et al., 2019). To address these issues, some innovative assisted extraction techniques have been developed, such as high-pressure assisted extraction, microwave-assisted extraction, and ultrasound-assisted extraction (Table 4). Table 4 Assisted extraction methods for white tea Assisted extraction method Parameters tested Results Reference High pressure Pressure: 200 MPa and 500 MPa; Time: 5 and 10 min Most efficient phenolic extraction from white peony was under the pressure of 200 MPa for 5 min Šeremet et al., 2021 Pressure: 300, 400, and 500 MPa; Solid to liquid ratios: 1, 2, and 3%; Time: 120, 360, and 600 s The optimal extracting conditions were 300 MPa, 2.2% solid to liquid ratio, and 10min Uzuner and Evrendilek, 2019 Microwave Power level: 114, 229, and 399 W; Time: 0.3-3.7 min; Ethanol concentration: 0-100%; Liquid/solid ratio: 15.9-184.1 mL/g Conditions for the highest amounts of total phenolics were 229W, 38.8% ethanol, 184 mL/g liquid/solid ratio, and 3 min of extraction Rehder et al., 2021 Ultrasound Ultrasound intensity: 40%, 70%, and 100%; Time: 5, 10, and 15 min; Solvent: ethanol, methanol, and combined ethanol/methanol The highest total phenolic content and free radical scavenging activity were obtained at 70% sonication intensity, 15 min, and methanol as solvent Ahmadi et al., 2022 Temperature: 25-55 °C; Time: 10-60 min; Volume of preconcentration solvent: 20-50 μL; Salt concentration: 5-15% The optimal extraction conditions for volatile compounds were 21 min, 32 °C, 27 μL extraction solvent, and 7.4% salt Sereshti et al., 2013 3.2.1 High pressure assisted extraction The high pressure can trigger various phenomena that induce a reaction in the direction of promoting volume decrease, which leads to the improvement of extraction efficiency (Khan et al., 2019). The highest total phenolic content (3,136 mg/L) was determined in the ethanolic (50%) extract of white peony under the pressure of 200 MPa for 5 min, while the highest EGCG (1,446 mg/L), ECG (354 mg/L) and caffeine (863 mg/L) contents were determined at the same pressure level but with a longer pressure holding time of 10 min (Šeremet et al., 2021). In another study, the optimal conditions were achieved using 300 MPa, 2.2% solid to liquid ratio, and 10 min brewing time, which resulted in 92% total antioxidant activity, 1,949 mg/L total phenolic content, and 17.5% total caffeine content (Uzuner and Evrendilek, 2019). High-pressure assisted extraction appears to be a promising
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