Journal of Tea Science Research, 2024, Vol.14, No.1, 19-43 http://hortherbpublisher.com/index.php/jtsr 29 alternative novel technique to minimize caffeine content while maximizing the amounts of total amino acids and phenolic compounds in cold-brewed white tea. 3.2.2 Microwave assisted extraction Microwaves are nonionizing electromagnetic waves within the frequency range of 300 MHz to 300 GHz, of which 915 MHz is considered the most useful for industrial applications due to its greater penetration depth, while 2,450 MHz frequency is generally used in domestic microwave ovens and for extraction applications (Routray and Orsat, 2012). It was reported that a microwave oven with a frequency of 2,450 Hz and a power of 229 W extracted the highest amounts of total phenolic and flavonoid compounds from a white tea blend with the predicted optimal conditions of 38.8% ethanol concentration, 3 min of extraction, and a 184 mL/g liquid/solid ratio (Rehder et al., 2021). 3.2.3 Ultrasound assisted extraction Ultrasound-assisted extraction of target compounds from a given matrix is a complex mechanism involving mass transfer and a variety of possible chemical reactions that affect yield and associated biological activities. In general, the effect of ultrasound on mass transfer is directly related to the sound energy introduced into the extraction system and the ultrasound frequency (Tiwari, 2015). The highest total phenolic content (68 mg/g) and free radical scavenging activity (78%) were observed for the following optimal conditions: 70% sonication intensity, 15 min sonication time and methanol as solvent (Ahmadi et al., 2022). Assisted extractions are promising alternatives for the conventional methods, but they may also have some drawbacks. For example, the usage of specific equipment complicates the extraction process whilst increases the energy consumption and cost expenditure. Even though several attempts have been made to improve the extraction efficiency of white tea, the literature is still limited regarding the combination of multiple techniques and the effects of both equipment settings and extraction conditions. Moreover, other techniques should be explored in the future, such as enzyme-assisted extraction, supercritical fluid extraction, accelerated solvent extraction, and subcritical water extraction (Raghunath et al., 2023). 3.3 Stability of white tea extract and stabilization techniques 3.3.1 Storage stability It is commonly observed that bioactive compounds in the extracted white tea tend to be destabilized during storage but the packaging material may play an important role in slowing down this process. The antioxidant activity of WTE decreased by 20%-50% after 3 months of storage at room temperature, regardless of the method used to measure the antioxidant activity (Pastoriza et al., 2017). However, extending storage to 6 months did not induce additional changes in the same study, possibly due to the oxidation of the sensitive phenolic compounds during the first 3 months, which protected the other antioxidants for longer periods. It was also reported that tea catechins degraded by at least 50% during the first month of storage in canned commercial soft drinks and accompanied by a decrease in biological activities (Zeng et al., 2017). Cold storage of tea beverages in polyethylene terephthalate bottles at 4 °C was reported to ensure a slower decline in catechin content in white, black, and green teas (Nekvapil et al., 2012). 3.3.2 Stabilization techniques To address the inherent drawbacks of tea polyphenols TPPs, e.g., low stability and bioavailability, the encapsulation of TPPs is desired to improve their stability against harsh environment for shelf-life extension and bio-efficacy enhancement (Yin et al., 2022). Polymeric nanoparticles (NPs) prepared by nanoencapsulation of WTE based on poly ε-caprolactone and alginate released 20% of the polyphenols in simulated gastric medium, and 80% after 5 h at pH 7.4, showing a good ability to control the delivery of polyphenols (Sanna et al., 2015). Furthermore, NPs protected tea polyphenols from degradation so that WTE retained its antioxidant activity. An increase in the thermal stability of liposomal WTE samples was registered according to the physical stability of
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