Journal of Tea Science Research, 2024, Vol.14, No.1, 19-43 http://hortherbpublisher.com/index.php/jtsr 27 significant increase in the phytochemical content was found after 5 min (the recommended infusion time for hot tea preparation) brewing at 90 °C for bagged white tea (Shannon et al., 2018). In contrast, the optimal extraction time was found to be over 40 min, when intact tea leaves were used (Lin et al., 2017). Thus, the size of white tea materials may also be an important parameter for the best brewing efficiency, with smaller ones being easier to extract. For a certain type of white tea, the extraction process is quite sensitive to temperature, even for hot tea brewed at more than 80 °C. When the brewing temperature of Fuding white tea increased from 80 °C to 100 °C (5 min), the EGCG, caffeine, and theanine contents increased dramatically from 78, 226, and 48 mg/L to 139, 388, and 75 mg/L, respectively, presumably because higher temperatures disrupted cell wall structures and facilitated the water penetration, along with diffusion of tea components (Zhang et al., 2017b). It is noteworthy that extraction at high temperatures above 85 °C for a prolonged time could induce two types of structural transformations, degallation and epimerization: the former hydrolyzes EGCG to gallic acid (GA) and the non-galloylated catechins, while the latter converts catechins from epi-catechins (EC, EGC, ECG, EGCG) to their corresponding trans-isomers (C, GC, CG, GCG) (Lin et al., 2017). Another important but relatively less recognized factor affecting brewing kinetics is water quality. Along with many other tea catechins, the EGCG content was the highest at 116 mg/L in the Fuding white tea infusion prepared with pure water with the lowest ionic level, compared to only 84 mg/L for tap water with the highest ionic level, and 101 mg/L for spring water with a medium ionic level (Zhang et al., 2017a). The common minerals in water, i.e., Ca2+, Mg2+, Na+, and K+, are known to lower the mass transfer rate from solid tea leaves to bulk liquid water in the brewing process (Murugesh et al., 2017). A lower pH favors the extraction of white tea polyphenols, which can be achieved naturally by adding lemon juice (Rusak et al., 2008). At higher pH, the stability of tea catechins may be compromised by the occurrence of degallation (Cao et al., 2021) and epimerization (Xu et al., 2017). In addition, the rate of auto-oxidation increases with pH, with the pyrogallol moiety on the B ring of EGC and EGCG being more susceptible to oxidation than the catechol moiety of EC and ECG (Murugesh et al., 2017). Since the white tea infusions are made ready-to-drink after the brewing process, the sensory attributes, including color, aroma, and taste, should be considered in addition to the physiochemical properties. Brewing conditions are particularly important determinants of the sensory properties of white tea, which does not contain flavor compounds produced by fermentation, but only natural chemical components of tea leaves (Pastoriza et al., 2017). Several studies have reported a positive relationship between the content of phenolic compounds and sensory characteristics. For example, since EGCG is the major contributor to bitterness, white tea drinks brewed with hard water containing less galloylated catechins had lower overall sensory acceptability and a darker color (Cao et al., 2021). In contrast, pure weakly acidic water with low concentration of dissolved ions produced white tea infusions with the highest catechin content and superior color, aroma, and taste (Zhang et al., 2017a). In terms of the temperature and time combination, brewing at 98 °C for 7 min was found to be the best condition to obtain a high content of antioxidant polyphenols while maintaining pleasant sensory properties (Pérez-Burillo et al., 2018). The water extraction technique has the advantages of low cost, toxin-free, and relatively simple setups. However, its efficiency may be limited due to poor selectivity for target bioactive molecules and loss of heat sensitive compounds during exposure to high temperature for a prolonged period. 3.1.2 Solvent extraction Organic solvents and their aqueous formulation have been studied for extraction of bioactive compounds from white tea with various efficiencies. It was reported that 40% aqueous ethanol was more effective in extracting tea phenolics and flavonoids than the 10% and 70% ethanol solvents at extraction times of 15 and 30 min, although the opposite trend was found for 5 min (Rusak et al., 2008). Similarly, 50% ethanol was reported to be the optimal concentration to maximize the extraction of total polyphenols, at an extraction time of 47.5 min and temperature of 65 °C (Peiró et al., 2014). The optimal solubility of white tea biomolecules plays a major role in their extractability and extraction efficiency. EGCG, the major white tea flavonoid, has a higher solubility in aqueous
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