Journal of Tea Science Research, 2024, Vol.14, No.1, 19-43 http://hortherbpublisher.com/index.php/jtsr 24 oxidation, the weakened biosynthesis may also contribute to the decrease in flavonoid levels (Chen et al., 2020b). The other step that white tea undergoes is the drying process, which can also alter heat-sensitive tea catechins if conducted at elevated temperatures (Wong et al., 2022). Withering would also change the levels of other important biomolecules in white tea, namely theanine and caffeine. Chen et al. (2020) found that even though the total free amino content increased during withering, theanine content slowly decreased because it could not be supplied by the proteolysis or transported from tea roots to leaves, whilst the metabolism of theanine continued. Similar results were also confirmed by Dai et al. (2017) and Zhou et al. (2022). In the same metabolomic studies, it was reported that the caffeine content increased significantly, which could be attributed to the abundant nucleosides and nucleotides provided by ribonucleic acid (RNA) degradation, precursors of caffeine synthesis. 2.3 Phytochemical degradation of white tea during storage It is widely believed in China that the aging for at least three years could greatly improve the health benefits of white tea, which is marketed at higher prices than teas produced and sold in the same year (Zhao et al., 2022). However, chemical analysis showed that some major bioactive molecules in white tea tend to decrease over storage time. The theanine content of longevity eyebrow reduced drastically from 6.4 mg/g to 0.43 mg/g over the storage period from 3 to 16 years, while the total catechin concentration decreased from 8.1 mM to 1.9 mM from 3 to 6 years but increased slightly to 3.2 mM on the 16th year (Ning et al., 2016). During storage, new chemicals are formed by using original white tea components as starting materials. Dai et al. (2018) reported that new compounds, 8-C N-ethyl-2-pyrrolidinone-substituted flavan-3-ols (EPSFs), were constantly formed from theanine and catechins during white tea storage for up to 16 years. Due to the loss of white tea catechins and other functional components, negative correlations were found between the antioxidative activities and their inhibitory effects on key enzymes related to diabetes mellitus type 2 (T2DM), at storage time of 1, 3, and 5 years (Xu et al., 2019). Similar results were reported by Xie et al. (2019), who found that the amount of EPSFs was a good predictive parameter for the storage time of white teas. Although some bioactive molecules of white tea are lost during storage, it is likely that the sensory development during aging contributed to its high market price. It was reported that the decrease and transformation of abundant flavonoids, tannins and amino acids were related to the reduced astringency, umami and increased browning of tea infusions (Fan et al., 2021). 3 Extraction of Active Biomolecules from White Tea White tea is a rich source of catechins, which have been widely used in the food industry as natural antioxidants to inhibit lipid oxidation. The specific chemical structure of tea catechins can neutralize oxidative stress by scavenging free radicals to delay the onset of lipid oxidation and by chelating metal ions to interfere with the catalysis of oxidative reactions (Musial et al., 2020). As natural antioxidants, tea catechins have the advantages of having higher potency, better safety, and more potential health benefits than their synthetic counterparts. The catechins, especially EGCG, have been found to have more than 4 times the relative potency of butylated hydroxyanisole, which is a synthetic antioxidant often added to foods to preserve fat or oil (Vuong et al., 2010). In addition to the strong antioxidant capacity of tea catechins, safety concerns about synthetic antioxidants and customers’ demand for natural ingredients have driven the food industry to favor the use of plant-derived phenolic antioxidants, e.g., tea catechins. Catechins, along with other active biomolecules in tea, such as caffeine and theanine, usually provide additional nutraceutical value to white tea extract (WTE) applications in foods. 3.1 Conventional extraction solvent properties, extraction times, temperature, etc. The chemical functionalities and health benefits of WTE depend on the content and composition of bioactive compounds. Thus, to produce high quality WTE, it is crucial to optimize the extraction conditions and understand the effects of various steps in the process. In general, the conventional extraction methods can be divided into two categories: water-mediated brewing and organic solvent-assisted solvent extraction (Table 3).
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