Journal of Tea Science Research, 2024, Vol.14, No.1, 19-43 http://hortherbpublisher.com/index.php/jtsr 23 processed into six typical tea types, green tea had the highest total catechin content of 131 mg/g, while white tea contained 75 mg/g of total catechin (Wang et al., 2018). In terms of caffeine and theanine, the caffeine content is similar among different tea varieties, while the theanine content of black tea is usually slightly lower than that of white tea. Specifically, the mean caffeine content of white, green, oolong, and black teas were 17, 16, 19, and 18 mg/g, respectively, and theanine content values were 6.3, 6.6, 6.1, and 5.1 mg/g (Boros et al., 2016). In contrast, another study reported a much higher caffeine content in white tea (4.9%) than green tea (2.9%) and a higher theanine content of white tea (1.9%) than that in black and green teas (Hilal and Engelhardt, 2007). The higher amount of theanine could be attributed to fresh young shoots used for white tea production, which are known to be rich in functional compounds like theanine (Sonawane et al., 2021). However, less theanine was detected in white tea than in green and black teas produced from the same batch of tea leaves when a prolonged withering process (36 h) was used (Dai et al., 2017). Similar results were reported by Wang et al. (2018), who found that comparing teas made from the same batch of tea leaves, the theanine content in white tea retained 63% of theanine in fresh tea leaves after 48 h withering, which was significantly lower than that of green tea (95%) and black tea (82%). In addition to the inherent variation among teas, the compositional inconsistency among different tea types may also result from diverse extraction procedures and assay protocols used to determine certain tea components. 2 White Tea Composition Affected by Harvest, Processing, and Storage From the fresh plant material to the white tea ready for consumption or other uses, tea leaves would go through processing and storage. Both steps determine the chemical composition and quality of white tea. Traditional research techniques and recent advances in the metabolomic approach have enabled compositional analysis of the enormous number of tea components and their changes (Li et al., 2022). 2.1 Phytochemical composition of white tea harvested from different seasons Tea leaves harvested in different seasons possess distinctive chemical profiles and various levels of bioactive molecules in white tea. Higher total phenolic content was observed in white tea harvested in the summer and early fall than in spring. Conversely, contrasting results were obtained for total flavonoid content, with higher values in spring than in summer (Paiva et al., 2021). Similarly, Fang et al. (2017) reported that in fresh tea shoots consisting of one apical bud and two adjoining leaves, the abundance of epicatechins and catechins were lower in March than September, with EGC content mostly influenced by the time of harvest. Interestingly, the same study found that theanine content was lower in May compared to March and September, and tea leaves contained richer caffeine in March than September. In contrast, catechins, theanine, and caffeine concentrations were reported to be higher in early spring silver needle tea than in late spring white peony tea and fall longevity eyebrow tea (Tan et al., 2017). The Xinyang white tea plucked in spring had higher levels of EGCG and theanine while fall tea had a remarkably higher level of caffeine (Ma et al., 2022). 2.2 Constituent transformation of white tea during processing The production of white tea involves only two steps: withering and drying. As the least processed type of tea, white tea retains most of the phytochemicals of Camellia sinensis L.. After the plucking, white tea undergoes wilting or withering, which usually takes place in the sun or on a rack in a heated room for 4-5 h to remove moisture, whereas oolong and black teas are withered for at least twice as long (Kosińska and Andlauer, 2014). The loss of moisture and resulting cell wall breakdown would lead to the release of oxidases and initiation of 5% oxidation catalyzed by oxidases, i.e., phenol oxidase and peroxidase (Hinojosa-Nogueira et al., 2021). Therefore, although white tea is considered as a type of non-fermented tea, it is actually slightly fermented through oxidation. In this process, polyphenol oxidase and native microflora initiate and catalyze the aerobic oxidation of tea catechins. They can dimerize, oligomerize, and even polymerize to a variety of derivatives, including theaflavins, theacitrins, theasinensins, theanaphthoquinones, and thearubigins (Dai et al., 2017). It was reported that the total polyphenol content in white tea decreased from 33% after 4 h to 28% after 93 hours of withering, while thearubigin slightly increased from 3.1% to 3.9% during the same period (Maulana et al., 2020). In addition to
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