Journal of Tea Science Research, 2024, Vol.14, No.2, 102-111 http://hortherbpublisher.com/index.php/jtsr 103 2 Chemical Properties of Tea Polyphenols 2.1 Structure and types of polyphenols in tea Tea polyphenols are a diverse group of naturally occurring compounds found in tea leaves, primarily consisting of catechins, theaflavins, thearubigins, flavonols, and phenolic acids (Figure 1) (Yan et al., 2020; Truong and Jeong, 2021). The most abundant and well-studied catechin in green tea is (-)-epigallocatechin-3-gallate (EGCG), which has been shown to possess a wide range of biological activities (Mokra et al., 2022b). Other significant catechins include epicatechin, epicatechin gallate, and epigallocatechin (Diao et al., 2019). The chemical structure of these polyphenols is characterized by multiple hydroxyl groups attached to aromatic rings, which contribute to their potent antioxidant properties (Truong and Jeong, 2021). Figure 1 Chemical structures of major tea polyphenols (Adopted from Truong and Jeong, 2021) 2.2 Mechanisms of antioxidant action Tea polyphenols exhibit their antioxidant action through several mechanisms. They act as direct antioxidants by scavenging reactive oxygen and nitrogen species, chelating transition metals, and inhibiting lipid, protein, and DNA oxidation (Truong and Jeong, 2021). Additionally, tea polyphenols can suppress pro-oxidant enzymes and induce endogenous antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GSH-Px) (Diao et al., 2019; Wang et al., 2019). These compounds also modulate cellular signaling pathways, including the nuclear factor erythroid 2-related factor 2 (NFE2L2) pathway, which plays a crucial role in the cellular defense against oxidative stress (Ma et al., 2022). 2.3 Stability and bioavailability of tea polyphenols The stability and bioavailability of tea polyphenols are critical factors that influence their effectiveness as antioxidants. Polyphenols in tea are relatively stable under acidic conditions but can degrade under alkaline conditions, which can occur during intestinal digestion (Baeza et al., 2018). The bioavailability of these compounds is influenced by their chemical structure, with galloylated catechins like EGCG being less bioavailable compared to non-galloylated catechins (Mokra et al., 2022b). Despite these challenges, regular consumption of tea provides a significant amount of polyphenols that can exert beneficial effects on health by preventing oxidative stress-related disorders (Baeza et al., 2018).
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