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In addition to water, tea is one of the most popular beverages consumed worldwide because of its many health-promoting benefits, such as antioxidation, anticancer, anti-inflammatory, and hypolipidemic activities. Tea contains a number of compounds, including the polyphenolic compounds, which are recognized as the main functional components in various types of teas. Catechins, which are the predominant polyphenolic compounds in green tea, can be oxidized during fermentation to produce different oxidized polyphenols, including theaflavins, thearubigins, and theabrownins. Theaflavins and thearubigins are present in high concentrations in fully fermented black tea, whereas theabrownins are the main polyphenolic compounds in post-fermented dark teas, such as Fuzhuan brick tea and Pu-erh tea. With the growing popularity of dark tea in China, the theabrownins, one of the main bioactive macromolecules in the dark tea, have received increasing attention because of its health benefits.
In the past few years, studies have provided important information on the functions and bioactivities of theabrownins, demonstrating that they can inhibit the activity of α-glucosidase in vitro, decrease blood glucose levels and increase glucose tolerance in mice under hyperglycemic stress. Additionally, they exhibit anti-osteoporotic, anti-tumor, and hypolipidemic effects in vitro and in vivo. Nonetheless, because of its complexity, the physicochemical properties of theabrownins are rarely studied. Since increasing attention has been paid to the health-promoting effects of post-fermented teas, theabrownins, which are the main bioactive compounds in these teas, deserve more investigations. Based on a few reported studies, theabrownins are a type of brown or reddish-brown pigment that are soluble in water but are insoluble in ethyl acetate, n-butyl alcohol or other organic solvents. In addition, theabrownins are heterogeneous macromolecules that polymerized, condensed, coupled, and oxidized from catechins, theaflavins, and thearubigins with other compounds, such as proteins, lipids, polysaccharides, and some other components. Because of their condensed nature, the molecules of theabrownins cannot be easily separated by chemical methods, the structural characteristics of theabrownins remain unclear, thus their applications remain limited.
It is observed that the molecular weights and the different sources of theabrownins have an impact on their structure and composition. Therefore, researches on relatively pure and specific theabrownins are necessary. In our previous study, Eurotium cristatum fermented loose tea, a type of fungal fermented dark tea, has exhibited potent hypolipidemic activity in high-fat-induced zebrafish, which is related to the activity of theabrownins. However, the physiochemical properties of theabrownins are unknown.