| Tea is one of the most popular beverages consumed worldwide because of its attractive aroma and taste. It is the brew prepared from the leaves of the plant Camellia sinensis. Freshly harvested tea leaves require processing to convent them into fermented (black), semi-fermented (oolong) and unfermented tea (green). Among them, black tea accounts for almost 80% of the world tea production. The major components in tea are tea polyphenols, especially catechins including (-)-Epicatechin (EC), (-)Epigallocatechin (EGC), (-)Epicatechin gallate (ECG) and (-)-Epigallocatehin gallate (EGCG). During the processing of tea, catechins undergo enzymatic and chemical oxidation lead to oxidized and polymerized polyphenols. Tea polyphenols have recently received much attention due to their pharmaceutical functions, such as antioxidative, antitumor, and anticarcinogenic activities. Tea polyphenols not only contribute to color and taste of tea beverages, but also have a series of chemical properties including ability to associate with protein, complex divalent transition metal cations, scavenge free radicals due to their phenolic hydroxyl groups. In this paper, the studies were conducted to understand oxidation mechanism, chemical properties of tea polyphenols such as scavenging free radicals and associating with protein. Meanwhile, the variation of tea polyphenols in Qimen black tea processing and their effects on sensory qualities of black tea were investigated.The catechins and their oxidative products should be firstly separated and purified to research their oxidation mechanism, chemical properties. In this paper, a simple and efficient method was developed for the isolation of catechins and theaflavins from tea extract. The crude catechins were submitted to high-speed countercurrent chromatography (HSCCC) and a broad range of HSCCC-compatible solvent systems was evaluated. Morever, partitioning behavior of the catechins was assessed by HPLC analysis of the two phases in solvent systems. The solvent system n-hexanes-ethyl acetate-water (0.5: 3.5: 6) showed favorable partitioningVIIbetween the two layers. The solvent system was successfully performed in separating five relatively pure individual catechins from 300mg of the crude extract in a single run. Compared with Sephadex LH-20 chromatography, HSCCC was a time saving, high efficient and low cost one. It was suitable for preparation of individual catechins. Meanwhile, a method combining HSCCC with Sephadex LH-20 chromatography was applied to separate and purify the individual theaflavins. The solvent system n-hexane-methanol-ethyl acetate-water (1:1:3:6) and the solvent 45% acetone were employed for HSCCC and Sephadex LH-20 chromatography respectively.Catechins and their oxidative products are a group of polyphenolic compounds, ubiquitously found in tea. They have been reported to exhibit a wide variety of biological effects, including antioxidant and free radical-scavenging activities. In this paper, total antioxidative activities of catechins and their oxidative products, their scavenging activities on H2O2 and peroxyl radical were investigated. The results showed that catechins and their oxidative products had different antioxidative activities, but their scavenging activities on H2C>2 were not high enough. Their scavenging activities ranked as EGCG>ECG>TFs>EGC>TP> TRS >TRS >TB>extraction of black tea>EC. Catechins and their oxidative products had distinct peroxyl radical scavenging activities. The order is TRs Extraction of black tea >TB> TRS >TFs>ECG>EGC>TP>EGC>EC. Comparing with catechins, peroxyl radical scavenging activities of their oxidative products were stronger. It suggested that oxidized and polymerized reaction couldn't reduce peroxyl radical scavenging activities of polypheonls. On the contrary, their scavenging activities could increase due to more phenolic hydroxyl groups.The interaction of catechins and their oxidative products with protein was also systematically stu...
|
PDF Full Text Request