Antioxidant and Anti-Cancer Activities of Green and Black Tea

Chapter 15

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Antioxidant and Anti-Cancer Activities of Green and Black Tea Polyphenols Jin-Woo Jhoo Department of Food Science and Technology in Animal Resources, Kangwon National University, 192-1 Hyoja-2, Chunchon, Kangwon 200-701, South Korea

Tea (Camellia sinensis) has attracted public attention because of accumulating scientific evidence linking its consumption to health benefits. As a popular beverage, tea consumption may provide beneficai biological activities, such as reducing the risk of mortality from cardiovascular disease ( C V D ) and delaying the onset of cancer. These biological activities are believed to arise from ability of tea polyphenols to effectively scavenge reactive oxygen species (ROS), as well as cancer chemopreventive effects associated with tea polyphenols and their molecular mechanisms. The biological activities of green and black tea polyphenols are summarized in this chapter.

Introduction Consumer interest in health promoting foods has increased, and people believe that functional foods can help the prevention of certain diseases. Much scientific evidence demonstrating a positive correlation between health benefits and the consumption of certain foods has been accumulated, and scientists are performing systematic research to understand the chemistry and physiological effects of these foods. For example, cruciferous vegetables are rich in sulfurcontaining phytochemicals known as glucosinolates which have been reported to

© 2007 American Chemical Society

In Antioxidant Measurement and Applications; Shahidi, F., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

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216 exhibit anticancer activity (7). Allium vegetables are rich in organosulfur compounds and epidemiological evidence indicates that the consumption of these vegetables may decrease the risk of certain cancers (2). In addition, flavonoids are widely regarded as bioactive components in foods with health benefits. Fruits, vegetables and many food products derived from them are good dietary sources of flavonoids. For example, the flavonol quercetin can be found in onion, apple and cranberry, as well as many other vegetables and fruits. Anthocyanidins and their glycoside are pigments in red grapes and berries. Tea is rich in flavan-3-ols such as catechins. Epidemiological studies provide strong evidence for the protective effects of dietary flavonoids against coronary heart disease and certain cancers (3-5). The principal hypothesis related to health benefits of flavonoids concerns the antioxidant properties of these compounds, specifically, their ability to scavenge reactive oxygen species (ROS). Flavonoids are polyphenolic compounds ubiquitous in plants. These compounds have a common three ring chemical structure: C - C - C . Various types of flavonoids exhibit different extents of oxidation in Β and C ring, and these oxidation states have been used for their classification (6). The redox chemistry of flavonoids can provide useful information to understand the antioxidative mechanism of flavonoids (7). Due to their low reduction potential, flavonoids do not require much energy to donate an electron. Several elements of the chemical structure of these molecules have proven effective for radical scavenging activity (7,5). In addition, flavonoids prevent oxidative reactions by chelating free copper and iron through bidentate ligands. For example, quercetin has three potential metal chelating sites on the A , Β and C-rings, of which the Α-ring site has the highest affinity. 6

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Constituents of Green and Black Teas Several types of tea products are commercially available, including green tea, oolong tea and black tea. Green tea is mostly consumed in China, Japan and the Middle East. Oolong tea, known as partially-fermented tea, is generally consumed in China and Taiwan. Black tea is manufactured through fermentation of the tea leaves. Fresh tea leaves contain four major tea catechins having unique biological activities. These compounds are (-)-epicatechin (EC), (-)-epicatechin gallate (ECG), (-)-epigallocatechin (EGC) and (-)-epigailocatechin gallate (EGCG) (Figure 1). They account for about 30% of the dry weight of tea leaves (P). The other components in tea, such as quercetin, kaempferol and myricitin and their glycosides, account for about 3% of the dry weight of tea leaves (7). To make green tea, tea leaves are steamed before it is dried to inactivate enzymes in the leaves. However, black tea manufacturing process requires additional fermentation process. The leaves are first subjected to a withering

In Antioxidant Measurement and Applications; Shahidi, F., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.


217 process to reduce their moisture content and are then ruptured through a rolling process. This process initiates the enzymatic oxidation of polyphenols in the tea leaves by allowing polyphenol oxidase to diffuse into the cytoplasm. The rolled leaves are then allowed to ferment for between 40 minutes and 3 hours (P). During the fermentation process, important chemical changes occur due to the action of polyphenol oxidase (PPO). PPO is responsible for oxidizing the dihydroxylated B-ring (catechol) and tri-hydroxylated B-ring (pyrogallol) of tea catechins to their o-quinones. Subsequent chemical reactions generate the various characteristic black tea pigments. Roberts (10) reported that black tea contains polyphenolic pigments that are not found in unprocessed tea leaves. These pigments are designated as brown acidic pigments and yellow neutral pigments or thearubigins and theaflavins, respectively. Four major theaflavins have been identified from black tea, namely, theaflavin, theaflavin-3-gallate, theaflavin-3 '-gallate, and theaflavin-3,3 'digallate (Figure 2), whereas thearubigins are a heterogeneous mixture of pigments. Theaflavins having orange or orange-red color and are formed from co-oxidation of selected pairs of tea catechins (Table 1). Theaflavins account for 2-6% of the dry, solid weight of brewed black tea. The relative proportions of the theaflavins were found to be as follows: theaflavin, 18%; theaflavin-3-gallate, 18%; theaflavin-3'-gallate, 20%; theaflavin-3,3'-digallate, 40%; and theaflavic acids plus isotheaflavin, 4% (P). Several minor pigments, namely, theaflavate A , theaflavate B, isotheaflavin-3'O-gallate, and neotheaflavin-3-

Antioxidant and Anti-Cancer Activities of Green and Black Tea

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