Tea, green

In the green tea manufacture, the development of oxidative processes is regarded as an adverse factor. The fresher the tea leaf used in manufacture, the better the tea produced. Since oxidative processes catalyzed by the leaf enzymes are undesirable, the enzymes are inactivated at an early stage and their reactions are replaced by thermochemical processes. In contrast to black tea manufacture, withering and fermentation stages are omitted in green tea processing. 

Yellow tea production does not include fermentation. Nevertheless, in withering, roasting, and firing, a portion of tannins undergoes oxidation, and, therefore, dry yellow tea is darker than green tea. 

Red tea is a partially fermented tea. Its special flavor which is free from the grassy note of gieen tea is formed during roasting and higher-temperature rolling. 

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Green tea, consumed hot or cold, is most common in Japan and China and is growing rapidly in the United States owing to the numerous reports of health benefits. The market has doubled from about 110 metric tons in 1993 to just over 227 t in 1996. Green tea consumption in the United States remains a small segment (less than 2%) of the total tea market. 

The total antioxidant activity of teas and tea polyphenols in aqueous phase oxidation reactions has been deterrnined using an assay based on oxidation of 2,2 -azinobis-(3-ethylbenzothiazoline-sulfonate) (ABTS) by peroxyl radicals (114—117). Black and green tea extracts (2500 ppm) were found to be 8—12 times more effective antioxidants than a 1-mAf solution of the water-soluble form of vitamin E, Trolox. The most potent antioxidants of the tea flavonoids were found to be epicatechin gallate and epigallocatechin gallate. A 1-mAf solution of these flavanols were found respectively to be 4.9 and 4.8 times more potent than a 1-mAf solution of Trolox in scavenging an ABT radical cation. 

DETERMINATION OF POLYPHENOLIC ENANTIOMERS IN GREEN TEA EXTRACT BY CAPILLARY ZONE ELECTROPHORESIS... 

Catechin and epicatechin are two flavanols of the catechin family. They are enantiomers. The capillary zone electrophoresis (CE) methods with UV-detection were developed for quantitative determination of this flavanols in green tea extracts. For this purpose following conditions were varied mnning buffers, pH and concentration of chiral additive (P-cyclodextrin was chosen as a chiral selector). Borate buffers improve selectivity of separation because borate can make complexes with ortho-dihydroxy groups on the flavanoid nucleus. 

Extraction of the catechin and epicatechin from the green tea was carried out by liquid extraction with methanol. 

Fig. Electropherogram of a) catechin standards, b) green tea sample prepared in 50 % methanol using liquid extraction...

Vayalil PK, Mittal A, Kara Y, Elmets CA, Katiyar SK (2004) Green tea polyphenols prevent ultraviolet light-induced oxidative damage and matrix metal-loproteinases expression in mouse skin. J Invest Dermatol 122 1480-1487... 

HU G, HAN c and chen j (1995) Inhibition of oncogene expression by green tea and (-)-epigallocatechin gallate in mice Nutrition and Cancer 24, 203-9. 

Commercial green tea leaves contain ascorbic acid (vitamin C) about 280 mg per 100 g dried leaves. Vitamin E in tea leaves is around 24-80 mg/lOOg dry weight but, because of its lipophilicity, solubility is low in tea infusions. The content of B vitamins in tea is around 8-15 mg/lOOg (Cheng and Chen 1994). 

Cancer is a major cause of premature death in modern society. In vitro and animal research indicates that tea, mainly green tea, may be effective against a wide variety of cancers. Activity has been observed in vitro at all three levels of cancer progression, namely initiation, promotion and transformation (Mitscher et al, 1997). 

Tea extracts have been demonstrated to inhibit a wide range of inflammatory responses and may be useful in treating chronic inflammatory states. For example, rheumatoid arthritis is an inflammatory disease that causes pain, swelling, stiffness and loss of function in the joints. The antioxidants in green tea may prevent or reduce the severity of these symptoms by reducing inflammation and slowing cartilage breakdown (Adcocks et al, 2002 Haqqi et al, 1999). 

Green tea consists of a wealth of simple phenolics (monomers), whereas black tea provides more complex polyphenols (dimers and polymers). It was found that with lipids the simple compounds were more effective antioxidants, while under aqueous conditions, polymers tended to have more activity. Weisburger (2001) suggested that polymers formed from a 2-5 unit polymerisation state seemed to be optimal, probably because the monomer is metabolised and excreted too rapidly, whereas the higher 6-10 unit polymers may suffer from difficulty in penetrating cellular membranes and be poorly absorbed. 

Tea can be consumed directly either by brewing loose leaves or tea bags or in a ready-to-drink form. In addition, extracts of tea (primarily green tea) may be prepared in a variety of physical forms in order to cover most application requirements. 

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