Doc:Tea

Latest revision as of 17:52, 20 January 2013

[edit] About Tea

Tea is made from leaves of Camellia sinensis var.sinensis or var.assamica. The former has small round leaves and the tree height is lower than 4m. It is cold resistant and is cultivated in Japan and China. The assamica has large pointed leaves and its tree height is over 10m. Its leaves contain more tannins and are used for black tea except for Darjeeling, which uses sinensis, in India and Sri Lanka. Both species share the same number of　chromosomes and　easily crossbreed. Their mix grow in Indochina.

Different tastes and flavors mainly come from manufactural differences. The degree of fermentation is as follows.

[edit] Classification of Tea

[edit] White Tea

White tea is covered with white hair, because of its special manufacturing process of "no crushing" (minimum treatment). Only 2,000 tons per year is manufactured in Fujian Province, China. The general grade is: Silver Needle > White Peony > Gongmei and Shoumei. Silver Needle with White Hair is made from bulky buds only, White Peony is made from the bud and 1 or 2 leaves. For Silver Needle, young buds are harvested, withered and quickly basket fried (40-50 ℃, 30 min for 250 g) to keep their fermentation minimal. For White Peony, frying temperature is 70-80 ℃ after withering.

[edit] Green Tea

It is non-fermented, i.e., produced by frying or steaming (fixation) the fresh leaves to inactivate polyphenol oxidases. Steaming is common in Japan whereas pan-frying (firing) is used in China. Steaming completely stops the transformation of flavan-3-ols, i.e., catechins by polyphenol oxidase whereas pan-frying is less efficient and allows some transformation. Assan (assamica) type contains too much tannins and not suitable for green tea. Almost all green tea is consumed in Japan, Vietnam, China, and Indonesia only.

[edit] Blue or Oolong Tea

It is produced by partial fermentation before drying. The process is called green leaf shaking (yaoqing), where mildly withered tea leaves are bruised at the edges by hand, and green leaf cooling (liangqing). Good oolong tea leaves have reddish edges with green centers. It is mainly produced in Fujian, Guangdong (both China), and Taiwan. Black oolong tea is usually roasted. Golden-colored oolong tea, produced in Taiwan, is lightly fermented oolong tea (closer to green tea).

[edit] Black Tea

Unlike green tea, leaves are withered and rolled to crush leaf cells and release polyphenol oxidases. Black tea in India, Sri Lanka, and Kenya is manufactured by a CTC (crushing, tearing, and curling) machine whereas in China an orthodox rotorvane is used. After rolling, leaves are fermented for 0.5 - 3 hours at 25-35 ℃ with high humidity (>95%). Leaves are fully oxidized and turn golden with floral aroma in this process.

[edit] Pu-erh Tea

Raw pu-erh tea is produced by pressing tea leaves and fermenting for years, sometimes for decades. Ripened pu-erh tea, which is more popular, is inoculated with black Aspergillus and fermented under an optimal condition for several months. From Yunnan Pu-erh tea, Aspergillus niger, A. gloucu, and species of Penicillium, Rhizopus, Saccharomyces, and Bacterium are found. A. niger is the most predominant, followed by Saccharomyces spp. ^[1] Ripened pu-erh tea contains less catechins than raw pu-erh tea, but more gallic acid as the degradation products of catechins (and others). Antioxidant activity is significantly higher for raw pu-erh tea.^[2]

1. ↑ Jeng KC, Chen CS, Fang YP, Hou RCW, Chen YS (2007) "Effect of microbial fermentation on content of statin, GABA, and polyphenols in pu-erh tea" J. Agric. Food Chem. 55:8787-8792
2. ↑ Ku KM, Kim J, Park HJ, Liu KH, Lee CH (2010) "Application of Metabolomics in the Analysis of Manufacturing Type of Pu-erh Tea and Composition Changes with Different Postfermentation Year" J. Agric. Food Chem. 58:345-352

[edit] Tea Composition

[edit] Catechins

The major phenolics in green tea include catechins (flavan 3-ols). They are

• (-)-epigallocatechin-3-gallate (EGCG; 59% of total catechins),
• (-)-epigallocatechin (EGC; 19%),
• (-)-epicatechin-3-gallate (ECG; 13.6%), and
• (-)-epicatechin (EC; 6.4%) ^[1].

• (+)-catechin
• (+)-gallocatechin, and
• (-)-epiafzelchin.

After drinking tea, catechins are absorbed in the small intestine, and the plasma level of catechins reach their peaks (between 1-10 µmol/L) in 2 - 4 h ^[7]. Only less than 2 % of ingested catechins were found in the plasma, and gallated catechins are less bioavailable than non-gallated forms ^[8]. After 24 h, plasma levels of EGCG and EGC return to baseline, but that of ECG remains elevated in methylated forms^[9].

1. ↑ McKay DL, Blumberg JB (2002) "The role of tea in human health: An update" J Am Coll Nutr 21:1-13
2. ↑ USDA Database for the Flavonoid Contents of Selected Foods, Beltsville 2003
3. ↑ Wu CD, Wei GX (2002) "Tea as a functional food for oral health" Nutrition 18(5):443-444
4. ↑ Kim S, Lee MJ, Hong J (2000) "Plasma and tissue levels of tea catechins in rats and mice during chronic consumption of green tea polyphenols" Nutr Cancer 37:41-48
5. ↑ Loktionov A, Bingham S et al. (1998) "Apolipoprotein E genotype modulates the effect of black tea drinking on blood lipids and blood coagulation factors: A pilot study" Br J Nutr 79:133-139
6. ↑ Belitz DH, Grosch W (1997) "Quimica de los Alimentos" Zaragoza Acribia
7. ↑ Yang CS, Chen L et al. (1998) "Blood and urine levels of tea catechins after ingestion of different amounts of green tea by human volunteers" Cancer Epidemiol Biomarkers Prev 7:351-354
8. ↑ Warden BA, Smith LS, Beecher GR, Balentine DA, Clevidence BA (2001) "Catechins are bioavailable in men and women drinking black tea throughout the day" J Nutr 131:1731-1737
9. ↑ Higdon JV, Frei B (2003) "Tea catechins and polyphenols: health effects, metabolism, and antioxidant functions" Crit Rev Food Sci Nutr 43:89-143

[edit] Caffeine

The caffeine content in different types of tea is: black tea > oolong tea > gree tea > fresh tea leaf ^[1]. More than 200 mg/day caffeine is not advisable and may produce nervousness, sleep disorders, vomits, headaches, epigastric pain, and tachycardia ^[2].

1. ↑ Lin Lin YS, Tsai YJ et al. (2003) "Factors affecting the levels of tea polyphenols and caffeine in te a leaves" J Agric Food Chem 51:1864-1873
2. ↑ Varnam AH, Sutherland JP (1994) "Beverages: Technology, Chemistry and Microbiology" Chapman & Hall (London)
3. ↑ Cabrera C, Artacho R, Gimenez R (2006) "Beneficial Effects of Green Tea-A Review" J Am Coll Nutr 25(2):79-99

[edit] Saponins

Tea synthesizes different saponins in leaves, seeds, and flower parts. In flower/bud, floratheasaponins are contained 1-2% of its dry weight^[1]. Therefore, tea containing flowers bubbles like a soap.

Cf. Category:Floratheasaponin

References

1. ↑ Yoshikawa M, Morikawa T, Yamamoto K, Kato Y, Nagatomo A, Matsuda H (2005) J Nat Prod 68:1360-1365