Tea
From Wikipedia, the free encyclopedia
For other uses, see Tea (disambiguation).
 Green Tea leaves in a Chinese gaiwan | |
| Type | Hot or cold beverage |
|---|---|
| Country of origin | China |
| Introduced | approx. 10th century BC.[1] |
Tea plant (Camellia sinensis) from Köhler's Medicinal Plants
There are at least six varieties of tea: white, yellow, green, oolong, black, and pu-erh[5] of which the most commonly found on the market are white, green, oolong, and black. All teas are made from the same species of plant, though different varieties may be used, and the leaves are processed differently, and, in the case of fine white tea, grown differently. Pu-erh tea, a post-fermented tea, is also often used medicinally.[5]
The term "herbal tea" usually refers to an infusion or tisane of leaves, flowers, fruit, herbs, or other plant material that contains no Camellia sinensis.[6] The term "red tea" refers to an infusion made from either black tea (mainly in Chinese, Korean, Japanese and other East Asian languages) or the South African rooibos plant (containing no Camellia sinensis).
Contents[hide] |
[edit] Cultivation and harvesting
Camellia sinensis is an evergreen plant that grows mainly in tropical and sub-tropical climates. Nevertheless, some varieties can also tolerate marine climates and are cultivated as far north as Pembrokeshire in the British mainland[7] and Washington in the United States.[8]Tea plants are propagated from seed or by cutting; it takes approximately 4 to 12 years for a tea plant to bear seed, and about 3 years before a new plant is ready for harvesting.[9] In addition to a zone 8 climate or warmer, tea plants require at least 127 cm. (50 inches) of rainfall a year and prefer acidic soils.[10] Traditional Chinese Tea Cultivation and Studies believes that high-quality tea plants are cultivated at elevations of up to 1,500 metres (4,900 ft): at these heights, the plants grow more slowly and acquire a better flavour.[11]
Only the top 1-2 inches of the mature plant are picked. These buds and leaves are called flushes.[12] A plant will grow a new flush every seven to fifteen days during the growing season, and leaves that are slow in development always produce better flavored teas.[9]
A tea plant will grow into a tree of up to 16 metres (52 ft) if left undisturbed [9], but cultivated plants are pruned to waist height for ease of plucking.[13]
Two principal varieties are used: the China plant (C. sinensis sinensis), used for most Chinese, Formosan and Japanese teas (but not Pu-erh); and the clonal Assam plant (C. sinensis assamica), used in most Indian and other teas (but not Darjeeling). Within these botanical varieties, there are many strains and modern Indian clonal varieties. Leaf size is the chief criterion for the classification of tea plants[14], with three primary classifications being: Assam type, characterized by the largest leaves; China type, characterized by the smallest leaves; and Cambod, characterized by leaves of intermediate size.[14]
[edit] Processing and classification
Main article: Tea processing
A tea's type is determined by the processing which it undergoes. Leaves of Camellia sinensis soon begin to wilt and oxidize, if not dried quickly after picking. The leaves turn progressively darker as their chlorophyll breaks down and tannins are released. This process, enzymatic oxidation, is called fermentation in the tea industry. Fermentation can be both aerobic or anaerobic, and mistakenly sometime is believe to necessitate involvement of bacteria. The fact is that fermenting agents can be yeast, bacteria or even only enzymatic (such as your muscles under heavy exercise). As far as tea production goes, most of the darkening of tea is believed to be because of enzymatic oxidation, hence why in the next step in processing the darkening is stopped at a predetermined stage by heating, which deactivates the enzymes responsible. In the production of black teas, the halting of oxidization by heating is carried out simultaneously with drying. Current biochemical studies have not been able to rule if fermentation takes place during the withering time in tea manufacturing.
Tea harvest on the eastern shores of the Black Sea, ca. 1905-15.
Tea is traditionally classified based on the techniques with which it is produced and processed.[15]
- White tea: Wilted and unoxidized
- Yellow tea: Unwilted and unoxidized, but allowed to yellow
- Green tea: Unwilted and unoxidized
- Oolong: Wilted, bruised, and partially oxidized
- Black tea: Wilted, sometimes crushed, and fully oxidized
- Post-fermented tea: Green tea that has been allowed to ferment/compost
[edit] Blending and additives
Main article: Tea blending and additives
Although single estate teas are available, almost all teas in bags and most other teas sold in the West are blends. Blending may occur in the tea-planting area (as in the case of Assam), or teas from many areas may be blended. The aim is to obtain better taste, higher price, or both, as a more expensive, better-tasting tea may cover the inferior taste of cheaper varieties.Some teas are not pure varieties, but have been enhanced through additives or special processing. Tea is highly receptive to inclusion of various aromas; this may cause problems in processing, transportation and storage, but also allows for the design of an almost endless range of scented and flavored variants, such as bergamot (Earl Grey), vanilla, caramel, and many others.
[edit] Content
Tea contains catechins, a type of antioxidant. In a freshly picked tea leaf, catechins can compose up to 30% of the dry weight. Catechins are highest in concentration in white and green teas, while black tea has substantially fewer due to its oxidative preparation.[16][17] Research by the U.S. Department of Agriculture has suggested that levels of antioxidants in green and black tea do not differ greatly, with green tea having an Oxygen Radical Absorbance Capacity (ORAC) of 1253 and black tea an ORAC of 1128 (measured in μmolTE/100g).[18] Tea also contains theanine and the stimulant caffeine at about 3% of its dry weight, translating to between 30 mg and 90 mg per 8 oz (250 ml) cup depending on type, brand[19] and brewing method.[20] Tea also contains small amounts of theobromine and theophylline.[21] Due to modern day environmental pollution fluoride and alluminum have also been found to occur in tea, with certain types of brick tea made from old leaves and stems having the highest levels. This occurs due to the tea plants high sensitivity to and absorption of environmental pollutants.[22][23]Dry tea has more caffeine by weight than coffee; nevertheless, more dried coffee is used than dry tea in preparing the beverage,[24] which means that a cup of brewed tea contains significantly less caffeine than a cup of coffee of the same size.
Tea has negligible carbohydrates, fat, or protein.
Although tea contains various types of polyphenols, "contrary to widespread belief, tea does not contain tannic acid".[25]
[edit] Origin and history
Main article: History of tea
Mass cultivation and trade of tea was begun by the Chinese. Through Buddhism the beverage was spread to Korea and Japan. Trade of tea by the Chinese to Western nations in the 1800s spread tea and the tea plant to numerous locations around the world.
[edit] Health effects
Main article: Health effects of tea
The health benefits of tea is a controversial topic with many proponents and detractors. An article from the Nutrition (1999, pp. 946–949) journal as related on PubMed states:In summary, the health benefits of tea have been shown in animal studies, but at doses much higher than regularly consumed by humans, at which dosage levels may prove to be harmful to health.The possible beneficial effects of tea consumption in the prevention of cancer and cardiovascular diseases have been demonstrated in animal models and suggested by studies in vitro. Similar beneficial effects, however, have not been convincingly demonstrated in humans: beneficial effects have been demonstrated in some studies but not in others. If such beneficial effects do exist in humans, they are likely to be mild, depending on many other lifestyle-related factors, and could be masked by confounding factors in certain populations. Another concern is that the amounts of tea consumed by humans are lower than the doses required for demonstrating the disease-prevention effects in animal models. Caution should be applied, however, in the use of high concentrations of tea for disease prevention. Ingestion of large amounts of tea may cause nutritional and other problems because of the caffeine content and the strong binding activities of tea polyphenols, although there are no solid data on the harmful effects of tea consumption. More research is needed to elucidate the biologic activities of green and black tea and to determine the optimal amount of tea consumption for possible health-beneficial effects.
In 2010 researchers found that people who consumed tea had significantly less cognitive decline than non-tea drinkers. The study used data on more than 4,800 men and women aged 65 and older to examine change in cognitive function over time. Study participants were followed for up to 14 years for naturally-occurring cognitive decline. (AAICAD 2010; Lenore Arab, PhD; UCLA[26])
Several of the potential health benefits proposed for tea are outlined in this excerpt from Mondal (2007, pp. 519–520) as following:
In a large study of over 11,000 Scottish men and women completed in 1993 and published in the 1999 Journal of Epidemiology and Community Health (1999, pp. 481–487), there was an increase in the risk of coronary disease with the regular consumption of tea, although it disappeared after adjustment for confounding factors (age and occupational status).Tea leaves contain more than 700 chemicals, among which the compounds closely related to human health are flavanoides, amino acids, vitamins (C, E and K), caffeine and polysaccharides. Moreover, tea drinking has recently proven to be associated with cell-mediated immune function of the human body. Tea plays an important role in improving beneficial intestinal microflora, as well as providing immunity against intestinal disorders and in protecting cell membranes from oxidative damage. Tea also prevents dental caries due to the presence of fluorine. The role of tea is well established in normalizing blood pressure, lipid depressing activity, prevention of coronary heart diseases and diabetes by reducing the blood-glucose activity. Tea also possesses germicidal and germistatic activities against various gram-positive and gram negative human pathogenic bacteria. Both green and black tea infusions contain a number of antioxidants, mainly catechins that have anti-carcinogenic, anti-mutagenic and anti-tumoric properties.
The IARC list teas as under Group 3 carcinogens since injection of black tea concentrates under the skins of mice showed some cancerous growths. However, it has not been possible to prove that tea affects humans in similar ways through consumption.[27]
ليست هناك تعليقات:
إرسال تعليق