Tea

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Southeastern Asia
(area adjoining Tibet, India, China, Burma)

On a global level, tea is the most popular beverage after water. Over 800 million cups of tea are consumed daily. Today India is the world's largest producer of tea, harvesting an estimated 700,000 metric tons annually, followed by China and Kenya with about 580,000 and 188,000 metric tons, respectively. But because most tea is produced and consumed locally, tea is not as important an international commodity as one might expect.
Abaxial leaf surface, flower, and fruit, and fruit of Camellia sinensis

     Abaxial leaf surface, flower,
    and fruit of Camellia sinensis

Many legends surround the origins of tea. In India, tea is described as a gift from Buddha, while in China its discovery is attributed to the Emperor Shen Nung in 2737 B.C. Whatever its origins, there is no disagreement that Camellia sinensis, the plant whose leaves are collected to make tea, is native to the area adjoining Tibet, India, China, and Burma.

The first comprehensive text on tea, First Tea Classic, was written by a Chinese man named Lo Yu in 780 A.D. In 801 A.D., tea was introduced to Japan, where it assumed a paramount role in Buddhist rituals. Each aspect of the elaborate Japanese tea ceremony, "sado," is linked to aspects of Zen Buddhism. Tea was first mentioned in European literature in 1559 by a Venetian author and in 1615 by an Englishman. By 1657, tea was being served in many of England's coffee houses. In 1773, American colonists protested the British import tax on tea by staging the Boston Tea Party on December 16. The British established commercial planting of tea in India in the early nineteenth century.

The general term "tea" is used to describe an infusion of leaves, roots, or flowers steeped in hot water before filtering. Specifically, the beverage we usually call tea is the result of an infusion of the terminal bud and top two leaves from C. sinensis steeped in hot water and filtered. These parts of the plant have been shown to have the highest content of caffeine, an alkaloid. Alkaloids are a major class of natural products that have a physiological effect in other organisms. They characteristically contain carbon, hydrogen, nitrogen, and in many cases, oxygen. Because of their potent pharmacological effects, alkaloids are the basis for many drugs. Caffeine increases heartbeat, blood pressure, and respiration. It has been shown to alleviate hunger, drowsiness, and fatigue, to promote the use of fat reserves for energy, and to act as a vasoconstrictor, helping relieve the pain of headaches caused by vasodilation. Caffeine also has a down side, with various amounts causing insomnia, nervousness, and irritability, depending on the individual. A 6-ounce cup of tea contains 10-50 mg of caffeine.

Chemical structure of caffeine.

C. sinensis is in the Theaceae family of dicots, which is represented by 28 genera and 520

Cultivated flower of Camellia sp. 

   Cultivated flower of Camellia sp.
species. C. sinensis, often called the "tea plant," is an evergreen tree that would attain a height of up to 34 feet if left alone and unharvested. However, for the best teas, the terminal buds and top two leaves of each branch are harvested, which encourages a bushier growth due to the removal of apical dominance and results in 3-4-foot shrubs. Harvesting, which can begin when the plants are 4 years old, is accomplished manually every 2 weeks during the growing season. In a possible attempt to reduce inbreeding depression, the flowers of Camellia are self-incompatible and therefore require cross-pollination, which is accomplished largely by various biotic forces.

Green tea, black tea, and oolong tea are all produced from C. sinensis, depending on the processing. To make green tea, the leaves are steamed after picking, rolled, and immediately dried, thereby retaining their green color. Because of the minimal processing, green tea retains more phytochemicals than other types of tea, including theanine, an amino acid linked with relaxing effects. In the United States, 95% of the tea consumed is black or fermented tea.

The steps in the processing of tea
in different parts of the world

Black tea is also the most popular on the international market. To produce black tea, the leaves are withered after collection. Withering is done under environmentally controlled conditions and involves reducing the water content of the leaves, which causes them to become flaccid as a result of decreased turgor pressure over the course of about 16 hours. The withered leaves are rolled, crushed, and torn to release the enzymes held within the individual cells, catalyzing fermentation. The fermented leaves are then dried. Oolong tea is semi-fermented and said to combine the taste and benefits of both green and black tea. The leaves are withered for only 6 hours, and then rolled, torn, and fermented for a short period of time. When dried, the smallest leaves are collected to make orange pekoe. Oolong tea is produced primarily in regions of eastern China and northern Taiwan. Any broken leaf pieces generally end up in tea bags. Tea bags were introduced by a New York tea wholesaler who sent samples to his customers in small silk bags instead of the traditional tins. Flavored teas are produced by adding flowers, essential oils, spices, or leaves during the withering process. For example, jasmine blossoms are added to make jasmine tea, and bergamot oil is added to make Earl Grey.

Historically, tea has been used as a medicinal product because it contains several B-complex vitamins and catechins, often called "tea flavonoids." Catechins are antioxidants, a highly desirable group of substances that tend to neutralize the damaging free radicals attacking the healthy cells in our bodies [see "Food of the Gods"]. A free radical is a molecule containing an oxygen atom that is missing an electron, which it seeks to replace by taking one from another molecule. The number of free radicals in the body is increased by such factors as radiation, poisons, cancer, and heart disease. Additional research is also looking into the cholesterol-lowering effects of drinking tea.

Whether you drink tea for its medicinal qualities, for a caffeine boost, or for pleasure, you are drinking a beverage surrounded in mystery and ceremony.

References, Websites, and Further Reading

Levetin, Estelle, and Karen McMahon. 1999. Plants and society, 2d ed. New York: McGraw-Hill Companies, pp. 263-66.

Mabberley, D. J. 1997. The plant-book, 2d ed. New York: Cambridge University Press.

Simpson, B.B., and Molly C. Ogorzaly. 2001. Economic botany: Plants in our world, 3d ed. New York: McGraw-Hill Companies.

Tijburg, L. B., et al. 1997. Effects of green tea, black tea and dietary lipophilic antioxidants on LDL oxidizability and atherosclerosis in hypercholesterolaemic rabbits. Cardiosource 135 (1):34-47.

Morphology of Theaceae, University of New Orleans
http://www.biodiversity.uno.edu/delta/angio/www/theaceae.htm

Related Reading in Stern, Introductory Plant Biology, 8th Edition

Chapter 2: The Nature of Life
The Elements: Units of Matter, pp. 15-17
Enzymes, p. 26

Chapter 3: Cells
Cell Size and Structure, pp. 32-33

Chapter 6: Stems
External Form of a Woody Twig, including terminal buds, p. 83

Chapter 7: Leaves
Human and Ecological Relevance of Leaves, pp. 122-24

Chapter 8: Flowers, Fruits, and Seeds
Differences Between Dicots and Monocots, p. 129
Structure of Flowers, including Table 8.1, pp. 129-30

Chapter 9: Water in Plants
Osmosis, including turgor pressure, pp. 151-52

Chapter 10: Plant Metabolism
Fermentation, p. 178

Chapter 11: Growth
Hormonal Interactions: Apical Dominance, pp. 195-96

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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