Tea leaf

Tea is an aromatic beverage commonly prepared by pouring hot or boiling water over cured or fresh leaves of the Camellia sinensis, an evergreen shrub native to East Asia.[3] After water, it is the most widely consumed drink in the world.[4] There are many different types of tea; some, like Darjeeling and Chinese greens, have a cooling, slightly bitter, and astringent flavour,[5] while others have vastly different profiles that include sweet, nutty, floral, or grassy notes. Tea has a stimulating effect in humans primarily by its caffeine content.[6]
Tea originated in the region encompassing today's Northeast India, north Myanmar, Southwest China and Tibet, where it was used as a medicinal drink by various ethnic groups in the region.[7][8] An early credible record of tea drinking dates to the 3rd century AD, in a medical text written by Hua Tuo.[9] It was popularised as a recreational drink during the Chinese Tang dynasty, and tea drinking spread to other East Asian countries. Portuguese priests and merchants introduced it to Europe during the 16th century.[10] During the 17th century, drinking tea became fashionable among the English, who started to plant tea on a large scale in India.
The term herbal tea refers to drinks not made from Camellia sinensis: infusions of fruit, leaves, or other parts of the plant, such as steeps of rosehip, chamomile, or rooibos. These are sometimes[11] called tisanes or herbal infusions to prevent confusion with tea made from the tea plant.Physically speaking, tea has properties of both a solution and a suspension. It is a solution of all the water-soluble compounds that have been extracted from the tea leaves, such as the polyphenols and amino acids, but is a suspension when all of the insoluble components are considered, such as the cellulose in the tea leaves.[77]Caffeine constitutes about 3% of tea's dry weight, translating to between 30 and 90 milligrams per 250-millilitre (8 1⁄2 US fl oz) cup depending on the type, brand,[78] and brewing method.[79] A study found that the caffeine content of one gram of black tea ranged from 22–28 mg, while the caffeine content of one gram of green tea ranged from 11–20 mg, reflecting a significant difference.[80] Tea also contains small amounts of theobromine and theophylline, which are stimulants, and xanthines similar to caffeine.[81]Fresh tea leaves in various stages of growthBlack and green teas contain no essential nutrients in significant amounts, with the exception of the dietary mineral manganese, at 0.5 mg per cup or 26% of the Reference Daily Intake (RDI).[82] Fluoride is sometimes present in tea; certain types of "brick tea", made from old leaves and stems, have the highest levels, enough to pose a health risk if much tea is drunk, which has been attributed to high levels of fluoride in soils, acidic soils, and long brewing.[83]The astringency in tea can be attributed to the presence of polyphenols. These are the most abundant compounds in tea leaves, making up 30–40% of their composition.[84] Polyphenols include flavonoids, epigallocatechin gallate (EGCG), and other catechins.[85][86] It has been suggested that green and black teas may protect against cancer[87] or other diseases such as obesity[88] or Alzheimer's disease,[89] but the compounds found in green tea have not been conclusively demonstrated to have any effect on human diseases.