Tea consists of the young leaves and leaf-buds of the tea shrub, Camellia Thea, Link. (N.O. Ternstroemiaceae), fermented and dried. The tea shrub is largely cultivated in China, Japan, Assam, Ceylon, and other tropical countries. The leaf-buds, together with two or sometimes three of the youngest leaves, are collected, allowed to wilt, and then rolled by hand or machinery until they acquire the twist characteristic of ordinary tea. They are then subjected to a process of fermentation, the leaves being allowed to reach a temperature of from 35° to 40° until the colour changes to a yellowish-brown; they are then rolled again, dried, and graded by sifting. In the production of green tea, the slightly wilted leaves are at once heated in a pan over an open fire; they are then rolled and fermented, during which the green colour is more or less completely retained. This difference is probably due to the destruction by heat of an oxydase (thease) which, in the case of black tea, acts upon the tannin, changing part at least of it into insoluble oxidation products, hence green tea contains more tannin than black. During the fermentation, which is common to both black and green tea, changes are induced, resulting in the production of the characteristic aroma and the destruction of a bitter principle. The nature and cause of these changes are not at present clear, but are probably due to the action of enzymes, which, together with thease, exist in the cell sap. The leaf of the tea plant is usually broadly lanceolate, firm in texture, rather thick, and tapering to a short petiole; the upper surface is glossy, and the under, when young, is pubescent, in older leaves nearly glabrous the serrated margin is slightly inrolled and bears characteristic, shrunken glandular teeth, which readily break off. The leaves may attain 15 centimetres in length, but those used for the production of tea seldom exceed 5 centimetres. These macroscopic characters usually suffice for the identification of even small fragments of tealeaves, but may, if necessary, be supplemented or replaced by the microscopic characters, viz., cells of the upper epidermis, small (50µ); those of the lower epidermis, larger (70µ), wavy, and accompanied by large stomata and long thick-walled, one celled simple hairs bent near the base; in the mesophyll, especially of older leaves, large, elongated branching, thick-walled, sclerenchymatous idioblasts occur. On incineration, tea yields from 5 to 7 per cent. of ash.
Constituents.—The principal constituents of tea are caffeine (1 to 5 per cent.), tannin (7 to 24 per cent.), and a trace of volatile oil; traces of theobromine and theophylline have also been detected. The percentage of caffeine usually shows little variation from an average of 4 per cent.; the commercial value of tea is, however, not determined by this factor alone, but by consideration of the size of the leaf, the presence of leaf-bud, and the taste of the infusion.
Action and Uses.—Tea is used as a source of caffeine, and the leaves are sometimes added to compounds for smoking or burning in asthma; some of the caffeine. is vaporised, and pyridine bodies .are formed during the slow combustion of die leaves. The action of freshly brewed tea is that of the hot water and the contained caffeine.