This section is from the book "A Text Book Of Materia Medica, Being An Account Of The More Important Crude Drugs Of Vegetable And Animal Origin", by Henry G. Greenish. Also available from Amazon: A Text Book of Materia Medica : Being an Account of the More Important Crude Drugs of Vegetable and Animal Origin.
In the manufacture of black tea the leaves are subjected to processes that vary somewhat in the different countries. In Assam where much of the tea consumed in England is produced, the leaf-buds together with two or three of the youngest leaves are collected and spread upon trays until they have lost their turgidity. The ' withered' leaves are then rolled between two flat surfaces worked by machinery, during which many of the cells of the mesophyll are broken, and their sap squeezed out and allowed to come into contact with the air; at the same time the leaves acquire the twist characteristic of ordinary tea. They are next ' fermented ' by exposing them on mats to the air; during this process, which is conducted at a temperature of 35°-40°, the colour changes from green to yellow and finally to coppery red, and the tea acquires its well-known odour. The fermented leaves are dried in a current of hot air, and graded by sifting by which a partial separation into leaf-bud, smaller, and larger leaves is effected.
The changes that take place during the fermentation are due to the action of an enzyme or more probably mixture of enzymes termed thease. Thease is very active at 54°, but is destroyed by a temperature of 76°-77°; it oxidises part of the tannin, converting it into a reddish brown, insoluble substance, while at the same time traces of volatile oil are produced and a bitter principle present in the fresh leaves is rendered insoluble. The quality of the tea produced depends largely upon the skill with which this process is conducted.
In the manufacture of green tea the leaves are subjected to a process of roasting in pans heated by direct fire, in which they are kept continually moving; they are then cooled, rolled into balls, and allowed to ferment. In this case the preliminary roasting probably destroys some at least of the various ferments of which the thease is composed, the tannin is not oxidised, and the leaves retain their green colour more or less unchanged.
1 Specific names are written with a small initial letter, except in the following cases:
(a) When the specific name was formerly a generic name, e.g. Camellia Thea. (6) When the specific name is the vernacular name, e.g. Pilocarpus Jaborandi.
(c) When the specific name is the proper name of a person used as a noun or as an adjective, e.g. Garcinia Hanburyi; Aloe Hanburiana.
(d) When the specific name is a noun, e.g. Eucalyptus Globulus.
The full-grown tea leaf is from 5 to 10 cm. long, dark green in colour, lanceolate or elliptical in outline and blunt at the apex, tapering at the base into a short stalk; the margin is distantly and shortly serrate, the serrations terminating in characteristic, glandular teeth which readily break off and are often absent from mature leaves. When quite young the leaves are covered with silky hairs, but as they mature these are lost, and the surface becomes glabrous. This difference is readily observed in commercial tea, the bud still bearing numerous hairs (' flowery ' Pekoe), while the larger leaves are glabrous or nearly so (Congou).
Fragments of the leaves may be readily recognised under the microscope by the small cells of the upper epidermis, those of the lower epidermis being rather larger and accompanied by numerous 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.1
The principal constituents of tea are caffeine and tannin. It contains in addition traces of theobromine, theophylline and volatile oil.
Caffeine or trimethylxanthine, C5H(CH3)3N402,H10, an alkaloid obtainable in colourless silky crystals melting at 237°, was isolated from coffee by Runge in 1820 and from tea by Oudry in 1827. It occurs also in mate (the leaves of Ilex paraguayensis, Lambert, which are largely used in the Argentine Republic as tea is in this country), in cola seeds, and in guarana. Tea contains from 1 to 5 per cent, (usually 3 to 4) of it apparently in combination with tannin, the combination being decomposed by water and the caffeine liberated. The tannin varies from 10 to 24 per cent. But the commercial value of tea is not determined by the percentage of caffeine or of tannin contained in it, but by a combined consideration of several factors, such as the appearance, the size of the leaves as indicating their age, the presence of ' tip' (unexpanded leaf-bud), and the taste of the infusion.
Theobromine, or dimethylxanthine, C5H1(CH3)2N402, and its isomer theophylline are closely allied to caffeine, but occur in small proportion only.
Caffeine, theobromine, and theophylline are derivatives of xanthin, C5H4N402, which, with a number of other substances, is regarded as containing the purin group, C5H4N4. Tea, coffee, and other drugs containing caffeine, theobromine, or a similar purin derivative form therefore a natural group the members of which are allied by the similarity of their chief constituents.
The following are the most important drugs belonging to the purin group:
Tea, the leaves of C. Thea, containing caffeine (1 to 5 per cent.), theobromine, and theophylline.
Coffee, the seeds of Coffea arabica, Linne (N.O. Rubiaceœ), containing caffeine (0.4 to 1.5 per cent.).
Cola Seeds, the seeds of Cola vera, Schumann (N.O. Sterculiaceœ), containing caffeine (2 to 2.5 per cent.) and traces of theobromine.
1 Compare Greenish, Foods and Drugs, p. 122; Greenish and Collin, Anatomical Atlas, p. 89.
Mate, the leaves of Ilex paraguayensis (N.O. Ilicineœ), containing caffeine (0.2 to 2.0 per cent.).
Guarana, the crushed seeds of Paullinia Cupana, Humboldt, Bonpland, and Kunth (N.O. Sapindaceœ), containing caffeine (2.5 to 5 per cent.).
Cocoa, the seeds of Theobroma Cacao, Linne (N.O. Sterculiacœ), containing theobromine (2 per cent.).
Shake 6 gm. of the dried but not powdered tea with 120 gm. of chloroform for 15 minutes, add 6 c.c. of solution of ammonia, shake frequently for half an hour and set aside till clear. Filter 100 gm. ( = 5 gm. of tea) into a flask, distil off the chloroform, add 3 c.c. of absolute alcohol, and dry on a water-bath. To the residue add 10 c.c. of 60 per cent, alcohol in which the caffeine is readily soluble, then 20 c.c. of water; shake vigorously, filter through a small filter, wash the residue and filter, evaporate, dry at 95°, and weigh. The weight of the residue multiplied by 20 gives the percentage of total caffeine.
Tea has been adulterated with foreign leaves, as well as with exhausted tea leaves that have been rolled and dried. For details concerning the detection of adulterations in tea, the student should refer to Allen's ' Commercial Organic Analysis,' Vol. III., Part II.
The chief pharmaceutical use of tea is as a source of caffeine which has a marked stimulant action on the nervous system and heart, and is also diuretic but less powerfully so than theobromine.
Abyssinian, Arabian or African tea consists of the leaves of Catha edulis Forskal (N.O. Celastrinœ); it contains the alkaloids cathine, cathinine and cathidine.
Bush tea is the leaves of various species of Cyclopia (N.O. Leguminosœ); it is used in S. Africa.
Marsh tea is the leaves of Ledum palustre, Linne (N.O. Ericaceœ).
Kaporie tea is the leaves of Epilobium angustifolium, Linne (N.O. Onagraceœ).
None of these contains caffeine.