Tea. Thea sinensis.
Tea. The, Fr. Thee, G.—The plant which furnishes tea—Thea sinensis L. (Fam. Theaceae) is an evergreen shrub. It has numerous alternate branches, furnished with elliptical-oblong or lanceolate, short petiolate, pointed leaves, which are serrate except at the base, smooth on both sides, green, shining, pinnately veined and having a prominent midrib. They are two or three inches long, and from half an inch to an inch in breadth. The flowers are either solitary or supported, two or three together, at the axils of the leaves. They are of considerable size, not unlike those of the myrtle in appearance, consisting of a short green calyx with five or six lobes, of a corolla with from four to nine large unequal snow-white petals, of numerous stamens with yellow anthers and connected at their base, and of a pistil with a three-parted style. The fruit is a three-locular and three-seeded capsule.
There are two principal varieties of T. sinensis, viz. viridis L., which is a large spreading shrub, having light green lanceolate leaves from 12 cm. long to 4 cm. wide; and a second variety, Bohea L., which is an upright shrub with dark-green elliptical leaves 6 cm. long and 3 cm. wide. The flowers of the latter occur in groups of two or three, whereas in viridis they are single. There are a great many varieties of tea, on the market, and their trade value is influenced by the peculiarities of mother plant and climate and the degree of ripeness and preparation of the leaves. As a rule only the youngest leaves are gathered. The so-called black and green teas are due to the method of preparation of leaves. In the first the chlorophyll is destroyed, whereas in the latter it is unaffected. Winton gives the following characters as common to all tea leaves: " The firm, rather thick texture; the glossy upper surface; the short stem into which the base of the leaf tapers; the thick margins, rolled a little towards the inner surface, with cartilaginous teeth; the veins which branch from the midrib. at angles usually greater than 45 degrees, and at some distance from the margin form loops uniting adjoining ribs. The teeth on the margin of the leaf are shrunken multicellular glands which break off readily from the old leaves." (For discussion of microscopical characteristics, see Winton and Moeller, "The Microscopy of Vegetable Foods."
The tea plant is a native of China and Japan, and is cultivated in both countries, but moat abundantly in the former. It has been more recently cultivated in India (Assam), Ceylon, Java and in South Carolina. In Japan it forms hedge rows around the rice and corn fields; in China, whence immense quantities of tea are exported, whole fields are devoted to its culture. It is propagated from the seeds. For particulars as to the cultivation of tea, see P. J., 1871, 386.
(An elaborate article on tea, with methods of cultivation, etc., by Win. B. Marshall, U. S. Nat. Museum, will be found in A. J. P., 1903, p. 79.)
The odor of the tea leaves themselves is very slight, and it is customary to mix with them the flowers of certain aromatic plants, as those of the orange, different species of jasmine, the rose, Osmanthus fragrans Lour. (Olea fragrans Thunb.), of the Oleaceae, and Camellia Sasanqua (Thunb.), in order to render them pleasant to the smell. The flowers are afterwards separated by sifting or otherwise. (See P. J., xv, 112.) Under the name .of flowers of tea was at one time sold a waste product consisting of the hair of young leaves, but the flowers themselves have been used to make a beverage. (P. J., lxxi, p. 453.)
Bush Tea and Honig Thee, used at Cape Colony, South Africa, as a substitute for tea, are the dried leaves and tops of several species of Cyclopia among which are C. subternata Vog., C. latifolia DC., C. genistoides Vent. and C. sessiliflora Eckl. and Zeyh. (Fam. Leguminosae.) According to the analysis of Henry G. Greenish, they do not contain theine, but a glucosidal body, cyolopin, C25H28O13. (P. J; xi, 549.)
The United States imported in 1915, 96,987,942 pounds of tea, valued at $17,512,619. The largest consumer is the United Kingdom, which in 1914 took 317,664,000 pounds, and the next Russia, which took 166,064,003 pounds. Numerous varieties exist in commerce, differing in the shape communicated by rolling, in color, in flavor, or in strength; but they may be all arranged in the two divisions of green and black teas, which, at least in their extremes, differ so much in properties that it is difficult to conceive that they are derived from the same species.
Under the name of tea oil there has appeared in the London markets a Chinese fixed oil said to be derived from Camellia drupifera Lour.. [C. oleifera Wall.), a detailed description of which, with tests, may be found in the P. J., vol. xvi, 634. A similar oil is prepared in Japan from the seeds of Camellia japonica L. (Ibid., 637, 764.)
Green tea is characterized by a dark green color, sometimes inclining more or less to blue or brown. It has a peculiar, refreshing, somewhat aromatic odor, and an astringent, slightly pungent and agreeably bitterish taste. Its infusion has a pale greenish-yellow color, with the odor and taste of the leaves. According to Warington, who examined numerous varieties of tea carefully, both by the microscope and chemical tests, many of the green teas imported into Great Britain owe their color to a powdery coating, consisting of calcium sulphate and Prussian blue; others to a mixture of these with a yellowish vegetable substance; and others, again, to calcium sulphate alone. (P. J., iv, 37.)
Black tea is distinguished by a dark brown color. It is usually less firmly rolled and lighter than the green, and contains the petioles of the plant mingled with the leaves. Its odor is fainter, and of a somewhat different character, though still fragrant. Its taste, like that of green tea, is astringent and bitterish, but is less pungent, and to many persons less agreeable. To hot water it imparts a brown color, with its sensible properties of taste and odor. These vary exceedingly in degree in the different varieties, and some black teas are almost wholly destitute of aromatic or agreeable flavor. According to Blyth, green tea is prepared from young leaves which are roasted over a wood fire within an hour or two after being gathered; while the black tea leaves, on the other hand, are allowed to lie in heaps for ten or twelve hours after they have been plucked, during which time they undergo a sort of fermentation; the leaves then pass through certain processes, and are slowly dried over charcoal fires. (Wynter Blyth, Foods, Composition and Analysis, 1903.) A sophisticated tea is largely exported from China, consisting of powdered tea mixed with sand and other earth, and agglutinated with gum; that which is to pass for black being colored with plumbago, and the green with the coating above referred to. On analysis, these teas were found to afford from. 35 to 45 per cent. of ashes, while the genuine yields only 5 per cent. A very full account of the adulteration of tea and the methods for the detection of the same may be found in Allen, Cow. Org. Anal., 2d edition.
The analyses below by Y. Kozai (Bulletin No. 7, Imperial College of Agriculture, Japan) have a special value, owing to the author's knowledge of tea manufacture. Unusual precautions were taken in sampling the leaves, to insure strictly parallel specimens being taken. The figures refer to the moisture-free leaves in each case.
|Analyses of Tea (percentage composition).
|Caffeine or Theine
|Tannin (as Gallotannic Acid)
|Other Nitrogen Free Extract
Rochleder found also a peculiar acid, which he calls boheic acid, C7H10O6. According to Stenhouse, the tannin of tea, though always accompanied by a little gallic acid, differs essentially from that of galls; not being, like it, a glucoside, but yielding, under the influence of sulphuric acid, a dark brown substance, almost insoluble in water. (See A. J. P., 1862, 254.) For article on the methods of analysis of tea and a series of analyses of different varieties, see A. J. P., 1887, 626. The volatile oil is citron-yellow, lighter than water, has a strong odor of the tea plant, solidifies easily by cold, and resinifies on exposure to air. It is probably one of the principles upon which depend the effects of tea upon the nervous system. Hence old teas are less energetic than the recently imported; and it is said that the fresh leaves have often produced dangerous effects in China. Nevertheless, the tannic acid is not without influence upon the system; and it is not improbable that the extractive contributes to the peculiar influence of this valuable product. Of these active ingredients, the volatile oil, tannic acid, and extractive are found most largely, according to the analysis of Mulder, in the green tea. Theine, C8H10N4O2, is a crystallizable principle discovered by Oudry. It was afterwards proved by Jobst to have the same composition as caffeine (see page 247). Kossel discovered, in 1888, a related principle, theophylline (dimethylxanthine), but only in very small amounts. It is now made synthetically and is official.
The proportion of theine found in tea varies considerably, the general range being from three to four per cent. India and Ceylon tea usually contain slightly more than four per cent.
Tea is astringent and gently excitant, and exerts a decided influence over the nervous system, evinced by the feelings of comfort and even exhilaration which it produces, and the unnatural wakefulness to which it gives rise when taken in unusual quantities or by those unaccustomed to its use. It is almost exclusively used as a beverage. Taken moderately, and by healthy individuals, it may be considered as practically harmless; but long continued in excessive quantity it is capable of inducing unpleasant nervous and dyspeptic symptoms. Green tea is decidedly more injurious in these respects than black, and should be avoided by dyspeptic individuals, and by those whose nervous systems are peculiarly excitable. It is rarely used as a medicine, but the infusion is popularly employed to relieve neuralgic headaches.