Tea, its history and mystery

by Joseph M. Walsh

Tea has been analyzed by many other chemists, but owing to a difference in the variety, character, quality, age, color and methods of preparation of the specimens submitted, the results have been as varied. The average composition in parts range as follows:—

Chemical Constituents.		Quantities.
	Theine,	3
	Tannin,	25
	Volatile Oil,	1
	Albuminoids,	15
	Mineral Matter,	6
	Gum and Glucose,	21
	Vegetable Fibre,	20
	Fatty Substances,	4
	Water of Absorption,	5
		———
Total,		100

Theine—Is the alkaloid of tea and is the substance to which it owes its refreshing and stimulating properties. It is a crystallizable matter, soluble in water, very bitter to the taste and characteristic alike of both tea and coffee, being to these beverages what quinine is to bark, and with the base of cocoa which has recently received the name of “theobromine,” it is also closely related. It is further remarkable as occurring in many other plants dissimilar in structure and character, grown in remote countries, but yet selected by the inhabitants on account of their yielding a slightly exciting and refreshing beverage, and to the presence of which the peculiar physiological action of tea on the animal economy is attributed. It was first discovered under the name of Caffeine by Runge, who originally found it in Coffee, and later by Oudry, who extracted an identical substance from tea, to which he gave the name of Theine. Strickler subsequently produced it from cocoa, naming it Theobromine. These bodies are evidently related to uric acid as like it, when exposed to the action of nitric acid and ammonia they yield a purple coloring matter, technically termed murioxide.

Theine is a substance which crystallizes very beautifully, forming white, silk-like crystals containing an atom of water of crystallization, the specific gravity of which is 1.23 at 1°C., and the 9 water of crystallization is not altogether evaporated by a temperature of 150°. As deposited from aqueous solutions it still contains an atom of water, but as deposited from solutions in alcohol or ether, or when sublimed it is anhydrous. It is much more soluble in hot than cold water or in alcohol or ether, and according to Peligot, one part of theine dissolves in 300 parts ether or in 93 parts water at ordinary temperatures. It is a base of the same class as aniline and urea, that is to say, it will combine with acids yielding crystalline compounds, but never neutralizing an acid. With chloride of platinum, chloride of gold and corrosive sublimate, the hydrochlorate of theine enters into combination, forming a double salt with each. As will be manifest from its formula—C₈—H₁₀—N₄O₂—theine is one of the most highly nitrogenous substances known to chemists, and connected with this high percentage of nitrogen (almost double that formed in any other albuminous substance) is its property of yielding an abundance of cyanides when fused with soda lime, which property distinguishes it from a number of organic bases, such as piperine, morphine, quinine and cinchonine. With the base of cocoa—which has received the name of theobromine—theine is also closely related, being nothing more than methylated theobromine. Strecker having produced it from theobromine by acting upon a silver derivative with iodide of methyl, in a sealed tube heated at 100°. Theine exists in tea, not in the free state, but in the form of tannate of tea, which appears to be dissolved by the excess of tannic acid contained by the tea leaf, and so it happens that the theine makes its appearance in the infusion instead of remaining in the exhausted leaves. The proportion of theine in tea has been variously given by different chemists. Mulder finding 0.43 per cent. in Green tea and 0.46 in Black, while Stenhouse found 1.05 and 1.27 in Green and Black respectively. Peligot found 2.34 and 3.0, and Zoller, whose research is comparatively recent, found 3.94 per cent. of theine in India tea. But it would be a mistake to regard these varying results as showing that the quantity of theine in tea is variable, as they serve only to illustrate the difficulties which stand in the way of a quantitative extraction of the theine, and the imperfection of the earlier methods. In Peligot’s paper, these difficulties are referred to, and by making an attempt to extract the theine from a sample of tea the chemist acquires a sense of the truth of them. The experiments of the latter, however, being of great interest to chemists merits a somewhat detailed description. He began by determining the total amount of nitrogen contained in the dried leaves of different kinds of China tea at 110°, finding 6.15 per cent. in 100 parts of Oolong, 6.58 in Congou, and 6.30 in Green tea, while from one sample of India he extracted only 5.10 per cent., proportions six times greater than had been heretofore obtained by any previous analysis. Next testing every soluble substance for nitrogenous matter, he proceeded by successive eliminations to ascertain the quantity of theine in 27 other different samples and found that Green teas contained on an average 10 per cent. of water, and Black only 8 per cent., and also that the latter contained about 43.2 of matter soluble in boiling water while the former averaged as high as 47.1, and that this soluble matter yielded only 4.35 of nitrogen in Black teas, and 4.70 per cent. in Green. It remaining to be determined whether this large percentage of nitrogen was wholly due to the theine or in part to some other principle, he next found that the precipitate with sub-acetate of lead contained no apparent quantity, and then testing the theine by a modification of Mulder’s process obtained from Green tea an average of 2.48 per cent., and from a mixture of Green and Black 2.70. But greatly as these quantities exceed those of all other chemists, they were still unable to account for the whole amount of nitrogen found in the infusion, so by adding mere acetate of lead and ammonia, separating them by filtration, and passing through it a current of sulphuretted hydrogen to precipitate the lead, and evaporating the liquid with a gentle heat he obtained an abundant supply of crystals of theine. This supply he still further increased by re-evaporation until the whole amounted to 3.48 per cent. of the entire. There still remaining a syrup containing some theine it was precipitated with tannic acid, the result being added to that already crystallized it yielded a total of 5.84 from Green tea in the natural state and 6.21 in the dried leaf. These experiments being further continued by boiling the exhausted leaves with potash, it showed a presence of caseine to the extent of 28 per cent. of the mass, the proportion of the latter substance in the raw leaf being only 14 to 15.

Theine is extracted from tea by boiling a quantity of the leaves in a considerably larger quantity of distilled water and the liquor squeezed out of the leaves which are to be boiled with a fresh quantity of water and again subjected to pressure, the process being repeated a third time. The several portions of the infusion expressed from the leaves are put in the same vessel, mixed together thoroughly and treated with an excess of acetate of lead and ammonia, which precipitates the tannin and coloring matter. The liquor is next filtered and the filtrate evaporated down to a small bulk, first over a naked flame and afterwards in a water bath, and on being allowed to cool the solution will deposit crude theine which is removed by filtration, and the filtration nearly dried up in the water bath, and the residue boiled with alcohol, which dissolves the theine out of it. From this hot alcoholic solution theine crystallizes on cooling, a final purification being effected by crystallization from ether and decolorizing with animal charcoal. A simpler but less effective method is to place the dust of finely powdered tea-leaves, or an evaporated watery extract on a watch glass and cover it with a paper cone and hold it over a spirit lamp or gas jet the vapor arising from the glass condenses on the interior of the cone and forms small crystals of theine. Concentrated sulphuric acid dissolves theine in the cold without the production of color, but if the alkaloid be treated with nitric acid evaporated to dryness, and the reddish-yellow residue moistened with a little ammonia it turns a splendid purple color. Again, if a solution of theine be evaporated with chlorine on a watch glass a reddish-brown residue is obtained, which if again treated with the vapor of ammonia it becomes a deep violet of which the chief precipitants will be phosphoric acid, iodine and platinum, forming a yellow and brown precipitate respectively.

Theine having no odor and only a slightly bitter taste it obviously has very little to do with the flavor of tea. It is, however, considered a very valuable constituent on account of the large percentage of nitrogen which it contains and to which is attributed the peculiar physiological action of tea on the animal economy, but what changes it undergoes in the human system has not yet been determined. When oxydized artificially it decomposes into methleamic (hydrocyanic) acid, a nitrogenous compound closely allied to caseine or gluten, and as hot water extracts but very little of this substance a large amount of it is wasted in the ordinary infusion, which might otherwise be saved by the addition of a little carbonate of soda to the water in preparing it.

Tannin—A large portion of the Tea-extract consists of tannin (tannic acid of a peculiar kind), there being much more in Green teas than in Black, ranging from 13 to 20 per cent. in the former, and 8 to 12 per cent. in the latter, but averaging 12 and 9.50 per cent. respectively, the difference being due to the fact that part of the tannin originally existing in the raw-leaf is destroyed during the process of fermentation to which Black teas are subjected in manipulation. It is a powerful astringent principle, puckering up the mouth when chewed, and to which tea owes its bitterness when overdrawn or boiled, constipating effect on the bowels, and the inky-black color which it imparts to water containing salts-of-iron. But whether it contributes in any degree to the exhilarating, satisfying or narcotic action of the tea has not yet been determined. Johnston thinks it probable that it does exert some such effect from the fact that a species of tannin is found in the Betel-nut, which when chewed produces a mild form of intoxication, but as to whether this property assists or retards digestion is still an unsettled question, the old maxim, “what is one man’s meat is another man’s poison,” being particularly true of this substance. Many persons finding that the use of tea while eating, or immediately after eating, has a soothing effect on their system, while the same persons after drinking coffee, under like circumstances, get nervous, and cannot digest their food properly. As there is no tannin in coffee, it stands to reason that the substance must have some influence on the digestive organs.

For the estimation of tannin in tea various processes are in use, a tritration by means of a standard solution of gelatine, which depends upon the well-known property possessed by gelatine of forming insoluble compounds with tannin being the most effective, but tedious and difficult. A much more simple and promising method consists in tritrating by means of a standard solution of lead, the point of saturation being indicated by the red color struck by an ammoniacal solution of ferricyanide of potassium, one drop of this solution being capable of coloring one milligram of tannin dissolved in 100 parts of water, the exact strength of the solution of lead being ascertained with a standard solution of tannin. In using the solution of lead, 10 drops of it are first diluted with 9 times its volume in water, and the tea infusion dropped into it from a graduated burette until the indicator strikes a red with the drop of the indicator. The infusion of tea is made by boiling 2 grains of the leaves in water and afterwards diluting it to 250, it being understood that the smaller the quantity of this infusion required to saturate the 10 parts of the lead solution, the higher the percentage of tannin in the sample of tea treated. This test is specially applicable for ascertaining whether Black tea in particular has been mixed with spent leaves. By taking the normal percentage of tannin in pure Black tea at 10 and the percentage of tannin in spent tea as 2, the difference is the extent of adulteration.

There is a great variability in the amount of tannin contained in the different varieties of tea, varying in quantity according to the country of production, kind, quality, and state of growth when picked. In six samples of China Oolong teas recently tested, the percentage of tannin extracted, after an infusion of thirty minutes, averaged only 7.44, an almost similar result being obtained from an examination of the finest Congou-China Green teas, ranging from 11.87 to 14.11 per cent., some Japan samples under the same conditions yielding on an average from 8 to 10. While with a sample of the finest Assam (India) a percentage of 17.73 of tannin by actual weight was extracted after an infusion of only fifteen minutes, two samples of India and Ceylon giving respectively 18.91 and 15.26, proving conclusively that India and Ceylon teas are much more heavily weighted with tannin than China and Japan teas. The percentage of tannin in the extract is also quite irregular, according to the quality of the tea, the ratio of tannin to the extract varying quite uniformly with the value of the tea, the percentage falling and rising with the percentage of the extract and cost of the tea.

Volatile Oil—Is the principle which imparts to tea its peculiar flavor and aroma, and upon the amount contained in the dried leaves depends the strength and pungency of the infusion. It is present only in very small quantities, but is, nevertheless, very potent in its effects, the proportions ranging, according to Mulder, from 0.6 per cent. in Black tea to 0.80 in Green, but averaging 0.75 in all good teas. It is found by distilling the tea with water, is lighter in body than water, citron-yellow in color, resinifying on exposure, solidifying with cold, and exerting a powerfully exciting or stimulating effect on the system. But there being no chemical analysis of this constituent extant, its exact effect on the human system is difficult to define. By some authorities it is claimed to produce wakefulness, acting, it is said, in the same manner as digitalis (fox-glove) which, when taken in overdoses, causes anxiety and inability to sleep. It is a well-known fact that Green teas produce these effects, while Black does not, the excessive fermentation to which the latter are subjected in the process of curing, dissipating the volatile oil to a greater extent, or, more properly, altering its general character not only in effect but also in flavor.