Popular Science Monthly/Volume 20/January 1882/The Chemistry of Coffee and Tea
|THE CHEMISTRY OF COFFEE AND TEA.
By ALBERT B. PRESCOTT,
PROFESSOR OF ORGANIC AND APPLIED CHEMISTRY IN THE UNIVERSITY OF MICHIGAN.
THE vegetation on the southern slopes of the eastern Himalayas, three or four thousand feet above the sea, though by no means luxuriant, is said to be very agreeable and of much interest to the botanist. Among the plants native to these slopes, planted in the course of nature during the preparation of the earth for man, and left wild with the elephant and the leopard, is a shrub growing from twenty to thirty feet high, and well worthy to be selected for pleasant foliage and fine flowers. The lanceolate leaves are from two to six inches long, and the flowers are large and white, very fragrant, in clusters of two or three in the axils of the leaves. This is the tea-plant, of the genus Thea, very nearly allied to the genus Camellia, of which the Japonica and other species from China and Japan are favorite cultivations of the greenhouse in Europe and this country. Nowhere in the world but on the borders of the Himalayas and in the wild regions of Assam is the tea-plant found growing uncultivated, but it was not discovered in this its natural habitation until the present century. As a cultivated plant, the Chinese have certainly had it since the fourth century, and they claim it to be indigenous to their own soil—just as confidently as they claim the parentage of numerous valuable articles. China has given tea to the world, and has furnished a favorable home to the plant, which is nevertheless quite as well suited in its native land, farther east. When it became known in England that the tea plant grew native in the highlands of the Himalayas, English companies engaged extensively in the cultivation of tea in that region, and finally, after the correction of notable failures in methods of culture and of cure, it appears that the finest teas of Asia are those of these mountain-plains and the choicest plants are of variety Assamica, lately propagated from the wild shrub of the mountains.
A child would ask the question, What is there so very good in the tea-plant or in its dry leaf? And a philosopher may well ask, What is there about it, that this article has had a commercial history since the early middle ages, and men toil to till it, and with infinite detail to pick it and dry it and roll it, and spread and stir and roast and toss it, and then carry it over the globe in quantities to distribute for every household and amounting in sum-total to a quarter of a billion of pounds every year? In the first place, what does the tea-leaf contain—the fresh leaf from the wild bush of the mountains, never gathered for use, or the leaf picked on the tea-farms and carried dry to your tea caddy—what kinds of compounds are in it, and can no other plant than this produce them?
To leave out a long story of progressive investigations, and to say nothing of the ways and means of analysis, the chief constituent to be named is the alkaloid theine. When separated from the other constituents of the tea-leaf, so as to be seen in its perfect purity, theine appears in snow-white, silky, filiform crystals, flexible and fragile, without odor, but having a mildly bitter taste. It dissolves readily in ten times its quantity of boiling water, and more slowly in a larger proportion of boiling water. It does not vaporize or decompose in the least at water-boiling heat; it melts at higher temperature and vaporizes slowly at about 360° Fahr., without decomposition. Chemically, it is a compound of the four organic elements, and is classified within the borders of the great group of alkaloids, but it differs distinctly from all other alkaloids (except its near relative, theobromine) in various chemical characteristics. It has, also, a much larger proportion of nitrogen than the other natural alkaloids. But it remains an important consideration that this crystalloid constituent of the tea-leaf is built on the chemical type of the alkaloids, a class of bodies which nature forms in plants but not in food-plants—bodies that include narcotics, stimulants, hypnotics, delyriants, poisons, antidotes, tonics; some of them affecting the whole nervous system, one to excite and another to depress, and others influencing only parts of the nervous system, for special functions of the body. There is an alkaloid that gives steady impulse to spinal nerves, causing continuous contraction of one set of the muscles of the body; another that diminishes nervous effort upon all the muscles, and so impresses the brain as to produce sleep along with dreamy activity of the mind; another that, among other effects, so controls certain muscles as to open wider the pupil of the eye, and another that promotes digestion by arousing force in the nervous supply of secretory functions. Again, in the observation of the effect of an alkaloid it is found that, while a small portion stimulates the nervous system, a large portion acts as a sedative, so that a difference in quantity of the potion causes a difference in kind of its effects. By far the greater number of alkaloids do exert some sort of specific effect on the nervous system, but even to this generalization there are exceptions, and we could really infer nothing from analogy as to the effect which theine should have on the system.
The administration of theine in small portions, to animals or to man, quickens the circulation and effects some degree of mental exhilaration and wakefulness. In final result, the excretion of carbonic-acid gas is diminished, and the flow of blood through the capillaries is retarded. Larger portions prove poisonous, causing painful restlessness, rigidity of the muscles, and general exhaustion. Not more than three or four grains at once can be properly taken for medicinal or experimental purposes. As to the effects of the habitual use of small portions of theine, the tea-drinking peoples of the world ought to be competent witnesses, and their testimony, pro or con, may be brought by any advocate before the judgment of the world.
But we have to ask whether the alkaloid theine is found elsewhere in the vegetable kingdom. The coffee-tree (Coffea Arabica, Rubiaceæ) appears to have been planted by Nature in the heart of tropical Africa. At present it grows wild in Liberia, and is cultivated in most regions of the tropical world. It is twenty to thirty feet high, though, like the tea-plant, in the course of culture it is cut down to the height of five or six feet. It has the general appearance of a cherry-tree. It is an evergreen, with leaves four or five inches long and one and a half or two inches wide, and bearing clusters of white and richly fragrant flowers, the fruit being dark-red when ripe, and each holding two seeds, the coffee-berries of commerce.
In chemical analysis coffee yields an alkaloid, at first named caffeine, but long since determined to be identical with the alkaloid of the tea-plant, so that while the terms theine and caffeine are both in use, chemists recognize them as synonyms. Theine is found not only in the seed—the "berry" of the coffee-plant but even more abundantly in the leaf, though the latter, of less agreeable flavor, has been little used, and is not in commerce. The use of the coffee-leaf is further discouraged by the fact that the plucking of the leaf is injurious to the crop of the berry.
The chemist also finds theine or caffeine in two South American plants, the maté and the guarana. The former, yerba maté, or "Paraguay tea" (Ilex Paraguayensis), is a small forest-tree, bearing fine white flowers, indigenous to Brazil and the Argentine Republic, where it has long been prepared for a beverage which is drunk throughout South America. The leaves and twigs are employed, sometimes with inclusion of the fruit. Brazil also furnishes guarana, an article known in medicinal commerce, and obtained from the seeds of Paulinia sorbilis, a climbing shrub. Its fruit is of the size of the grape, and has a single seed. The seeds are bruised into a pulp, molded into cylindrical cakes, and dried. So prepared, guarana is in habitual use in South America, infused as a beverage, and also taken solid as an adjunct to food.
One more plant has to be named as a source of theine—the tree of the cola-nut, of Western Africa (Cola acuminata). The tree, of which two varieties are found, one with broad and one with narrow leaves, has been cultivated somewhat in tropical countries. The nuts are of the size of a pigeon's-egg, of a brownish color, a fragrant odor, and aromatic, bitter taste. The natives prize them very highly, using them in the solid state as a condiment with food, and to sustain strength during continued exertion when the supply of food is deficient. The cola-nut is rich in theine, as first shown by the analysis of Professor Attfield, at London, 1864.
So far, then, only five plants on the globe have been found to contain the alkaloid theine or caffeine, but the chemist finds an alkaloid very closely allied to this one, though of distinct individuality, namely, theobromine. This interesting chemical compound is homologous with the alkaloid of tea and coffee, differing in the molecule by one atom of carbon and two of hydrogen, theine having the structure C8H10N4O2, and theobromine C7H8N4O2. The relation is the same as that between any two contiguous members of the methylic alcohol series. Theine is constituted as methyl-theobromine, C7H7 (CH3) N4O2, and the chemist finds it an easy task to change theobromine into theine. Now, theobromine can be artificially produced from xanthine (C5H4N4O2); indeed, theobromine is dimethyl-xanthine, and theine is trimethyl-xanthine. In other words, xanthine may be presented as a chemical ancestor of the active principle of tea and coffee, and so it becomes of interest to inquire into the origin of xanthine, so called. In reply, it must be said that xanthine is not found in plants, but it is found in chemical laboratories; it is not a myth, and it can be made from a good many things that are abundant on the face of the earth. It can be made from uric acid, and it can be made (simply by action of nitrous acid) from guanine, which is abundant enough in guano. Guanine is found here and there in animal excrementitious material, is produced by spiders, and occurs in the pancreatic gland of the horse, and in organs of the salmon. The extensive group of bodies related to xanthine and theine have been found a very rich field of chemical research. Theine itself is a body of a very simply distinctive chemical structure, from which numerous derivatives are readily constructed. Hitherto, however, the chemical synthesis of theine from inorganic elements fails by lack of some short link whereby to form uric acid from any one of its own derivatives, nearly all which can be artificially formed. But the change of guanine into theine is easily accomplished. It is perfectly practicable to bring guano material to the laboratory, and send away the same atomic elements transformed into the snow-white, silky crystals of theine. Given only a sufficient demand for the pure stimulant principle of tea and coffee, and a market value high enough above the cost of its vegetable sources, and it might then safely be predicted that not many months would elapse before companies with thousands of capital stock would engage successfully in the chemical manufacture of theine from guano. Then, very likely, rival companies would establish the claim to manufacture a still purer article from certain of the waste substances of the world—articles more accessible than guano.
But it must not be forgotten that theobromine, just mentioned as an intermediate body in the chemical transmutation of guanine into theine, is itself an alkaloid of well-known vegetable origin. It has been found in only one plant (the Theobroma cacao), a beautiful evergreen tree of Mexican nativity, extensively cultivated in South America, growing twenty to thirty feet high, bearing small flowers in clusters on the large branches, and yielding fruit in a purple-yellow pod, seven or eight inches long, each holding twenty to forty seeds—the "cocoa beans" of commerce. From these are prepared all the forms of chocolate, "cocoa," and "soluble cocoa," now in use over the world as a beverage, a competitor for the favor accorded to the more nearly equal claims of coffee and tea. At the discovery of Mexico and Peru, the Spaniards found chocolate from time beyond record an habitual beverage, and the chocolate-tree in extensive cultivation, in both countries.
The chemist has yet to find, if he can, a plant on the globe containing the alkaloid theine, or its chemical associate, theobromine, that has not been, by some of the races of men in some of the ages of the world, brought into use as a refreshing beverage, or adjunct to food, prized for adding cheerfulness to nourishment, and giving solace to fatigue. However the peoples of the world—while wholly unconscious of the identities of chemical science—have been guided in their search over forest and field for a herb to infuse, in a beverage that should cheer but not inebriate, it is certain that no hint of common qualities is to be gained from the external aspect of the plants so brought into use. Between the leaf of the tea-plant and the long pod of the cacao-tree there is no visual resemblance to indicate a common constituent, or to suggest a parallel use. As unlike as their general appearance, even more diverse are the botanical characteristics of the caffeine-containing plants, representing, as they do, diverse families in the vegetable kingdom.
In the modes of preparation for use, the sources of caffeine present some coincidences, with many diversities. The coffee-berry, except in some Eastern countries, is roasted, an operation to which its valued fragrance is almost wholly due. The roasting develops from inodorous constituents a very diffusible empyreumatic oil of very appetizing odor. In the more elaborate preparation of the tea-leaf a degree of roasting appears to be always necessary for development of agreeable flavor, while a certain extent of fermentation precedes the roasting in the manufacture of black teas. For black tea the leaves are withered a little, rolled to liberate the juice, then left in balls for just enough fermentation, then sun-dried, and subjected to a careful firing in a furnace. For green teas the fresh leaves are first withered in hot pans, then rolled to liberate the juices, and slightly roasted in the pans, now sweated in bags, and returned to the pans for a final slow roasting, with stirring for eight or nine hours, beginning at the temperature of 160° Fahr. and falling to 120° at the close. These operations are here given as now conducted by the planters in India, and as proposed for tea-culture in the Southern United States, and are considerably abridged from the time-established routine of minute detail in China. The teas of commerce contain a distinct essential oil, a constituent which does more than all others to obtain the esteem and fix the value of each grade of tea in the market. The question of the effect of the curing process upon the essential oil—to liberate or generate or modify or dissipate it—is an inquiry of no little interest and consequence, and deserves more chemical investigation than it has received. In the manufacture of all the forms of chocolate, the cacao seeds, cleared of the shell-coat, are roasted to begin with. Without further treatment they constitute the "cacao-nibs." When ground, pressed to remove a part of the oil, made with sugar into a paste, flavored with vanilla, spices, etc., and cast in molds, we have "chocolate" proper. When left in the pulverulent form, or molded in porous and friable cakes, the article is presented as cacao or "cocoa," or "soluble cocoa"—the proportion of oil being diminished either by its removal under pressure, or by adding some form of starch or farina. "Flake cocoa" is made by crushing the roasted seed, shell and all. In the preparation of the South American maté, the leaves and twigs are subjected to a "curing" process, and roasted under a covering of earth to develop an aromatic principle which gives flavor to the beverage. The guarana, also, is dried or lightly roasted, though acquiring only slight fragrance. The African cola-nut is strongly aromatic as taken from the tree, and this seems to be the only one of the six chosen foods furnishing theine or theobromine that is not subjected to dry heat in its preparation.
Tracing, so far in our study, the common constituent of coffee and tea, in its occurrence in various vegetable products which have been appropriated, after similar methods of treatment, to the uses of adjunct food, it is full time to inquire what else is to be found in the tea-leaf, the coffee-berry, and the chocolate-nib, besides a certain stimulant, alkaloid. With entire justice to the remaining components they may be here classified in three groups—nutritious substances, astringents, and aromatics.
Tea yields from thirty to fifty per cent of its substance to solution by prolonged boiling in water; but, as the beverage is properly prepared, probably not over twenty to twenty-five per cent of the solids are dissolved. This soluble portion includes dextrine, glucose, gum, and potassium salts, besides tannin and the alkaloid. The nutrient portion of tea is comparatively small. In coffee we have ten to thirteen per cent of fixed oil, and about the same proportion of legumine, with gums and extractive matters, and two per cent of soluble potassium salts. The roasting reduces the glucose from six or seven to one or two per cent, liberates the oily matter, and so modifies the legumine as to render it measurably soluble. The beverage seldom contains over ten or twelve per cent of the roasted berry in absolute solution, so as to be retained in the liquid when perfectly clear, but some of the finer powder is likely to remain in suspension, and the oil is held with the liquid, while hot, to a still greater extent. In quantity, the food constituents of roasted coffee are generous enough, but in digestibility it is more than probable that they are deficient. As to the cacao-seed, its oil, constituting half its weight, is a substance well known in its separated state as cacao-butter, and is an agreeable and very wholesome form of oleaginous food. In quantity it ought not to be unduly diminished, either by removal with the press or by addition of farina, and it should constitute as much as twenty-five per cent of all of the cacao preparations. Besides the oil, a good portion of starch and about eighteen per cent of albuminoids are found in cacao-seeds, and all these are to a large extent obtained in chocolate beverages. The butter of these creamy liquids is less likely to cause disturbance of digestion than the roasted albuminoids they contain.
The astringents of tea and coffee are tannins, properly classified along with the numerous varieties of tannic acid, the astringent principles of plants. In tea the proportion is large, ranging, according to the lowest statements, from nine to twelve per cent, and placed by some authorities as high as thirty and forty per cent. Dragendorff reports green teas to bear a higher percentage of tannic acid than black, and this difference may be due to a decomposition of tannin during the fermentation in the manufacture of black teas. But it is important to know that only a small part of this tannin is extracted from the leaf in the suitable preparation of the beverage. Some experiments with tea as it is prepared for the table gave, for a five fluid-ounce cup of the liquid, in ten instances an average content of a trifle over one grain of tannin. Other experiments with tea after five minutes' steeping gave in twelve instances an average of tannin equal to only 100 per cent of the dry leaf; while the same teas, on thirty minutes' gentle steeping, yielded tannin amounting to an average of 22 per cent of the dry leaf. In another case, by thirty minutes' active boiling, so much tannin as 112 per cent of the tea-leaf was obtained in solution. Black teas are necessarily steeped longer than green, as they yield all their soluble matters more slowly. In coffee the proportion of tannin is not over one third, perhaps on an average not over one fourth or one fifth, of that in tea. The cacao-nib and its preparations are free from tannin. Guarana is heavily charged with a specific variety of tannic acid, and the mate is even more astringent than the tea-leaf.
The fragrant principle of tea, the essential oil, already referred to, has not been separated in notable quantities, but it is recognized as a diffusible stimulant, transient and harmless in its effect on the system, and certainly attracting no little favor to the tea-cup. The diffusible flavorous substance of coffee is a product of its roasting, and the same is true of the proper fragrance of cacao, to which other odors are often added. These vaporous bodies are so easily dissipated by a prolonged steeping, and especially by an active boiling, that brief infusions of tea and of coffee are likely always to be preferred.
In the quantity of the alkaloid, theine or caffeine, the tea-leaf is over twice as rich as the coffee-berry. The medium proportion is that of 2 to 22 per cent in tea, a little under 1 per cent in coffee, and about 12 per cent (theobromine) in the cacao-nib while the guarana preparation has 5 per cent, and the mate H per cent. Attfield found 2 per cent of alkaloid in the cola-nut. From a pound of tea, then, there can be obtained at least 140 grains, or over a quarter of an ounce, of the crystallized alkaloid, about enough to balance a silver quarter-dollar and a dime. The theine in a pound of tea is twenty-five or thirty times as much as could be taken at once without notable disturbance of the nervous system. A pound of coffee contains twelve or fifteen times as much theine as one ought to take at once.
The greater portion of the alkaloid enters into solution in making the common hot infusions of coffee and tea. Tea yields its alkaloid to hot water upon even brief application; coffee, especially when but lightly roasted, requires longer steeping for the extraction of the theine. It may be said, then, of long steeping of coffee and tea, that it is not required in obtaining the chief portion of the stimulant principle; that it only serves in the case of coffee to increase somewhat the nutrient matters; that, unless in tightly closed vessels, it wastes the fragrance, the qualities held in choicest esteem and highest value; and that, when used with teas, it renders the beverage so strongly astringent as to be both disagreeable and unwholesome.
The opinion is general, though not universal, that coffee is a stronger stimulant than tea, comparing them as American beverages. On this question, the analyst can only aver that a given weight of tea leaf yields about three times as much of the common stimulant principle as the same weight of roasted coffee. How many cups of tea may be made from a pound of the dry leaf, or how few cups of coffee are made from a pound of the browned berry; and how long either quantity, dispensed under regulations both prudent and generous, will continue to supply a flowing cup in daily service of an individual—these questions may be respectfully referred to authorities more competent than chemists. The proper judges can comprise none other than the gracious autocrats of the tea-table themselves; and when they shall have vouchsafed perfectly definite replies, regarding these desired numbers and quantities, then in a very short time some assurance can be given of the stimulating effects of coffee in comparison with tea.
But, pending all conclusion from chemical identity and parallel quantity of constituents, any sufferer from coffee may resort to tea, and any injury from either may be cited against the closest chain of reasoning which chemistry can present. Most people have a small stock of experimental science of their own, sufficient for personal theory, if not wholly in accord with personal practice. And now regarding the effects of coffee in comparison with tea, let it be remembered that sleeplessness during the hours of repose may come from irritation in the stomach as well as from stimulation in the brain. The rich, oily, and albuminoid matters of roasted coffee, and especially the empyreumatic oil to which its flavor is due, are liable to derange the digestion of many persons, while tea is seldom subject to this disadvantage, and its diffusive fragrant oil may be helpful to the action of the digestive powers. In countries where the people rarely have indigestion, it is not found that coffee prevents sleep more than tea, but this report is often made in America, where the digestion is not the best in the world. The astringent influence of tea, unless prepared with brief steeping, may produce ultimate injury, but seldom causes immediate disturbance. To the action of the stimulant alkaloid itself, of course some persons and some nations are more susceptible than others, but we may be sure that it is only from this action that increased mental activity and the wakefulness of nervous exhilaration are obtained, alike in the use of coffee and of tea. And it is doubtless far more from this action of the one alkaloid found in both beverages, than from any other constituents, that their habitual use becomes injurious to many persons.
The consumption of coffee and tea in the United States in one year (1877) provides about 82 pounds of coffee and 13 pound of tea to each inhabitant. The importation of coffee, in Europe, is stated to furnish for each inhabitant, in France, 210 pounds; in Belgium, 132 pounds; in Holland, 21 pounds; in Sweden, 6 pounds; in Norway, 9 pounds; in Austria, 2 pounds; in Russia, 5 pound; in Italy, 1 pound. In Great Britain, the consumption of coffee has been on a decrease for the last thirty years. The tea consumption in that country amounts to 2 pounds to each individual. The total amount of tea consumed on the globe would give about 4 pound to each of its inhabitants.
The grades of coffee in the market come from different varieties of the tree, and from different countries where it is cultivated. The various kinds of tea result primarily from differences in the age of the leaf when gathered. The choicest teas are from the younger and more succulent leaves, the earliest leaf producing flowery pekoe, the next orange pekoe, then pekoe and souchong, and from the oldest leaves bohea proper. Differences of manufacture have already been noted for black and green teas. The processes of manufacture in China and India are necessarily modified to enable the tea to bear sea transportation without injury, and it must be accepted that the finest tea can only be obtained in tea-growing lands. When American enterprise shall have devised such means of preparation as may dispense with the present lavish use of hand-labor in Asia, then we may have tea ranking above tobacco in the products of the Southern United States.
The adulterations of tea comprise mineral matters, foreign leaves, and spent tea. The food-analysts of Great Britain have fixed the maximum limit of eight per cent of mineral matter, including three per cent to be soluble in water, and a minimum limit of thirty per cent aqueous extract. The color-facing of turmeric, prussian-blue, and gypsum, though not excluded as fraudulent, is characteristic of poorer qualities of green teas. The adulterations of coffee, sold in the ground condition, are multifarious, all sorts of roasted grains, nuts, and shells having been taken for this use, but the chiccory-root has been the most extensive admixture, and in Great Britain has been in actual demand by the consumer. Even the entire coffee-berry is sometimes counterfeited—a suggestion from the legendary wooden nutmegs of New England invention. But we may trust that the great body of tea and of unground coffee in commerce is nearly or quite innocent of adulteration. No positively hurtful impurities are apt to be found, but not the less all falsifications of these or other articles of food should be severely punished. It is clearly a misdemeanor to tamper with the food which any man may select by its distinctive name, in his own discretion, for his personal use.
Notwithstanding the adoption of theine-containing beverages by mankind at large, we can not hesitate to commend that robust habit which discards all dependence on adventitious food, even on so mild a stimulus as that of the tea-cup, and preserves through life the fresh integrity of full nervous susceptibility. And probably there was never a time when there were so many persons as now who are disposed, by conviction and by a desire for a stalwart physical independence, to refuse to fix any habit that holds the nervous system.
But, taking the world and its discomforts as we find them, it must be granted that the thralldom of tea is comparative physical freedom, and we can gladly give voice to those who praise these comfortable beverages. "Near the fire," says De Quincey, "paint me a tea-table—place only two cups and saucers upon it-and beside them paint me an eternal tea-pot. . . . For tea," he says, "will always be the favorite beverage of the intellectual, and for my part I would have joined Dr. Johnson against any impious person who should presume to disparage it."