CHEMICAL AND DIETETIC CHARACTERS OF COFFEE AND TEA. 567

By evaporating the solution, amalic acid may be obtained in colourless crystals, which are insoluble in alcohol; they assume a violet colour if moistened with baryta water. On evaporating the solution, cholestrophan may be obtained in pearly scales, resembling those of cholesterin in appearance, and hence its name (from cholesterin, paivw, to appear); it may be sublimed unaltered. According to the researches of Rochleder (Liebig's Annal., lxxiii. 123), the products of the oxidation of caffeine resemble those obtained from uric acid by similar treatment, and the two classes of compounds are homologous, or rather substitution-products from the same organic group; for amalic acid corresponds to alloxantin, in which two atoms of hydrogen have been displaced by two of methyl; and cholestrophan bears a similar relation to parabanic acid :—

Amalic acid (dimethyl-alloxantin) ¤ ̧н(ЄH ̧) ̧Ñ‚→4

Parabanic acid.

2 3

Cholestrophan (dimethylo-para-] €(CH),N ̧ ̧.

banic acid)

2

2,

Strecker, by heating dry parabanate of silver with anhydrous methyl iodide in a sealed tube, has, indeed, effected its conversion into cholestrophan (methyl-parabanic ether) and iodide of silver :

Ag2¤ ̧Ñ‚Ð ̧ +2 ¤H ̧I = (¤H ̧) ̧¤ ̧Ñ‚Ð ̧+2 AgI.

2 3

3

3

Amalic acid also possesses the property, like alloxantin, of staining the fingers pink; and if exposed to the vapour of ammonia, it produces a compound homologous with murexid, which, like the latter compound, is distinguished by its magnificent purple colour :

In the latter case the difference between the two compounds is 2 (Є,H), because murexid is the result of a reaction of ammonia upon two atoms of alloxantin, and caffeo-murexid in like manner is derived from two of amalic acid.

(1407) Chemical and dietetic characters of Coffee and Tea.Caffeine is a compound which presents a high degree of interest, both from the metamorphoses of which it is susceptible, and from the circumstance that it forms an ingredient in three substances; namely, tea, coffee, and a third substance of less importance, maté (or Paraguay tea, obtained from the Ilex Paraguayensis),

568

CHEMICAL AND DIETETIC CHARACTERS OF COFFEE AND TEA.

one or other of which constitutes a portion of the daily diet of three-fourths of the human race. The circumstance that these different shrubs should have been selected by different nations, for the purpose of yielding a beverage, when infused with boiling water, shows that the ingredient which they furnish is one which is adapted in a special manner to some craving of the human frame. Although these bodies differ so widely in flavour, they all contain the same azotised principle, the physiological effects of which have not been submitted to the detailed examination which they well deserve. The best series of experiments upon this point are due to Julius Lehmann, who watched for some weeks the effects of roasted coffee, as well as of its essential oil, and of caffeine, upon two individuals in good health (Liebig's Annal., lxxxvii. 205). The use of coffee as an article of diet appears to exercise an important influence in retarding the waste of the tissues of the body; since it was observed that during its use the proportion of phosphoric acid and of urea excreted by the kidneys was much smaller than when the coffee was omitted, the diet being in all other respects the same in both cases. Thus, it was found that when an infusion of three-quarters of an ounce of roasted coffee-berries was taken daily for a fortnight, the average proportion of urea and of phosphoric acid which passed off by the kidneys in 24 hours, was less by one-third than it was during a similar period in which the coffee was omitted. The empyreumatic oil of the coffee was found to exert a stimulating effect upon the nervous system. When too large a quantity of this essential oil was taken, it produced loss of sleep, nervous excitement, and symptoms of congestion of the brain. In smaller quantities it produced a gentle perspiration, removed the sensation of hunger, and acted upon the bowels as a laxative. It likewise reduced the amount of urea and of phosphoric acid in the urine. It was also concluded from the production of similar alterations in the quality of the urine, when a solution of pure caffeine was substituted for the ordinary infusion of coffee, that caffeine likewise exerted a power of retarding the disintegration of the constituents of the animal frame.

Tea, coffee, and the Paraguay tea-plant, in the form of which they are used dietetically, all contain the following principles :1. Caffeine. 2. One of the forms of tannic acid. 3. An aromatic essential oil, which is different in each plant, and is the main cause of their different and peculiar flavours.

Constituents of Coffee.-When the coffee berry is first gathered and dried in the air, it has but little fragrance, and only a slightly

CONSTITUENTS OF COFFEE.

569

bitter and astringent taste. The raw coffee berry is hard and horny; it swells up with difficulty even when heated with boiling water; it contains in its green state about the same quantity of soluble matter as after it has been roasted, but the amount varies greatly in different specimens.

Coffee swells considerably during roasting, the increase in volume amounting to one-third or even to one-half of its original bulk; at the same time it loses in weight to an extent varying from 15 to 25 per cent. upon the weight of the berries before roasting; the amount of this loss depending upon the extent to which the roasting is carried. The roasting should be stopped as soon as the berry has become friable. The agreeable bitter aromatic taste of coffee is developed during this operation. It is worthy of remark that inferior coffee becomes improved in quality by keeping, and if roasted after it has been kept for some years, it yields an infusion of a finer flavour than if roasted immediately after it has been gathered. Both the essential oil and the bitter flavour are developed from one of the soluble constituents of the berry, since the aqueous extract of the raw berry when roasted acquires the full odour and taste of coffee. The proportion of this aromatic oil in roasted coffee is very minute; not exceeding, according to Payen, one five-thousandth of the weight of the coffee. Coffee also contains 14 or 15 per cent. of fixed oil. The astringent acid, caffeic acid, as it has been termed, is allied to the quinic; it constitutes about 5 per cent. of the dry berry: it does not blacken a solution of ferrous sulphate, but turns it green, and it does not precipitate solutions of gelatin; it is dissolved by strong sulphuric acid, and produces a red liquid. When exposed to the air in contact with alkalies, caffeic acid absorbs oxygen, and forms a yellow insoluble resin.* A peculiar reaction occurs with caffeic acid when I part of its concentrated solution is heated with 4 parts of finely-powdered black oxide of manganese, and 1 part of oil of vitriol diluted with an equal bulk of water. Yellow crystals of quinone (1337) are deposited upon the neck and sides of the retort, and the acid liquid which distils over is saturated with quinone, and contains formic acid. (See a Report on the adulteration of coffee by Graham, Stenhouse, and Campbell, Q. J. Chem. Soc., ix. 52.) The acid of Paraguay tea, and that of the leaves of the holly tribe, also furnish quinone

Vlaanderen and Mulder have recently made a minute investigation of the caffeic acid, which gives a yellow precipitate with salts of lead, and to which they assign the formula, when anhydrous, of 1,H,? A second acid termed caffelic acid, EH,,, gives a white precipitate with salts of lead.

The saline matters found in coffee contain more than half their weight of potash (51.5 to 55.8 per cent.), chiefly in the form of carbonate and phosphate; they are almost entirely free from silica. The sugar in raw coffee is in the form of cane sugar (Graham, Stenhouse, and Campbell), and its quantity varies from 6.2 to 7.7 per cent. The process of roasting not only expels the hygroscopic moisture, but it considerably

greater part of the

modifies the properties of the caffeic acid, and converts nearly the whole of the sugar into caramel. The solid portion left after infusing the ground berry in water contains a large amount of nutritious substance, in the form of an azotised principle resembling casein or legumin. This may account for the practice among the Arabs and other oriental nations, of swallowing the undissolved portions of the grain with the decoction: in the countries of Central Asia the use of animal food is rare, and hence the inhabitants have been led to vary the mode of preparing their coffee in such a manner as to enable them to use the substance, in some measure, as a substitute for the nitrogenous constituents supplied by a flesh diet.

The dried leaves of the coffee-plant also contain as much as 1 per cent. of caffeine (Stenhouse), as well as an astringent vegetable acid, and a large proportion (13 per cent.) of an azotised principle allied to gluten. The dried coffee-leaf has an agreeable aroma; it is used largely in the form of infusion by the natives of Sumatra.

Constituents of the Tea-plant.-The dried leaves of the Thea

*Containing 39 per cent. of caffeine; 35 parts, therefore, indicate 136 of caffeine. This salt has not been found by other chemists.

GREEN TEA-BLACK TEA.

571

Sinensis, which constitute our ordinary tea,' contain about 45 per cent. of soluble matter; but, according to Péligot, tea does not usually give up more than about one-third of its weight of soluble matter to boiling water. The most important constituents of the leaf, in the form in which it is consumed in Europe, are:-1. The essential oil, to which it owes its peculiar aroma, and the proportion of which, according to Mulder, is about o'79 in green, and o'60 per cent. in black tea. This oil exerts a most powerfully stimulating and intoxicating effect. In China, tea is seldom used till it is a year old, on account of the well-known intoxicating effects of new tea, due probably to the larger proportion of essential oil contained in the freshly-dried leaf. 2. Caffeine, or theine, the quantity of which varies considerably in different varieties of tea: in different kinds of green tea furnished to Péligot, the amounts ranged from 2:2 to 4'1 per cent. 3. A nitrogenized compound analogous to casein, amounting to 14 or 15 per cent. of the weight of the leaf. Almost the whole of this material is thrown away in the spent leaves, which when dry contain not less than 28 per cent. of their weight of this substance.* 4. The astringent principle, which is a modification of tannin; it occasions a precipitate in solutions of gelatin, and produces a black with ferrous sulphate: it constitutes from 13 to 18 per cent. of the dried leaf. The ash of tea varies from 5'3 to 5'6 per cent. of the dried leaf; a portion of this ash is probably due to the colouring matter which the Chinese are in the habit of adding to their green teas, for the foreign market; this colouring matter is in some cases indigo, in others a mixture of Prussian blue and pipe-clay.

It appears from the observations of Mr. Fortune, that either green or black tea can be obtained at pleasure from the same plant. Green tea is prepared from the young leaves, which, within an hour or two after they have been gathered, are roasted in pans over a brisk wood fire. After four or five minutes' roasting the leaves become flaccid, and are rolled by the hands upon a wooden table; they are then again thrown into the drying pans, where they are kept in rapid motion, and in about an hour, or an hour and a half, are completely dried.

Black tea is allowed to lie in heaps for ten or twelve hours after the leaves have been gathered; they are then tossed about for

The Mongol Tartars economize a large portion of this nutritive matter by boiling the powdered tea with the alkaline water of their steppes, to which a quantity of salt is added; by this means a notable proportion of the gluten is dissolved or suspended in the liquid, which they commonly eat thickened with butter, milk, and baked flour (Lehmann).