FOREIGN OR SPURIOUS LEAVES IN TEA.
Another reprehensible form of adulteration is the substitution or admixture of foreign or spurious leaves obtained from other plants, which resemble in structure but differ widely in character from the true tea-leaf, such as those of the willow, plum or ash. Millions of pounds of these leaves are annually picked, cured and colored in the same manner as tea in China, and used for the purpose of increasing the bulk and reducing the cost, while in England, particularly, the leaves of the birch, elm, willow, chestnut, poplar and hawthorne have been extensively used for the same purpose. The coloring material used in the latter country differs from that used in China and Japan being still more dangerous and injurious to health. This form of adulteration, however, is trivial when compared with the former one, but, nevertheless, the expert and analyst are frequently called upon to deal with it to a much greater extent than most people imagine.
Such foreign leaves in tea may be best detected by their botanical character or by the absence of the special structural marks which distinguish the genuine tea-leaf from that of all other leaves in the vegetable kingdom, for while the true leaf bears a strong resemblance to that of the willow, ash and plum, it varies, however, in size form and structure. The border of the true tea-leaf is more regularly serrated, the serration stopping just short of the stalk, and the venations are very characteristic, the veins running out from the mid-rib almost parallel to one another, but altering their course before the border of the leaf is reached, and turning so as to leave a bare space just within the border of the leaf. So that in making an examination of a sample of tea for the purpose of ascertaining whether these distinctive characteristics are present in the leaves under treatment, it will be found convenient to pour hot water on them so as to soften, uncurl and spread them out more easily, as otherwise considerable difficulty will be experienced owing to the brittleness of the tea-leaves in the dry state.
TRUE TEA-LEAVES.
CHINESE TEA-LEAF.
JAPANESE TEA-LEAF.
The leaf of China Green tea is much broader than that of Black in proportion to its length, but not so thick, and somewhat accuminate or curled at the apex, that of Black being elliptical, oblong and flat in shape, long and pointed, that of Green being much shorter and rounder in form.
INDIA TEA-LEAF.
CEYLON HYBRID TEA-LEAF.
JAVA TEA-LEAF.
SKELETON OF GENUINE TEA-LEAF.
But in order to better detect the presence of spurious leaves in tea a keener knowledge of the botanical formation of the true tea-leaf is requisite, for which purpose the use of a microscope will be found an invaluable aid. Tea-leaves in general construction bear a strong resemblance to those of the willow and many other plants of the kind, vary widely in size and form, being much smaller, more deeply serrated, and ending more regularly just short of the stalk. The venations are very characteristic, the veins running out from the middle rib, almost parallel with each other, altering their course before reaching the extremities, and turning so as to leave a bare space within the border. When infused and unfolded it is of a bright-green color, the loopings together of the principal veinings in the true leaf being very characteristic. While the spurious leaf is either of a greenish-yellow or reddish-brown color, and irregular in form under the same conditions or when deprived of its cosmetics.
To still better distinguish between them treat a sample of the suspected tea as in making an ordinary infusion to soften and expand the leaves, then separate and uncurl them and lay flat on a piece of glass or other smooth surface for comparison with the genuine leaf. Next see that they agree in description and formation, but more especially in the venations and serrations. Or soak the leaves in hot water, and carefully unroll and closely examine their formation and structure and then compare closely. The epidermis of the lower surface of the true leaf can be with a little caution detached in small portions with a sharp razor, and then analyze the frame or skeleton of the leaf in a little water or glycerine, under a microscope, comparing the venations and serrations with those of the genuine tea-leaf. Still another simple and inexpensive test is to boil a few of the suspected leaves for a minute or two on a watch glass, with a little distilled water, and add an equal portion of burnt magnesia, treating the whole until it is reduced to a large-sized drop. If no crystalline sublimate is obtained therefrom by the operation the leaves cannot be those of genuine tea.
Chemically an examination of the ash of tea-leaves also affords some criteria which may also be utilized for the purpose of identifying the true tea-leaf. For instance, in common kinds of wood, such as oak, deal and pine, the proportion of ash is a few-tenths per cent. of the whole, and by taking wood in its ordinary air-dried condition it contains some 20 to 30 per cent. of moisture. Leaves, on the other hand, contain 10, 20 and even 30 times as much mineral matter, there being doubtless a connection between this abundance of mineral matter and the active chemical changes which take place in the leaves during the growth of the plant. In tea-leaves, therefore, as in leaves in general, the ash amounts to a considerable percentage, usually averaging about 6 per cent., so that a chemical examination of the ash forms an important part of the analysis of tea. The proportion of ash in all teas is tolerably constant, genuine tea rarely yielding so little as 5 and never exceeding 6 per cent. of ash on incineration. Therefore, to determine the amount of ash, weigh a small quantity of the tea in its ordinary commercial state and burn it in a platinum crucible and then re-weigh the resultant ash; by observing a few simple precautions very constant and accurate results are attainable by this method. The crucible should be clean and bright, the lid fitting precisely; an iron triangle, covered with a tobacco pipe may be employed to support the crucible during ignition. The operation is then commenced by igniting the empty crucible, and allowing it to cool, being placed for that purpose on a piece of porcelain or iron, immediately weighing the crucible on becoming cold. Next place one or two grams of tea in the crucible, weighing both together, and ignite over a spirit lamp or good gas-burner, stirring the contents with a platinum wire for a few minutes. When ignition is complete the crucible is covered with the lid, let cool again and immediately weighed a second time. If there is any doubt as to the completeness of the ignition, the crucible must be again ignited, and if there is no difference in weight, the ignition is thereby proved finished. As is obvious, in order to obtain accurate results it is indispensable that the crucible should be in the same condition when it is weighed empty and when weighed with the ash, and this is insured by the preliminary ignition and rapid re-weighing, as above described.
The following determinations of the percentages of ash in spurious leaves most used in the adulteration of tea, dried after the manner of tea, may serve to illustrate, the leaves being gathered towards the end of August:—
To these may be appended the determinations of Paraguay tea at 28 and the ordinary tea of commerce at 5.92 per cent.; while in Peligot’s analysis the average proportions of ash in true tea-leaves is given as follows:—
| Kind of Leaf. | Per cent. of Ash. | |||
| China | Tea, | 5.5 | ||
| Japan | “ | 5.5 | ||
| Java | “ | 5.3 | ||
| India | “ | 6.06 | ||
| Ceylon | “ | 6.06 | ||
Proving, as has been mentioned, that genuine tea-leaves as brought direct from the producing countries, or such as is a fair commercial article, does not yield less than 5, or sensibly more than 6 per cent. of ash on incineration. When the ash much exceeds 6 per cent. the first question to be considered is whether it is accidental or if the high yield of ash would be maintained if a larger quantity of the sample were incinerated. The composition of the ash of genuine tea-leaves has also been carefully studied, yielding on analysis the following constituents:—
This analysis is especially important, inasmuch as the tea which furnished the ash was of guaranteed purity, so that no question of the possibility of its sophistication could arise. On examining this analysis it will be observed that tea-ash contains a quantity of iron, also some manganese, the presence of manganese being so marked in tea-ash that on subsequent treatment of the ash with water a deep-green solution of manganate is obtained. Owing to this presence of manganese in tea-ash it also invariably evolves chlorine very perceptibly when it is treated with hydrochloric acid.
If the analysis of the tea-ash is referred to it will be noted that more than one-half of it should be soluble in water, so that for all practical purposes a complete analysis is not requisite, a determination of the ratio of soluble to insoluble parts of the ash being sufficient. Such a determination is made by boiling the ash several times with a little water, filtering and washing the residue in the filter, drying the precipitate, igniting and weighing it. The weight of the insoluble part of the ash may then be subtracted from the weight of the entire ash, in which manner both the percentage of the soluble and insoluble ash will be conveniently arrived at, in which case the following determination of the percentage of “soluble” and “insoluble ash” in 100 parts of spurious leaves and pure tea when completely dried will be of interest here:—
| Kind of Leaf. | Soluble Ash. | Insoluble Ash. | |
| Ash, | 3.19 | 7.48 | |
| Plum, | 5.66 | 4.24 | |
| Willow, | 4.16 | 5.18 | |
| Teas of commerce, | 3.55 | 2.47 | |
Proving that in spurious leaves the ratio of soluble or insoluble ash is very different from what it is in genuine tea-leaves, and that an ash of such composition cannot be very soluble in water.
Peligot has also pointed out that tea leaves differ from other leaves by their extraordinary richness in nitrogen, the prepared leaf being by the process of curing rendered still more nitrogenous than the raw or spent leaves, the former averaging, according to his experiments, 6 per cent. of nitrogenous matter, and the latter 4.37, so that if the tea-leaf be unique in containing such a high percentage of nitrogen, it is obvious that a determination of nitrogen in tea may also be useful as a method of identification.
The tea-extract also yields a comparatively large quantity of ammonia when it is boiled with potash or permanganate of potash, and it is probable that this characteristic may also prove very valuable in the testing of tea. A solution containing about 10 per cent. of solid potash, free from ammonia and nitrogenous matter is required for the purpose, and easily obtained. So that a deficiency of theine, a deficiency of nitrogen, and a deficiency of ammonia are all indicative of the presence of foreign leaves in tea.
SPENT OR EXHAUSTED LEAVES IN TEA
Are principally used in the adulteration of Black teas, and is effected by adding or substituting leaves that have been at least once used, and from which all the vital properties have been extracted. The Chinese being inveterate tea-drinkers, large quantities of these leaves are always to be had for the purpose, they are re-dried and subjected to a treatment of gypsum or terra-japonica, in order to make them retain their curl, and then glazed or “faced” with a preparation composed of either graphite or silica to enhance their appearance in the hand; a decoction obtained from catechu or logwood being next added to impart a tea-like color to the liquor when infused. This vile compound is known to the Chinese as “Bastard tea,” and is rarely sold alone, being used principally for mixing or blending with pure teas.
The presence of spent or exhausted leaves in either Green or Black tea is best determined by estimating the amount of tannin contained in the liquor after infusion, and for which experiment various tests are in use. A large proportion of the tea-extract is found to consist of tannin (tannic acid), there being much more in Green than in Black tea, the larger portion of that originally existing in the latter being dissipated by the extra fermentation to which this variety is subjected in curing and firing. Green teas contain on an average about 15 per cent. of tannin, Black teas never exceeding 10 per cent. This rate, however, varies considerably from different causes, such as age, quality, soil and climatic condition of the districts of growth, the main average being 12 and 9 per cent. respectively in pure teas. Spent or exhausted leaves, on the other hand, contain only 2 per cent. on an average at the highest estimate, a difference of 7 to 10 per cent. of tannin, as will be observed, in favor of pure teas.
For the purpose of estimating the percentage of tannin contained in tea, the simplest method is to make an infusion of the leaves and pour it into a cup or glass and add to it a small quantity of a standard solution of plumbic acetate. The acetate will cause the tannin to form a precipitate, which must be removed and weighed in a small scale fitted for the purpose; then by taking the nominal percentage of tannin contained in pure teas at 12 in Green and 10 in Black and 2 per cent. in Spent or exhausted leaves, the difference will be the percentage of adulteration, the extent being indicated by the lessened proportion of tannin in the same ratio. The presence of Catechu (soluble salts of iron) is best ascertained by making an ordinary infusion of the leaves, allowing the liquor to cool and pouring it into a cup or glass. Next add a preparation of neutral plumbic acid and separate the precipitate formed by the introduction of the chemical by filtration, and adding a little argentic nitrate to the filtrate. If catechu be present the residue will turn a dark-brown, the liquid meanwhile acquiring a deep yellow hue, while under the same conditions the liquor of pure tea will remain unaffected. But if the cosmetic be extensively employed, a weak solution of ferric-chloride will cause the precipitate to turn light-green or it may be detected under the microscope if heavily coated.
Spent and spurious leaves may also be readily detected by the “ash test,” through the following deductions: Pure teas, as shown above, contain from 5 to 6 per cent. of ash on incineration, 2 per cent. of this being soluble in boiling water; any increase of these parts is a certain indication of the presence of foreign or exhausted leaves in the sample treated. The percentage of ash contained in spent leaves, ranging from 10 to 30, and in spurious leaves from 40 to 50, in many instances, while the residue of teas adulterated with mineral matter have been known to reach as high as 75 to 80 per cent. of the incinerated sample. The presence of logwood is best exposed by the addition of a few drops of sulphuric acid to an infusion made with the leaves in the ordinary way. If any of the dye be present this acid will cause the liquid to turn a deep red, but if the tea be uncolored the liquor will remain entirely unaffected. Graphite being visible to the naked eye is easily distinguished by its characteristic glossy nature, or can be separated by treating the leaves with boiling water in the usual manner and evaporating the infusion. The substance, if present, will form a deposit at the bottom of the vessel or will be found adhering to its sides if used in large quantities. While Silica is readily recognized by the increased amount of ash insoluble in the water obtained by calcimining a sample of the tea so adulterated, as above described.