Tea and Its Adulteration
Among alimentary substances probably no article is subjected to more adulteration than tea. The sophistications practised may be conveniently divided into three classes:
1. Additions made for the purpose of giving increased bulk and weight, which include foreign leaves and exhausted tea-leaves, and also certain mineral substances, such as metallic iron, sand, brick-dust, etc.
2. Substances added in order to produce an artificial appearance of strength in the tea decoction, catechu, or other bodies rich in tannin, and iron salts being chiefly resorted to for this purpose.
3. The imparting of a bright and shining appearance to the tea by means of various coloring mixtures or “facings,” which adulteration, while sometimes practised upon black tea, is much more common with the green variety. This sophistication involves the use of steatite (soap-stone), sulphate of lime, China clay, Prussian blue, indigo, turmeric, and graphite; chromate of lead and copper salts being but very rarely employed. The compound most frequently used consists of a mixture of soap-stone (or gypsum) with Prussian blue, to which a little turmeric is sometimes added.
Genuine tea is the prepared leaf of Thea sinensis. It contains: moisture, 6% to 10%; theine, 0.4% to 4.0%; tannin, (green) 20%, (black) 10%; ash, 5% to 6%; soluble extractive matters, 32% to 50%; and insoluble leaf, 47% to 54%.
The presence of foreign leaves, and, in some instances, of mineral adulterants, in tea is best detected by means of a microscopic examination of the suspected sample. The genuine tea-leaf is characterized by its peculiar serrations and venations. Its border exhibits serrations which stop a little short of the stalk, while the venations extend from the central rib, nearly parallel to one another, but turn just before reaching the border of the leaf (see Fig. 16). The Chinese are said to employ ash, plum, camellia, velonia, and dog-rose leaves for admixture with tea, and the product is stated to be often subjected in England to the addition of the leaves of willow, sloe, beech, hawthorn, elm, box-poplar, horse-chestnut, and fancy oak (see Figs. 17, 18, and 19). For scenting purposes chulan flowers, rose, jasmine, and orange leaves are frequently employed. In the microscopic examination the sample should be moistened with hot water, spread out upon a glass plate, and then submitted to a careful inspection, especial attention being given to the general outline of the leaf and its serrations and venations. Most foreign leaves will, in this way, be identified by their botanical character. The presence of exhausted tea-leaves may also often be detected by their soft and disintegrated appearance. If a considerable quantity of the tea be placed in a long glass cylinder and agitated with water, the coloring and other abnormal bodies present frequently become detached, and either rise to the surface of the liquid as a sort of scum or fall to the bottom as a deposit. In this way Prussian blue, indigo, soap-stone, gypsum, sand, and turmeric can sometimes be separated and subsequently recognized by their characteristic microscopic appearance. The separated substances should also be chemically tested. Prussian blue is detected by heating with a solution of caustic soda, filtering, and acidulating the filtrate with acid, and then adding chloride of iron, when, in its presence, a blue color will be produced. Indigo is best discovered by its appearance under the microscope; it is not decolorized by caustic alkali, but it dissolves in sulphuric acid to a blue liquid. Soap-stone, gypsum, sand, metallic iron, etc., are identified by means of the usual chemical tests. A compound, very aptly termed “Lie-tea,” is often met with. It forms little pellets consisting of tea-dust mixed with foreign leaves, sand, etc., and held together by means of gum or starch. This, when treated with boiling water, falls to powder. In the presence of catechu the tea infusion usually becomes muddy upon cooling; in case iron salts have been employed to deepen the color of the liquor, they can be detected by treating the ground tea-leaves with acetic acid and testing the solution with ferrocyanide of potassium. Tea should not turn black upon immersion in hydrosulphuric acid water, nor should it impart a blue color to ammonia solution. The infusion should be amber-colored, and not become reddened by the addition of an acid.
TEA ASSAY.
In the following tea assay proper the estimation of theine is not included. The processes suggested for this determination are rather unsatisfactory; and there appears, moreover, to exist no direct relation between the quality of tea and the proportion of theine contained. The tests here mentioned, in connection with those already given, will, it is believed, usually suffice to indicate to the analyst the presence of spent leaves, inorganic coloring matters, and other mineral adulterations.
Tannin.—A good process for the estimation of tannin in tea has been published by Allen (Chem. News, vol. xxix. p. 169 et seq.) A standard solution of lead acetate is prepared by dissolving 5 grammes of the salt in distilled water and diluting the liquid to 1,000 c.c. As an indicator, 5 milligrammes of potassic ferricyanide are dissolved in 5 c.c. of water, and an equal volume of strong ammonia-water added. The exact strength of the lead solution is to be determined by means of a solution of pure tannin of known strength. Two grammes of the tea to be tested are powdered, boiled with water, and, after filtering and thorough washing, the decoction is made up to a volume of 250 c.c.; 10 c.c. of the lead solution are now diluted with 90 c.c. of boiling water, and the tea infusion is gradually added from a burette until a few drops of the liquid, when filtered and added to a little of the indicator placed upon a porcelain slab, causes a pink coloration to appear; 125, divided by the number of c.c. of tea infusion found to be necessary to produce the pink color, will give directly the percentage of tannin in the sample examined. As previously stated, green tea contains 20% of tannin, and black tea 10%. In spent tea, however, only about 2% of tannin is present; and, although any tea deficient in this constituent could be fortified by the addition of catechu, its determination often affords indications of value.
The Ash—a. Total Ash.—5 grammes of the sample are placed in a platinum vessel and heated over a Bunsen burner until complete incineration has been accomplished. The vessel is allowed to cool in a desiccator, and is then weighed as quickly as possible. In genuine tea the total ash should not be much below 5% or much above 6%, and it should not be magnetic; in “faced” teas the proportion of total ash is often 10% or 15%; in “lie-tea” it may reach 30%, and in spent leaves it may fall as low as 3%, the ash in this case being abnormally rich in lime salts and poor in potash salts. Tea-dust sometimes contains 10% of total ash without necessarily being considered bad in quality. In the proposed United States tea-adulteration law (1884) a maximum of 8% of total ash is allowed for tea-leaf.
b. Ash insoluble in water.—The total ash obtained in a is washed into a beaker and boiled with water for a considerable time. It is then brought upon a filter and the insoluble residue washed, dried, ignited, and weighed. In unadulterated tea it will not exceed 3% of the sample taken.
c. Ash soluble in water.—This proportion is obtained by deducting ash insoluble in water from the total ash. Genuine tea contains from 3% to 3.5% of soluble ash, or at least 50% of the total ash, whereas in spent or exhausted tea the amount is often but 0.5%.
d. Ash insoluble in acid.—The ash insoluble in water is boiled with dilute hydrochloric acid and the residue separated by filtration, washed, ignited, and weighed. In pure tea the remaining ash ranges between 0.3% and 0.8%; in “faced” teas, or in teas adulterated by the addition of sand, etc., it may reach the proportion of 2% to 5%. Fragments of silica and brick-dust are occasionally to be found in the ash insoluble in acid.
The Extract.—Two grammes of the carefully-sampled tea are boiled with water until all soluble matter is dissolved, water being added from time to time to prevent the solution becoming too concentrated. The solution is poured upon a tared filter, and the remaining insoluble leaf repeatedly washed with hot water until the filtered liquid becomes colorless. The filtrate is now diluted to a volume of 200 c.c., and of this 50 c.c. are taken and evaporated in a weighed dish over the steam-bath until the weight of the extract remains constant; its weight is then determined. Genuine tea affords from 32% to 50% of extract, according to its age and quality; in spent tea the proportion of extract will be greatly reduced.
Insoluble Leaf.—The insoluble leaf obtained in the preceding operation, together with the weighed filter, is placed in an air-bath and dried for at least eight hours at a temperature of 110° C.; its weight is then determined. In unadulterated tea the amount of insoluble leaf ranges between 47% and 54%; in exhausted tea it may reach a proportion of 75%.
It should be noted that in the foregoing estimations the tea is taken in its ordinary air-dried condition. If it be desired to reduce the results obtained to a dry basis, an allowance for the moisture present in the sample (an average of 8%), or a direct determination of the same, must be made.
The following tabulation gives the constituents of genuine tea so far as the ash, extract, and insoluble leaf are involved:
Total ash—ranges between 4.7% and 6.2%.
Ash soluble in water—ranges between 3% and 3.5%; should equal 50% of total ash.
Ash insoluble in water—not over 2.75%.
Ash insoluble in acid—ranges between 0.3% and 0.8%.
Extract—ranges between 32% and 48%.
Insoluble leaf—ranges between 43% and 58%.
The table below may prove useful as indicating the requirements to be exacted when the chemist is asked to give an opinion concerning the presence of facing admixtures or of exhausted or foreign leaves in a sample of tea:
Total ash—should not be under 4.5% or over 7%.
Ash soluble in water—should not be under 40% of total ash.
Ash insoluble in water—should not be over 3%.
Ash insoluble in acid—should not be over 1%.
Extract—should not be under 30%.
Insoluble leaf—should not be over 60%.
Note.—The British Society of Public Analysts adopt:
Total Ash (dry basis)—not over 8% (at least 3% should be soluble in water).
Extract (tea as sold)—not under 30%.