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: