This section is from the book "Beverages And Their Adulteration Origin, Composition, Manufacture, Natural, Artificial, Fermented, Distilled, Alkaloidal And Fruit Juices", by Harvey W. Wiley. Also available from Amazon: Beverages And Their Adulteration.
Teas are subjected to many forms of sophistication. Chief among these once were reckoned artificial coloring and facing, that is, the attachment of heavy bodies to the surface of the tea to increase weight. Facing and coloring may be practised together, since the facing material may be a coloring body, or the coloring itself be a heavy material to add weight. In general, facing is practiced by treating the prepared leaves with a mixture containing Prussian blue, turmeric or indigo. In general, it may be assumed that this coloring is intended to conceal damage or inferiority, but perhaps, in some cases, it is not. Very often, however, tea leaves which have been damaged in the manufacture, or which from their age or imperfection are found to be of an inferior grade, are faced solely to improve their appearance. The addition of heavy bodies to increase weight is no longer practised.
It may be stated, as a general rule, that in tea-growing countries the teas which are used for home consumption are not subjected to the treatment as described above. It is very common, however, in preparation of teas for export to subject them to this treatment.
Whatever may be said in extenuation of the coloring matters of tea on the ground that they are not injurious to health in the quantities used, it must not be forgotten that they do not in any case ever add anything to the value of the product.
Before facing to increase weight ceased, the addition of chemicals that cost only a few cents per pound to tea which is sold at prices ranging from 50 cents to $5 per pound, could not fail to be extremely remunerative to those who practise the process. The materials which were used cannot be wholly regarded as free from injurious effects; especially is this the case with Prussian blue, which is one of the most common of the materials used for facing. Sometimes very large quantities of these facing materials, such as Prussian blue, soapstone, and other substances, have been found, the quantities having been reported from 1 to 3 percent. This is very much above the average, which may be regarded as not exceeding one-half of 1 percent of the total weight of the treated tea.
Whether or not a tea has been faced can be determined by chemical and microscopical methods. The microscope is usually sufficient for the purpose. Following are methods of detecting ordinary facing materials:1
This substance is easily detected by means of the microscope. Shake the leaves in a glass cylinder with water and examine the detached particles with the microscope. If the coloring matter sought is present, transparent particles of a brilliant blue may be seen. Prussian blue may often be identified by the microscope on the leaf mounted as an opaque object. The particles detached as above may be examined chemically as follows: Treat with hot sodium hydroxid solution, acidulate with acetic acid, and add ferric chlorid. If Prussian blue is present in the facing the characteristic blue precipitate will be formed. The powdered tea leaf may be examined by the chemical method, but it is advisable to remove the tannin by precipitation with gelatin solution and filtration through powdered kaolin, after acidulating with acetic acid. The color of Prussian blue is discharged by sodium or potassium hydroxid.
1Spencer, Bulletin No. 13, Division Chemistry, Part 7, page 881.
Under the microscope indigo appears of a greenish blue. Its color is not discharged by sodium hydroxid. a distinction from Prussian blue. Indigo forms a deep blue solution with sulphuric acid.
Turmeric - Turmeric is identified by means of the microscope. According to Hassell turmeric consists of characteristic yellow cells of a rounded form which are filled with peculiar shaped starch granules. On the addition of an alkali the cells turn brown, swell up, and the outlines of the starch granules become visible.
The microscope is employed in the detection of plumbago. A thin slice of the tea leaf will exhibit numerous bright particles if plumbago facing has been used.
These substances, employed with the coloring matter in facing teas, may be separated by shaking the leaves in a cylinder with water. The sediment is examined by the usual qualitative methods for these substances.