4. A slight excess of ammonium hydroxide is added to 50 c.c. of the wine, a piece of white woollen fabric introduced, and the liquid boiled until the alcohol and ammonia are expelled. By this treatment it will be found that most aniline colouring matters, if present, become attached to the wool. Their presence can be corroborated by removing the fabric, washing and pressing it, and then dissolving it, with constant stirring, in a hot solution of potassium hydroxide. When solution has taken place, the liquid is allowed to cool, and one-half its volume of alcohol is added, then an equal volume of ether. The mixture is vigorously shaken, and, after remaining at rest for some time, the supernatant ethereal solution is removed, introduced into a test-tube, and a drop or two of acetic acid added. In presence of fuchsine, its characteristic colour will now become apparent. Methyl violet and aniline blue are separated by an analogous process.
5. Logwood and cochineal may be detected by agitating 100 c.c. of the suspected wine with manganic peroxide, and filtering. The filtrate afforded by pure wine will be colourless.
6. In Dupré’s process,[105] cubes of jelly are first prepared by dissolving 1 part of gelatine in 20 parts of hot water, and pouring the solution into moulds to set. These are immersed in the wine under examination for 24 hours, then removed, slightly washed, and the depth to which the colouring matter has permeated is observed: pure wine will colour the gelatine very superficially; the majority of other colouring principles (e.g. fuchsine, cochineal, logwood, Brazil wood, litmus, beetroot, and indigo) penetrate the jelly more readily and to a far greater degree. Dilute ammonium hydroxide dissolves from the stained cake the colouring matter of logwood and cochineal, but not that derived from fuchsine or beetroot.
7. The colouring principle of genuine wine when subjected to dialysis, does not pass through the animal membrane to any decided extent, while that of logwood, cochineal, and Brazil wood easily dialyses.
8. Many of the foreign dyes added to wine are precipitated by a solution of basic plumbic acetate. The precipitate obtained upon treating 10 c.c. of the sample with 3 c.c. of this reagent is collected on a filter and washed with a 2 per cent. solution of potassium carbonate, which dissolves cochineal, sulphindigotic acid and aniline red. The latter is separated upon neutralising the solution with acetic acid, and shaking with amylic alcohol, which, in its presence, will acquire a rose colour. The liquid is next acidulated with sulphuric acid, and again agitated with amylic alcohol, by which the carminamic acid, originating from cochineal, is isolated. Any remaining indigo (as well as the carminamic acid) is to be subsequently identified by means of its spectroscopic reactions. Upon treating the portion of the plumbic acetate precipitate which remains undissolved by potassium carbonate with a dilute solution of ammonium sulphide, the colouring matter of pure wine and of logwood is dissolved. If, in presence of logwood, the original sample is shaken with calcium carbonate mixed with a little calcium hydroxide solution and filtered, the filtrate will exhibit a decided red tint, but, if the wine treated be pure, little or no coloration will be produced.
9. An artificial colouring for wine, known as rouge végétale, is not uncommonly employed. According to Amthor,[106] its presence can be recognised as follows:—100 c.c. of the wine are distilled until all alcohol is removed. The residual liquid is strongly acidulated with sulphuric acid, and agitated with ether. Some woollen yarn is next introduced into the ethereal solution, which is then evaporated over the water-bath. In presence of rouge végétale, the wool will acquire a brick-red colour, which turns violet upon treatment with ammonium hydroxide.
10. Cauzeneuve and Lepine[107] state that acid aniline red, “naphthol-yellow S,” and roccelline red are harmless, whereas safranine and ordinary Martius’ yellow are decidedly poisonous.
The presence of “Bordeaux red”[108] is recognised by first adding sodium sulphate to the suspected wine, then a solution of barium chloride: the artificial dye is carried down with the precipitated barium sulphate, from which it can be extracted by means of sodium carbonate solution. The brownish-red liquid thus obtained acquires a deep red colour if acidulated with acetic acid, which it readily communicates to silk upon boiling. Natural red wine fails to produce a coloration under the same circumstances.
For the detection of the presence of artificial colouring matter the following process is used in the Municipal Laboratory in Paris:—Preliminary tests are made—
1st. By soaking pieces of chalk in an aqueous solution of egg-albumen; these are dried and applied for use by dropping a little of the wine upon them, and noting the coloration produced. Natural coloured wine usually causes a greyish stain, which, in highly coloured varieties, may verge to blue.