That preserved vegetables are made of a bright and attractive green colour by impregnation with copper, from the deliberate use of copper vessels for this purpose, is a fact long known. Green peas especially have been coloured in this way, and a number of convictions for this offence have taken place in England.
§ 800. The “Coppering” of Vegetables.—The fact that green vegetables, such as peas, beans, cucumbers, and so forth, preserve their green colour, if boiled in copper vessels, has long been known. In this “coppering” the French have been more active than the English traders; the French operate in two different ways. One method is, to dip from 60 to 70 litres of the green vegetables in 100 litres of 0·3 to 0·7 per cent. of copper sulphate, to leave them there for from five to fifteen minutes, then to remove them, wash and sterilise in an autoclave. A second method is to put the vegetables into a copper vessel, the wall of which is connected with the negative pole of an electric current, the positive pole dips in a solution of salt in the same vessel, the current is allowed to pass for three minutes, and the vegetables are afterwards sterilised. Fruits are simply allowed to stand with water in copper vessels, the natural acidity of the juice dissolving sufficient copper.
The amount of copper taken up in this way is appreciable, but yet not so much as might be expected; the prosecutions for selling “coppered” peas in England have been based upon quantities varying from 1 to 3 grains per lb.; the highest published amount of copper found in peas artificially coloured is 0·27 per kilo., or 18·9 grains per lb.
The reason why vegetables preserve their green colour longer when treated with a copper salt has been proved by Tschirch[869] to be owing to the formation of a phyllocyanate of copper.
[869] Das Kupfer, Stuttgart, 1893.
Phyllocyanic acid is a derivative of chlorophyll, and allied to it in composition; the formula of C24H28N2O4 has been ascribed to it. Under the action of acids generally, mineral or organic, chlorophyll splits up into this acid and other compounds. Copper phyllocyanate, (C24H27N2O4)2Cu, contains 8·55 per cent. of copper; it forms black lamellæ, dissolving easily in strong alcohol and chloroform, but insoluble in water; it is a little soluble in ether, insoluble in petroleum ether, and dissolved neither by dilute acetic acid, nor by dilute nor concentrated hydrochloric acid. The compound dissolves in caustic alkali on warming. In alcohol it forms a beautiful non-fluorescent solution. A solution of 1 : 100,000 is still coloured strongly green.
This solution, in a stratum of 25 mm. thick, gives four absorption bands when submitted to spectroscopic observation, and Tschirch has worked out a process of estimation of the amount of copper phyllocyanate based upon the disappearance of these bands on dilution.