GREENING VEGETABLES WITH COPPER SALTS. 1043 



and in minute quantities, is also probable. Green vegetables in the 

 presence of minimum amounts of copper compounds do not turn yel- 

 low in cooking, but preserve a fresh, green color. This phenomenon is 

 well known to all canners and has been in use since the beginning of 

 the art. For greening pickles, the use of copper kettles has been known 

 for centuries. The cause of the phenomenon has never been satisfac- 

 torily elucidated, though Tschirch has proposed an explanation. 1 This 

 is that in the process of cooking the chlorophyl is converted by the 

 weak vegetable acids into two bodies, one of a basic nature and one of 

 an acid phyllocyanic acid. By this change the vegetables lose their 

 original color and become brown. If, however, a salt of copper is pres- 

 ent this combines with the phyllocyanic acid, forming a body of intense 

 tinctorial power, being comparable in this respect with eosin. Solu- 

 tions of one part in 200,000 show a blue color. The salt is soluble in 

 alcohol, but not in water or dilute acids. It contains 9.2 per cent of 

 copper oxid. Tschirch, from physiological investigations, was of the 

 opinion that the compound is not harmful and that the presence in 

 canned goods of 100 mg of copper oxid in the form of the alcohol- 

 soluble salt should be made legally allowable. 



Since the earliest development of the art of canning, it has been a 

 maxim in the canneries that in order to secure the best results green 

 peas should always be cooked in copper kettles and that the kettles 

 should not be too clean. Vegetable juices, even when only faintly acid, 

 exercise a remarkable solvent action on copper oxid, and, in turn, in their 

 presence copper oxidizes readily ; so that when cooking is done in copper 

 kettles and too much cleanliness is not exercised, vegetables readily 

 absorb enough copper to produce the desired efl'ect. The amount nec- 

 essary is very small, only 20 to 30 mg per kilo, or, in other words, 20 

 to 30 parts per million, of the vegetable. Lately, that is, within a few 

 years, the preservers have learned that copper kettles are not neces- 

 sary, but that the desired effect can be as well produced by the direct 

 addition of copper sulphate or acetate, and this is now the usual prac- 

 tice. In 1890 an Alsatian firm ( J. Clot & Cie) patented a method of 

 greening peas by which the copper vessel containing them was placed in 

 connection with one wire from a dynamo, while the other wire was con- 

 nected with an electrode hanging in the fluid surrounding the food. 

 The current passing through the mixture dissolved enough copper to 

 color the peas. A translation of this patent is given on page 1162. 



For many years there has been a dispute as to whether copper might 

 be called a normal constituent of food. Many cases of its occurrence 

 in the animal kingdom are known, and it is of course pretty certain, in 

 view of this fact, that it must be a constituent of some vegetable tis- 

 sues. One noteworthy occurrence of copper in the higher animals was 

 discovered by Prof. A. H. Church in 1869. 2 The wing feathers of a 



1 Ztschr. Nahr. Hyg., 1891, 5, 221. 



2 Chem. News, Amer. Rep. 1869, 5, 61 ; Chem. News, 1892, 65, 218. 



