COFFEE
Coffee is often colored with such substances as Scheele’s green, chrome yellow, iron oxide, Prussian blue, indigo and turmeric. Imitation coffee beans have been made of wheat flour, bran, rye, chicory and peas.
Allen’s Preliminary Test.—A good preliminary test for ground coffee is to sprinkle some of it on the surface of cold water. The oil of true coffee prevents the particles from being readily soaked, and so they float for some time. Chicory and most of the other adulterants of coffee contain no oil, but do contain caramel, which is quickly extracted by the water producing a zone of brown color about such particles. They become soaked and quickly sink. The liquid containing pure coffee diffuses uniformly without coloring the water to any perceptible degree. Chicory and similar roots give a dark brown, turbid infusion. Roasted cereals do not impart so distinct a color to water.
Coloring Matter
Shake the coffee beans in cold water and make the regular qualitative tests for the inorganic coloring matters—Scheele’s green may be identified by testing for copper and arsenic; chrome yellow, by testing for lead chromate; iron oxide may be detected by its characteristic tests.
Organic coloring matter is best extracted with alcohol. Prussian blue may be detected by dissolving it from the sediment with hot caustic alkali, acidifying with hydrochloric acid, treating it with a drop of ferric chlorid. If present, ferric ferrocyanide, a blue precipitate, will be formed. Indigo is not discharged by sodium hydroxid, while Prussian-blue is. It will form a deep blue solution with sulfuric acid.
[Test for turmeric] as under mustard.
Imitation Coffee Beans
Most imitation coffee, as already stated, is heavier than water. Coffee contains no starch, so the imitation beans made of cereals may be detected by testing for starch.