XI. COLORING MATTERS IN FOODS.— TENTATIVE. 



(An italicized number, following immediately the name of a dye, is the 



number by which that dye is designated in "A Systematic Survey of 



the Organic Colouring Matters", 1904, by Arthur Green, 



based on the German of Schultz and Julius.) 



1 PIGMENTS. 



The insoluble pigments, ultramarine, lampblack, etc., are most commonly used 

 as facings and may be separated by washing the sample with water and allowing 

 the washings to settle. The particles of coloring matter can be identified by micro- 

 scopic examination and by treatment of the residue or purified coloring matter 

 with chemical reagents. A large proportion of the common pigments other than 

 lakes, such as the yellow, brown and red ochres and umbers, are derivatives of the 

 heavy metals and contain iron, manganese, etc. Others, such as various green and 

 blue compounds, including the green chlorophyll derivatives, contain copper. 

 These pigments may be identified by the usual tests for the respective metals. The 

 analytical properties of the insoluble coloring matters are described in various 

 standard works, some of which are listed in the bibliography, especially "Farb- 

 stofftabellen by Schultz'." 



Soluble Coloring Matters and Their Lakes, 

 coal tar dyes. 



2 Wool dyeing test^. 



(a) WiJies, fruit juices, distilled liquors, flavoring extracts, vinegars, beers, sirups, 

 non-alcoholic beverages and similar products. — Dilute 20-200 cc. of the sample with 

 1-3 volumes of water and boil or heat on the steam bath with a small piece of white 

 woolen cloth (nun's veiling). When the mixture contains much alcohol, heat until 

 most of the alcohol has been removed; in other cases, take out the wool after 

 5-15 minutes and rinse with water. Then treat the liquid with 3 or 4 drops of con- 

 centrated hydrochloric acid for each 100 cc. and warm again for 10-20 minutes with 

 a clean piece of wool. The basic dyes go on the fiber best from neutral or faintly 

 ammoniacal solutions and, if present, will appear on the first piece of wool. Acid 

 colors dye from neutral solutions but more readily from those containing free acid. 

 If the wool takes up any considerable amount of coloring matter in either case, the 

 presence of coal tar dyes is indicated. The lichen colors' (Archil, Cudbear, Litmus) 

 go readily on wool, however, and many other natural colors, such as Turmeric, will 

 dye the fiber, if present in considerable amount. On the other hand, a few coal 

 tar dyes, especially Auramine O and Naphthol Green B, are quite unstable and, if 

 present in small amounts, may give no distinct dyeing. Acid dyes are much more 

 frequently used than basic dyes and in most cases may be removed from wool with- 

 out much decomposition by "stripping" the latter with dilute ammonia^ By the 

 action of the alkali, many natural colors are destroyed, while others remain for the 

 most part on the fiber. If the behavior with wool in neutral and acid solutions 

 indicates the presence of acid dyes, rinse the colored cloth thoroughly with water, 

 cover with 2% ammonium hydroxid solution in a casserole, boil for a few minutes, 

 remove the cloth and squeeze out the adhering liquid. Boil the ammoniacal solu- 



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