COLORING MATTER. Ill 



XVII. COLORING MATTER. 



By L. M. TOLMAN, 

 Bureau of Chemistry, U. S. Department of Agriculture. 



1. GENERAL DISCUSSION. 



The food chemist has two problems in connection with coloring matter the analy- 

 sis of dyes used for food colors and the detection and identification of the color used 

 in a food. The first will require an estimation of the hea\ 7 y metals present and a 

 determination of the general group to which the color belongs. The second will 

 require the detection of the presence of the color, the determination of the group to 

 which the color belongs, and the presence or absence of poisonous metals. 



The complete examination of dyes is too large a subject to take up in these methods, 

 and one will have to refer to such works as Schultx and Julius, on Organic Coloring, 

 Allen's Commercial Organic Analysis, and others that go into the subject in an 

 exhaustive manner. The determination of the general nature of the dye can be made 

 by the use of Rota's scheme, which is the simplest of the many different methods 

 proposed and is quite satisfactory, although it requires a great deal of care and experi- 

 ence. 



The detection of the color in a food product and its identification are more difficult. 

 It must be separated in a somewhat pure condition and then tested. Almost all the 

 methods for separating added color from the food will take up some of the natural 

 color of the food as well. 



As will be seen in tables for the extraction of fruit colors (p. 113), amyl alcohol 

 extracts the coloring matter from many fruits, and these extracts may easily be mis- 

 taken for added colors. 



Some of the highly colored fruit juices will dye wool, and the color will be perma- 

 nent; but these will not be mistaken lor coal-tar dyes if the double-dyeing method 

 is followed. 



In the methods of manufacture of coal-tar dyes many become contaminated with 

 poisonous metals, such as arsenic, copper, xinc, tin, and lead. There i> always the 

 possibility of the presence of arsenic, as sulphuric acid is used at one stage or another 

 in the preparation of nearly every dye. 



Some colors have metallic atoms in their molecule, such as malachite green, which 

 is a double chlorid of /inc in combination with the organic -roup. 



Many vegetable colors are sold as lakes of tin or alum. Other colors are known 

 to have a toxic action, such as picric acid and naphthol yellow. 



Mixtures of two or more dyes are often added to foods. This can sometimes be 

 shown by a system of fractional dyeing, when- the dyes are taken up at different 

 rates by the fabric. In examining mixtures of red, orange, and blue dyes, which are 

 widely sold for coloring wine, the writer found that the woolen cloth took up the 

 red much faster than the orange, and the blue slowest; so that the first piece of 

 cloth dyed was red; the second, a lighter shade; the third, greenish, and the fourth, 

 Muish. 



2. DKTKH.M i NATION OK HKAVY MKTALS. 



Directions for this determination are given under Vegetables ( p. .V_M. 



3. DKTKHM i NATION OF COAL-TAR COLOKIM; MATTKKS in DYKINC, WOOL. 



(a) MKTIIOl) OF SOSTKONI A N I < A KI'KNTI ERI. !l 



From 10 to L'O grams of the sample are dissolved in 100 ec of water, filtered if neces- 

 sary, acidified with from 2 to 4 cc of 10 per cent solution of hydrochloric acid, and 

 a piece of woolen cloth, which has been washed in a very dilute solution of boiling 



Ztsch. until, riirm.. isw. :&;,. :;MT ; I. s. Dcpt. of Agr., Div. of Chem., 13ul. 4(i revised, p. 68, 



