XI] COLORING MATTERS IN FOODS 163 



Erythrosine become paler or colorless with acids so that the effects of acid reducing 

 agents are not so readily apparent. Neutral solutions of Naphthol Yellow S are 

 decolorized by sodium hyposulphite and other reducing agents, the color not return- 

 ing with air or oxidants. An evanescent deepening of the shade may take place 

 immediately upon the addition of the hyposulphite. Erythrosine and Light Green 

 S F Yellowish become paler with sodium hyposulphite, the color being partially 

 restored upon the addition of potassium persulphate. 



In hot solutions containing an excess of sodium tartrate the dyes named 

 are readily decolorized by titanium trichlorid". In the case of Indigo Carmine if 

 the reducing agent has been added carefully and an excess avoided, the blue color 

 readily returns on shaking with air. With Erythrosine and Light Green S F Yel- 

 lowish the color is scarcely restored by air but on cooling and adding potassium 

 persulphate returns imperfectly. The reduction products of the other dyes do 

 not give colored solutions again on oxidation disregarding a slight yellowish or 

 brownish tint that may sometimes appear. 



Indigo Carmine is extracted in small proportions from slightly acid solutions by 

 shaking with dichlorhydrin. Most of the other common bluish dyes are triphenyl- 

 methane derivatives and are relatively more soluble in this liquid than in the aque- 

 ous layer. A small portion (1 cc.) of the solution obtained in the separation, 5, 

 may be used directly. 



Ponceau SR gives in neutral or faintly acid solutions a bluish red, flocculent pre- 

 cipitate with barium chlorid or acetate, practically all of the dye being removed from 

 solution. Some of the solution obtained in the separation, 5, may be used in this 

 test, first neutralizing the free hydrochloric acid with sodium acetate; or better, it 

 may be evaporated to dryness on the steam bath to remove the acid and the residue 

 taken up with a little water. The solution should contain 0.005% or more of the 

 dye. 



Naphthol Yellotv S, in solutions containing an excess of ammonia or sodium car- 

 bonate, becomes intensely rose-red on the addition of sodium hyposulphite, the color 

 gradually fading again as complete reduction takes place. 



Erythrosine differs from most of the common dyes by containing iodin. To test 

 for this, acidify the solution with sulphuric acid, shake with ether, separate the 

 ether solution of the color and evaporate to dryness in a platinum dish after the 

 addition of a few drops of sodium carbonate solution or sufficient to form the deep 

 red sodium salt. Hold the dish containing the residue in the Bunsen flame until 

 organic matter is destroyed, take up the residue with water, acidify with sulphuric 

 acid and test for iodin in one of the usual ways, such as with chlorin water and carbon 

 disulphid or tetrachlorid, or with starch paste and an oxidizing agent. It is useless 

 to test for iodin with very small amounts of dye but in most cases sufficient coloring 

 matter can be separated from the food product to give satisfactory results. 



16 Natural Coloring Matters. 



The natural coloring matters as a class show much less tendency to dye animal 

 fiber than do the common synthetic colors. In many cases the crude products used 

 contain a number of colored substances and a complete separation can scarcelj' be 

 attempted. Most of the natural coloring matters, in dilute solution, are sensitive 

 to alkalies, some to acids, hence such reagents must be used with care. 



