92 MILK PRODUCTS 



amination under polarized light with crossed 

 nicols {i. e., dark field), when the process butter 

 appears mottled, owing to the presence of 

 crystals. 



Butter Colors. — Butter and butter-substitutes 

 are usually artificially colored. Turmeric and 

 annatto or azo-colors allied to methyl-orange are 

 used. 



Azo-colors. — These may be detected by the 

 test devised by Geisler. A small amount of the 

 sample, or, better, the fat filtered from it, is mixed 

 on a porcelain plate with a little fullers' earth. 

 Azo-colors give promptly a red mass; if they are 

 not present, the mixture becomes only yellow or 

 light brown. All samples of fullers' earth are 

 not equally active, and tests should be made with 

 different samples by using fat known to contain 

 the azo-compound until a good specimen of the 

 earth is secured. 



For the detection of very minute quantities of 

 the color, the sample may be dissolved in light 

 petroleum, and the fullers' earth added to the 

 solution, when the pink color will appear as a 

 distinct ring or zone at the edge of the deposited 

 layer of the reagent. 



Low has proposed the following test for the 

 yellow azo-color: A few c.c. of the filtered 

 fat are mixed in a large test-tube with an 

 equal volume of a mixture of one part strong 



