62 MILK ANALYSIS 



to a separator, the casein, water, and salt removed, and the re- 

 mainder washed three times, at least, with ether to remove the 

 fat. The curd is collected on a filter, washed with water, and 

 the nitrogen determined by treating the precipitate with the 

 filter by the Kjeldahl- Gunning method. The filtrate from the 

 curd is made slightly acid with acetic acid, boiled, the pre- 

 cipitated proteids collected on a filter, and the total nitrogen 

 determined. The factor 6.38 may be used in each case for 

 converting the nitrogen into proteids. 



A distinction between ordinary and process butter may 

 often be made by microscopic examination 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. Preparations of turmeric and annatto or 

 azo-colors allied to methyl-orange are used. The latter forms 

 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, while if they are not present, the mix- 

 ture 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 cubic centimeters of the filtered fat are mixed in a large 

 test-tube with an equal volume of a mixture of one part strong 

 sulfuric acid and four parts glacial acetic acid. The contents 

 of the tube are then heated almost to boiling and thoroughly 



