292 METHODS OF ANALYSIS [Chap. 



To test for formaldehyde proceed as directed under X, 17, 18, 19, 20, 21, 22, 



or 



23, applying the test directly to the milk. 



COLORING MATTERS. 



21 Leach Method. — Tentative. 



Warm about 150 cc. of milk in a casserole over a flame and add about 5 cc. of 

 acetic acid, then slowly continue the heating nearly to the boiling point while stir- 

 ring. Gather the curd, when possible, into one mass with a stirring rod and pour 

 off the whey. If the curd breaks up into small flecks, separate from the whey by 

 straining through a sieve or colander. Press the curd free from adhering liquid, 

 transfer to a small flask and macerate for several hours, preferably overnight, in 

 about 50 cc. of ether, the flask being tightly corked and shaken at intervals. De- 

 cant the ether extract into an evaporating dish, remove the ether by evaporation 

 and test the fatty residue for annatto as directed in XI, 24. 



The curd of an uncoloredmilk is perfectly white after complete extraction with 

 ether, as is also that of a milk colored with annatto. If the extracted fat-free curd 

 is distinctly colored an orange or yellowish color, a coal tar dye is indicated. In 

 many cases upon treating a lump of a fat-free curd in a test tube with a little strong 

 hydrochloric acid the color changes to pink, indicating the presence of a dye simi- 

 lar to aniline yellow or butter yellow or perhaps one of the acid azo yellows or 

 oranges. In such cases, separate and identify the coloring matter present in the 

 curd as directed under XI. If aniline yellow, butter yellow, or other oil-soluble 

 •dye is present, the greater part will be found in the ether extract containing the 

 fat. In such cases proceed as directed under XI, 3. 



In some cases the presence of coal tar dyes can be detected by treating about 

 10 cc. of the milk directly with an equal volume of hydrochloric acid (sp. gr. 1.20) 

 in a porcelain casserole, giving the dish a slight rotary motion. In the presence of 

 some dyes the separated curd acquires a pink coloration. 



CREAM. 



22 SOLIDS.-OFFICIAL. 



Proceed as directed in 1 , employing 2-3 grams of the sample. 



23 ASH.— OFFICIAL. 

 Proceed as directed under 2. 



24 TOTAL NITROGEN.-OFFICIAL. 

 Proceed as directed under 3. 



LACTOSE. 



25 Gravimetric Method. — Official. 



Proceed as directed under 1 1 . 



FAT. 



26 Extraction Method. — Official. 



Weigh 4-5 grams of the homogeneous sample into a Rohrig tube or similar appa- 

 ratus, dilute with water to about 10.5 cc. and proceed as directed under 12. 



27 Babcock Method. — Official. 



Weigh 9 or 18 grams, depending upon the fat content of the sample, into a stand- 

 ard Babcock cream bottle and proceed as directed under 15. 



