418 CITY MILK SUPPLY 



81. It shall be determined by the Kjeldahl method, using the Gunning or 

 some other reliable modification, and employing the factor 6.25 in reckoning the 

 protein from the nitrogen. 



Kjeldahl Method. Five cubic centimeters of milk are measured carefully into a 

 flat-bottom 800-c.c. Jena flask, 20 c.c. of concentrated sulphuric acid (C. P.; sp. gr., 

 1.84) are added, and 0.7 gram of mercuric oxid (or its equivalent in metallic mercury); 

 the mixture is then heated over direct flame until it is straw-colored or perfectly 

 white; a few crystals of potassium permanganate are now added till the color of the 

 liquid remains green. All the nitrogen in the milk has then been converted into 

 the form of ammonium sulphate. After cooling, 200 c.c. of ammonia-free distilled 

 water are added, 20 c.c. of a solution of potassium sulphide (containing 40 grams 

 sulphide per liter), and a fraction of a gram of powdered zinc. A quantity of semi- 

 normal HC1 solution more than sufficient to neutralize the ammonia obtained in 

 the oxidation of the milk is now carefully measured out from a delicate burette 

 (divided into Ko c.c.) into an Erlenmeyer flask and the flask connected with a dis- 

 tillation apparatus. At the other end the Jena flask containing the watery solution 

 of the ammonium sulphate is connected, after adding 50 c.c. of a concentrated soda 

 solution (1 Ib. "pure potash" dissolved in 500 c.c. of distilled water and allowed 

 to settle) ; the contents of the Jena flask are now heated to boiling, and the distillation 

 is continued for 40 min. to an hour, until all ammonia has been distilled over. 



The excess of acid in the Erlenmeyer receiving flask is then accurately titrated 

 back by means of a tenth-normal standard ammonia solution, using a cochineal solu- 

 tion as an indicator. From the amount of acid used the per cent, of nitrogen is ob- 

 tained; and from it the per cent, of casein and albumen in the milk by multiplying 

 by 6.25. The amount of nitrogen contained in the chemicals used is determined by 

 blank experiments and deducted from the nitrogen obtained as described. (Farring- 

 ton and Woll, "Testing Milk and Its Products," p. 221.) 



82. Coloring Matter and Preservatives. All certified milks and creams shall 

 be free from adulteration, and coloring matter and preservatives shall not be 

 added thereto. 



83. Tests for the detection of added coloring matter shall be applied when- 

 ever the color of the milk or cream is such as to arouse suspicion. 



Test for Coloring Matter. The presence of foreign coloring matter in milk is easily 

 shown by shaking 10 c.c. of the milk with an equal quantity of ether; on standing, 

 a clear ether solution will rise to the surface; if artificial coloring matter has been added 

 to the milk, the solution will be yellow colored, the intensity of the color indicating 

 the quantity added; natural fresh milk will give a colorless ether solution. ("Testing 

 Milk and Its Products," Farrington and Woll, p. 244.) 



84. Tests for the detection of formaldehyde, borax, and boracic acid shall 

 be applied at least once in each month. Occasionally application of tests for 

 the detection of salicylic acid, benzoic acid, and the benzoates are also 

 recommended. 



Test for the Detection of Formaldehyde. Five cubic centimeters of milk is measured 

 into a white porcelain dish, and a similar quantity of water added; 10 c.c. of HC1, 

 containing a trace of Fe 2 Cl 6 is added, and the mixture is heated very slowly. If 

 formaldehyde is present, a violet color will be formed. ("Testing Milk and Its 

 Products," Farrington and Woll, p. 249.) 



Test for Boracic Acid (Borax, Borates, Preservaline, etc.). One hundred cubic centi- 

 meters of milk are made alkaline with a soda or potash solution, and then evaporated 



