506 MILK 



79. The protein standard for certified cream shall correspond to the pro- 

 tein standard for certified milk. 



80. The protein content shall be determined only when any special con- 

 sideration seems to the medical milk commission to make it desirable. 



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

 or some other reliable modification, and employing the factor 6.25 in reckon- 

 ing the protein from the nitrogen. 



Kjeldahl Method. — Five cubic centimeters of milk are measured care- 

 fully 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 per- 

 manganate 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 sulphid (containing 40 grams sulphid 

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

 normal HCl solution more than sufficient to neutralize the ammonia obtained 

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

 buret (divided into ^ c.c.) into an Erlenmeyer flask and the flask connected 

 with a distillation 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 pound "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 forty 

 minutes 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 solution as an indicator. From the amount of acid used the 

 per cent, of nitrogen is obtained; and from it the per cent, of casein and 

 albumen in the milk by multiplying by 6.25. The amount of nitrogen con- 

 tained in the chemicals used is determined by blank experiments and de- 

 ducted from the nitrogen obtained as described. (Farrington and Woll, 

 Testing Milk and Its Products, p. 221.) 



82. Coloring-7naiter and Preservatives. — AU 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 

 whenever 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 

 colormg-matter has been added to the milk, the solution will be yellow col- 

 ored, 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 formaldehyd, borax, and boric acid shall 

 be applied at least once each month. Occasionally appUcation of tests for the 

 detection of salicylic acid, benzoic acid, and the benzoates is also recom- 

 mended. 



Test for the Detection of Formaldehyd. — Five cubic centimeters of milk is 

 measured into a white porcelain dish, and a similar quantity of water added; 

 10 c.c. of HCl, containing a trace of FejCle, is added, and the mixture is heated 

 very slowly. If formaldehyd is present, a violet color will be formed. (Test^ 

 mg Milk and Its Products, Farrington and Woll, p. 249.) 



Test for Boric Acid (Borax, Borates, Preservaline, etc.). — One hundred 

 cubic centimeters of milk are made alkaline with a soda or potash solution, 

 and then evaporated to dryness and incinerated. The ash is dissolved in 

 water, to which a little hydrochloric acid has been added, and the solution 



