316 PRINCIPLES AND PRACTICE OF MILK HYGIENE 



79. The protein standard for certified cream shall corre- 

 spond to the protein standard for certified milk. 



80. The protein content shall be determined only when any 

 special consideration 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 reckoning the protein from the nitrogen. 



KjeldaM Method. Five cubic centimetres are measured 

 carefully into a flat-bottom 800 c.c. Jena flask, 20 c.c. of concen- 

 trated sulphuric acid (C. P.; sp. gr., 1.84) are added, and 0.7 

 gramme of mercuric oxide (or its equivalent in metallic mer- 

 cury) ; 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 grammes sulphide per litre), 

 and a fraction of a gramme 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 1/20 c.c.) 

 into an Erlenmeyer flask and the flask connected with a distil- 

 lation 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 40 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 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 



