266 Principles of Legislative Milk Control. 



it coming out. As a considerable amount of heat is generated by the action of the 

 sulphuric acid on the milk, the test bottle should be wrapped in a cloth. 



The shaking of the sample must be done thoroughly and quickly, and the test 

 bottle inverted several times, so that the liquid in the neck becomes thoroughly mixed. 

 By pressing in the rubber stopper the height of the liquid can be brought to about the 

 zero point on the scale. 



If only a few samples have to be analyzed and the room is warm, the test 

 bottles can be put into the centrifuge without any preliminary heating, otherwise 

 the test bottles must be warmed for a few minutes (not longer) in the water bath 

 at a temperature of 60 to 65 C. When the temperature rises higher than this, 

 say above 70 C., the rubber stopper is liable to be blown out of the test bottle. 

 After the test bottles have been heated they are arranged symmetrically in the 

 centrifuge and whirled for 3 to 4 minutes at a speed of about 1,000 revolutions per 

 minute. When the centrifuge has a heating arrangement attached to it, the preliminary 

 warming is not, of course, necessary. When the test bottles are taken out of the 

 centrifuge, they are again placed in the water bath at a temperature of 60 to 65 C., 

 and left there for several minutes before being read; where the centrifuge is heated, the 

 tubes can be read off as taken from the centrifuge. 



By carefully screwing in the rubber stopper, or even by pressing it, the lower 

 limit of the fat column is brought onto one of the main divisions of the scale, 

 and then, by holding the test bottle against the light, the height of the column of 

 fat can be accurately ascertained. The lowest point of the meniscus is taken as the 

 level when reading the upper surface of the fat in a sample of whole milk, and the 

 middle of the meniscus for separated milk. 



If the column of fat is not clear and sharply defined, the sample must be again 

 whirled in the centrifuge. 



Each division on the scale is equivalent to 0.1 per cent, so it is very easy to 

 read to 0.05 per cent, or, with a lens, to 0.025 per cent. If the number which is 

 read off is multiplied by 0.1, then the percentage quantity of fat in the milk is obtained; 

 e. g., if the number on the scale was 36.5, then the percentage of fat is 3.65. (Milk 

 and Dairy Products, Barthel; translated by Goodwin, p. 71.) 



77. Before condemning samples of milk which have fallen out- 

 side the limits allowed, the chemist shall have determined, by 

 control ether extractions, that his apparatus and his technique are 

 reliable. 



78. Protein standard. The protein standard for certified 

 milk shall be 3.50 per cent, with a permissible range of variation 

 of from 3 to 4 per cent. 



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. 



~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 

 1/20 c. c.) into an Erlenmeyer flask and the flask connected with a distillation appa- 

 ratus. At the other end the Jena flask containing the watery solution of the ammonium 



