MEDICAL MILK COMMISSIONS AND CEETIFIED MILK. 33 
the thumb being kept on the stopper to prevent it coming out. As a consider- 
able 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 thor- 
oughly 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 
botttles can be put into the centrifuge without any preliminary heating, other- 
wise 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 ar- 
rangement 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 denned, 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 outside 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 correspond to the protein 
standard for certified milk. 
80. The protein content shall be determined only when any special considera- 
tion 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 ^y 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 am- 
monium sulphate is connected, after adding 50 c. c. of a concentrated soda solu- 
tion (1 pound " pure potash " dissolved in 500 c. c. of distilled water and allowed 
