CONTROL OF THE PUBLIC MILK SUPPLY 417 



stand with the mouths uppermost; then, with the pipette designed for the purpose, 

 or with an automatic measurer, 10 c.c. of sulphuric acid are filled into the test bottle, 

 care being taken not to allow any to come in contact with the neck. The few drops 

 remaining in the tip of the pipette should not be blown out. Then 11 c.c. of milk 

 are measured with the proper pipette and allowed to flow slowly onto the acid, so that 

 the two liquids mix as little as possible. Finally, the amyl alcohol is added. (It is 

 important to use the reagents in the proper order, which is sulphuric acid, milk, 

 amyl alcohol. If the sulphuric acid is followed by amyl alcohol and the milk last, 

 then the result is sometimes incorrect.) A rubber stopper, which must not be dam- 

 aged, is then fitted into the mouth of the test bottle, and the contents are well shaken, 

 the thumb being kept on the stopper to prevent 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 tempera- 

 ture of 60 to 65C. When the temperature rises higher than this, say above 70C., 

 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 min. at a speed of about 1,000 revolutions per min. 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 65C., 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 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. 



27 



