CERTIFIED MILK 505 



added and mixed. Then 9 c.c. of concentrated sulphuric acid is added in 

 portions of about 1 c.c.; after each addition the liquids are mixed by giving 

 the bottle a gyratory motion. If the fluid has not lost all of its milky color 

 by this treatment, a little more concentrated acid must be added. The 

 neck of the bottle is now immediately filled at about the zero point with 1 

 part sulphuric acid and 2 parts water, well mixed just before using. Both 

 the liquid in the bottle and the diluted acid must be hot. The bottle is then 

 placed at once in the centrifugal machine; after rotation from one to two min- 

 utes, the fat will collect in the neck of the bottle and the percentage may be 

 read off. 



(c) Gerber's Test. This test is applied as follows: The test bottles are 

 put into the stand with the mouths uppermost; then, with the pipet 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 con- 

 tact with the neck. The few drops remaining in the tip of the pipet should 

 not be blown out. Then 11 c.c. of milk are measured with the proper pipet 

 and allowed to flow slowly on to 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, 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 three to four min- 

 utes at a speed of about 1000 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 on to 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 men- 

 iscus 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 per- 

 centage 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 extrac- 

 tions, that his apparatus and his technic 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. 



