416 CITY MILK SUPPLY 



70. The chemical analyses shall be made by a competent chemist designated by 

 the medical milk commission. 



71. Method of Obtaining Samples. The samples to be examined by the chemist 

 shall have been examined previously by the bacteriologist designated by the medical 

 milk commission, as to temperature, odor, taste, and bacterial content. 



72. Fat Standards. The fat standard for certified milk shall be 4 per cent., with a 

 permissible range of variation of from 3.5 to 4.5 per cent. 



73. The fat standard for certified cream shall be not less than 18 per cent. 



74. If it is desired to sell higher fat-percentage milks or creams as certified 

 milks or creams, the range of variation for such milks shall be 0.5 per cent, on 

 either side of the advertised percentage and the range of variations for such creams 

 shall be 2 per cent, on either side of the advertised percentage. 



75. The fat content of certified milks and creams shall be determined at least 

 once each month. 



76. The methods recommended for this purpose are the Babcock (a), the 

 Leffmann-Beam (6), and the Gerber (c). 



(a) Babcock test. The Babcock test is based on the fact that strong sulphuric acid 

 will dissolve the non-fatty solid constituents of milk, and thus enable the fat to sepa- 

 rate on standing. It can be conducted by any of the Babcock outfits which are pur- 

 chasable in the market. 



"The test is made by placing in the special test bottle 18 grams (17.6 c.c.) of milk. 

 To this is added, from a pipette, burette, or measuring bottle, 17.5 c.c. commercial 

 sulphuric acid of a specific gravity of 1.82 to 1.83. The contents of the bottle are 

 carefully and thoroughly mixed by a rotary motion. The mixture becomes brown 

 and heat is generated. The test bottle is now placed in a properly balanced centrifuge 

 and whirled for 5 min. at a speed of from 800 to 1,200 revolutions per min. Hot 

 water is then added to fill the bottle to the lower part of the neck, after which it is 

 again whirled for two minutes. Now, enough hot water is added to float the column 

 of fat into the graduated portion of the neck of the bottle, and the whirling is repeated 

 for a minute. The amount of fat is read while the neck of the bottle is still hot. The 

 reading is from the upper limits of the meniscus. A pair of calipers is of assistance in 

 measuring the column of fat." (Jensen's "Milk Hygiene," Leonard Pearson's trans- 

 lation.) 



(6) Leffman-Beam test. The distinctive feature is the use of fusel oil, the effect 

 of which is to produce a greater difference in surface tension between the fat and the 

 liquid in which it is suspended, and thus promote its readier separation. This effect 

 has been found to be heightened by the presence of a small amount of hydrochloric 

 acid. 



The test bottles have a capacity of about 30 c.c. and are provided with a graduated 

 neck, each division of which represents 9.1 per cent, by weight of butter fat. 



Fifteen centimeters of the milk are measured into the bottle, 3 c.c. of a mixture of 

 equal parts of amyl alcohol and strong hydrochloric acid 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 one part sulphuric acid and two 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 minutes, 

 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 



