﻿32 BULLETIN 1, U. S. DEPARTMENT OF AGRICULTURE. 



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 (&), and the Gerber (c). 



(a) Babcock test. — The Babcock test is based on the fact that strong sul- 

 phuric acid will dissolve the nonfatty solid constituents of milk, and thus enable 

 the fat to separate on standing. It can be conducted by any of the Babcock 

 outfits which are purchasable 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 mix- 

 ture becomes brown and heat is generated. The test bottle is now placed in a 

 properly balanced centrifuge and whirled for 5 minutes at a speed of from 800 

 to 1,200 revolutions per minute. 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 read- 

 ing 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 

 translation.) 



(b) Leffmann-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 sepa- 

 ration. 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 treat- 

 ment, 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 bot- 

 tle 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 col- 

 lect 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 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 re- 

 agents 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 damaged, 

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



