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 nor 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 S00 
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 miuute. 
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, 
