While still hot, the bottles are placed in a centrifugal machine and whirled for at least five minutes. The most convenient machine, where it is available, is the one driven by a jet of steam. The revolutions of the centrifugal should be at least 700 per minute for a twenty inch and 1,200 for a twelve inch wheel. After five minutes the bottles are removed and filled to the upper mark or nearly so with hot water, replaced in the machine and whirled for at least one minute. The fat will then be found in a clearly defined column in the graduated neck of the bottle. In reading the scale, the extreme limits between the lowest point marked by the lower meniscus and the highest point marked by the edge of the upper meniscus are to be regarded as the termini of the fat column.
In testing cream by the babcock process, it may either be diluted until the column of fat secured is contained in the graduated part of the neck or specially graduated bottles may be used.
Condensed Milk: In applying the babcock test to condensed milk, it is necessary to weigh the sample and to use only about eight grams.[465] This quantity is placed in the bottle and dissolved in ten cubic centimeters of water and the analysis completed as above. The reading noted is multiplied by eighteen and divided by the weight of the sample taken.
Skim Milk: In determining the fat in skim milk and whey, it is desirable to use a bottle of double the usual capacity, but with the same graduation on the neck. The percentage of fat noted is divided by two.
Cheese: Five grams are a convenient quantity of cheese to employ. To this quantity in the bottle are added fifteen cubic centimeters of hot water and the flask gently shaken and warmed until the cheese is softened. The treatment with acid and whirling are the same as described above. The noted reading is multiplied by eighteen and divided by five.
474. Solution in Amyl Alcohol and Hydrochloric and Sulfuric Acids.—Leffmann and Beam have proposed to aid the solution of the casein in sulfuric acid by the previous addition to the milk of a mixture of equal volumes of amyl alcohol and hydrochloric acid.[466] In this process the same graduated flasks may be used as in the babcock process, or a special flask may be employed. In this case the graduation of the neck is for fifteen cubic centimeters of milk, and each one and a half cubic centimeters is divided into eighty-six parts. The quantity of milk noted is placed in the flask, together with three cubic centimeters of the mixture of amyl alcohol and hydrochloric acid, and well shaken. To the mixture, sulfuric acid of 1.83 specific gravity is added until the belly of the flask is nearly full and the contents well mixed by shaking. When the casein is dissolved, the addition of the sulfuric acid is continued until the flask is filled to the upper mark and again the contents mixed. It is well to close the mouth of the flask with a stopper while shaking. The bottle is placed in a centrifugal and whirled for a few moments, when the fat is collected in the graduated neck and its volume noted.
The process is also known in this country as the beimling method.[467] The fat separated in the above process is probably mixed with a little fusel oil, and therefore it is advisable to use the specially graduated bottle instead of one marked in absolute volumes.[468]
The method, when conducted according to the details found in the papers cited, gives accurate results, but is somewhat more complicated than the babcock process and is not now used to any great extent in analytical work.
475. Method of Gerber.—The method proposed by Gerber for estimating fat in milk is based on the processes of Babcock, Beimling and Beam already described. The tubes in which the decomposition of the milk and the measurement of the fat are accomplished are of two kinds, one open at only one end for milk and the other open at both ends for cheese. They are closed during the separation by rubber stoppers.[469]
