The solvent employed is a mixture of sodium and potassium hydroxids, containing in one liter 125 and 150 grams, respectively, of these alkalies. The sample of milk is mixed with half its volume of the reagent and placed in boiling water for two hours. By this treatment the casein is dissolved and the fat saponified. After cooling to about 60°, the soap is decomposed by the addition of equal parts of sulfuric and acetic acids. The tubes containing the mixture are again placed in boiling water for an hour and they are then filled with boiling water to within one inch of the top. The tubes may either be furnished with a graduation or the column of fat be measured by a scale.
464. Method of Thörner.—The process of Short is conducted by Thörner as follows:[454]
Ten cubic centimeters of milk measured at 15° are saponified, in tubes fitting a centrifugal, by the addition of one and a half cubic centimeters of an alcoholic potash lye, containing 160 grams of potassium hydroxid per liter, or one cubic centimeter of an aqueous fifty per cent soda lye. The saponification is hastened by setting the tubes in boiling water, where they remain for two minutes. The soap formed is decomposed with a strong acid, sulfuric preferred, the tubes placed in the centrifugal and whirled for four minutes, when the fat acids will be formed in the narrow graduated part of the tube and the volume occupied thereby is noted after immersion in boiling water. Thörner’s process is not followed in this country, but is used to a considerable extent in Germany.[455]
465. Liebermann’s Method.—In this method, fifty cubic centimeters of milk, at ordinary temperatures, are placed in a glass cylinder twenty-five centimeters high and about four and a half internal diameter; five cubic centimeters of potash lye of 1.27 specific gravity are added, the cylinder closed with a well fitting cork stopper and thoroughly shaken.[456] After shaking, fifty cubic centimeters of petroleum spirit, boiling point about 60°, are added. The cylinder is again stoppered and vigorously shaken until an emulsion is formed. To this emulsion fifty cubic centimeters of alcohol of ninety-five per cent strength are added, and the whole again thoroughly shaken. After four or five minutes the petroleum spirit, containing the fat, separates. In order to insure an absolute separation of the fat, however, the shaking may be repeated three or four times for about one-quarter minute, waiting each time between the shakings until the spirit separates.
Of the separated petroleum spirit twenty cubic centimeters are placed in a small weighed flask. The use of the flask is recommended on account of the ease with which the petroleum spirit can be evaporated without danger of loss of fat. Instead of the flask a weighed beaker or other weighed dish may be employed.
The petroleum spirit is carefully evaporated on a water-bath and the residue dried at 110° to 120° for one hour. The weight found multiplied by five gives the content of fat in 100 cubic centimeters of the milk. The percentage by weight can then be calculated by taking into consideration the specific gravity of the milk employed.
The results obtained by this method agree well with those obtained by the paper coil method, when petroleum spirit instead of sulfuric ether is used as the solvent for the fat. Sulfuric ether, however, gives an apparently higher content of fat because of the solution of other bodies not fat present in the milk.
466. Densimetric Methods.—Instead of evaporating the separated fat solution and weighing the residue, its density may be determined and the percentage of fat dissolved therein obtained by calculation, or more conveniently from tables. The typical method of this kind is due to Soxhlet, and until the introduction of modern rapid volumetric processes, it was used perhaps more extensively than any other proceeding for the determination of fat in milk.[457] The reagents employed in the process are ether saturated with water and a potash lye containing 400 grams of potash in a liter. The principle of the process is based on the assumption that a milk made alkaline with potash will give up all its fat when shaken with ether and the quantity of fat in solution can be determined by ascertaining the specific gravity of the ethereal solution.
Fig. 108. Areometric Fat Apparatus.