The tube should have a content of about twenty-five cubic centimeters below the upper mark on the neck. In use 10.4 cubic centimeters of milk and a sufficient quantity of the mixed acids to fill it nearly to the upper mark are placed in the tube, together with a piece of pumice stone, and the mixture boiled. On cooling below 100°, the fat will separate and the volume thereof may be measured in the constricted portion of the tube. The volume of the fat may be converted into weight on multiplying by 0.88 at 60°, or more conveniently the percentage of fat be taken from a table. In practice, the tube is filled with the milk and acid mixture nearly up to the neck, its contents well mixed and additional acid mixture added until the liquid is raised in the tube above the neck. After mixing a second time, the contents are boiled for five minutes and the fat allowed to collect in the expanded part of the tube above the neck. When the fat has collected, the mixture is boiled gently a second time for a few minutes. By this treatment the fat is mixed with the upper portions of the acid liquid and clarified. The clearing of the fat may be hastened by sprinkling over it a little effloresced sodium sulfate. The fat is brought into the graduated neck by opening a small orifice in the belly of the tube, which is closed by means of a rubber band. When the temperature has reached 60°, the space occupied by the fat is noted and the numbers obtained express the percentage of fat in the sample.
This process is illustrative of the principle of analysis, but is no longer used in analytical determinations.
471. The Lactocrite.—One of the earliest methods for fat estimation in milk, depending on the solution of the casein and the collection of the fat by means of whirling, is based on the use of a centrifugal machine known as the lactocrite. This apparatus is modeled very like the machine usually employed for creamery work,[461] and at one time was extensively used, but it has now given place to less troublesome and expensive machines. The acid mixture for freeing the fat of casein is composed of glacial acetic acid carrying five per cent of sulfuric. The samples of milk are heated with the acid mixture in test tubes provided with stoppers and short glass tubes to return the condensation products. The hot mixture is poured into a small metallic cylindrical cup holding about three cubic centimeters. This cup fits by means of an accurately ground shoulder on a metal casing, carrying inside a heavy glass graduated tube of small internal diameter. The excess of the milk mixture escapes through a small aperture in the metallic screw cap of the metal holder. The metal holder is cut away on both sides in order to expose the graduations on the glass tube. The glass tube is held water-tight by means of perforated elastic washers. Thus prepared the tubes are inserted in the radial holes of a revolving steel disk previously heated to a temperature of 60°. The whirling is accomplished in a few minutes by imparting to the steel disk a speed of about 6,000 revolutions per minute. At the end of this operation the fat is found in a clear column in the small glass tube and the number of the divisions it occupies in this tube is noted. Each division of the scale represents one-tenth per cent of fat.
This apparatus is capable of giving accurate results when all its parts are in good working order. In this laboratory the chief difficulty which its use has presented is in keeping the joints in the glass metal tube tight.
This description of the apparatus is given to secure an illustration of the principle involved, a principle which has been worked out in later times into some of the most rapid and practical processes of estimating fat in milk.
472. Modification of Lindström.—Many modifications have been proposed for conducting the determination of fat by means of the lactocrite, but they do not involve any new principle and are of doubtful merit. In the modification suggested by Lindström, which has attained quite an extended practical application, the solvent mixture is composed of lactic and sulfuric acids and the butyrometer tubes are so changed as to permit the collection of the fat in the graduated neck after whirling, by means of adding water. The apparatus is also adjusted to secure the congelation of the fat column before its volume is noted.[462] The analyst can read the fat volume at his leisure when it is in the solid state and is not confused by changes of volume during the observation. The best acid mixture has been found to be composed of 100 volumes of lactic, an equal amount of acetic and fifteen volumes of sulfuric acids.[463]
Fig. 109.
Babcock’s Butyrometer
and Acid Measure.
473. The Babcock Method.—Among the many quick volumetric methods which have been proposed for the determination of fat in milk, none has secured so wide an application as that suggested by Babcock.[464]
The chief point of advantage in the use of this method is found in effecting the solution of the casein by means of sulfuric acid of about 1.83 specific gravity. By this reagent the casein is dissolved in a few moments without the aid of any other heat than that generated by mixing the milk with the reagent. The bottle in which the separation is made is shown in [Fig. 109]. The graduations on the neck are based on the use of eighteen grams of milk. To avoid the trouble of weighing, the milk is measured from a pipette graduated to deliver eighteen grams of milk of the usual specific gravity. While it is true that normal milk may vary somewhat in its density, it has been found that a pipette marked at 17.6 cubic centimeters delivers a weight which can be safely assumed to vary but slightly from the one desired. The graduated bottle holds easily thirty-five cubic centimeters of liquid in its expanded portion and the volume of milk just noted is mixed with an equal volume of sulfuric acid, conveniently measured from the lip cylinder shown in the [figure]. The complete mixture of the milk and acid is effected by gently rotating the bottle until its contents are homogeneous. The final color of the mixture varies from dark brown to black.