Part 92

The apparatus is arranged as shown in Fig. 108, whereby it is easy to drive the ethereal fat solution into the measuring vessel by means of the bellows shown. In the bottle, seen at the right of the engraving are placed 200 cubic centimeters of milk, ten of the potash lye and sixty of the aqueous ether. The milk and potash are first added and well shaken, the ether then added, and the contents of the bottle are shaken until a homogeneous emulsion is formed. The bottle is then set aside for the separation of the ethereal solution, which is promoted by gently jarring it from time to time. When the chief part of the solution has separated, a sufficient quantity of it is driven over into the measuring apparatus, by means of the air bulbs, to float the hydrometer contained in the inner cylinder. After a few moments the scale of the oleometer is read and the percentage of fat calculated from the table. All the measurements are made at a temperature of 17°.5. The temperature is preserved constant by filling the outer cylinder of the apparatus with water. If the room be warmer than 17°.5, the water added should be at a temperature slightly below that and vice versa. The oleometer carries a thermometer which indicates the moment when the reading is to be made.

The scale of the oleometer is graduated arbitrarily from 43 to 66, corresponding to the specific gravities 0.743 and 0.766, respectively, or to corresponding fat contents of 2.07 and 5.12 per cent, in the milk, a range which covers most normal milks.

In the use of the table the per cents corresponding to parts of an oleometer division can be easily calculated.

TABLE FOR CALCULATING PER CENT OF FAT IN MILK BY AREOMETRIC METHOD OF SOXHLET.

Reading of Per cent fat Reading of Per cent fat oleometer. in milk. oleometer. in milk.

43 2.07 55 3.49 44 2.18 56 3.63 45 2.30 57 3.75 46 2.40 58 3.90 47 2.52 59 4.03 48 2.64 60 4.18 49 2.76 61 4.32 50 2.88 62 4.47 51 3.00 63 4.63 52 3.12 64 4.79 53 3.25 65 4.95 54 3.37 66 5.12

=467. Application of the Areometric Method.=—Soxhlet’s method, as outlined above, with many modifications, has been extensively used in Europe and to a limited degree in this country, and the results obtained are in general satisfactory, when the sample is a mixed one from a large number of cows and of average composition.

The author has shown that the process is not applicable to abnormal milk and often not to milk derived from one animal alone.

The chief difficulty is found in securing a separation of the emulsion. This trouble can usually be readily overcome by whirling. Any centrifugal machine, which can receive the bottle in which the emulsion is made, may be employed for that purpose.

Since the introduction of more modern and convenient methods of fat determination, the areometric method has fallen into disuse and perhaps is no longer practiced in this country. It is valuable now chiefly from the fact that many of the recorded analyses of milk fat were made by it, and also for its typical character in representing all methods of analysis of fat in milk based on the density of ethereal solutions.

=468. The Lactobutyrometer.=—A typical instrument for measuring the volume of fat in a milk is known as Marchand’s lactobutyrometer. It is based on the observation that ether will dissolve the fat from milk when the casein is wholly or partly dissolved by an alkali, and further, that the fat in an impure form can be separated from its ethereal solution by the action of alcohol. Experience has shown that all the fat is not separated from the ethereal solution by this process, and also that the part separated is a saturated solution in ether. The method cannot be rigorously placed in the two classes given above, but being volumetric demands consideration here chiefly because of its historical interest.

The instrument employed by Marchand is a tube about thirty centimeters long and twelve in diameter, closed at one end and marked in three portions of ten cubic centimeters each. The upper part is divided in tenths of a cubic centimeter. The superior divisions are subdivided so that the readings can be made to hundredths of a cubic centimeter.

The tube is filled with milk to the first mark and two or three drops of a twenty-five per cent solution of soda lye added thereto. Ether is poured in to the second mark, the tube closed and vigorously shaken. Alcohol of about ninety per cent strength is added to the upper mark, the tube closed, shaken and allowed to stand in a vertical position, with occasional jolting, until the separation of the liquids is complete. In order to promote the separation the tube is placed in a cylinder containing water at 40°.

When the separation is complete the milk serum is found at the bottom, the mixture of alcohol and ether in the middle and the fat at the top. The mixture of ether and alcohol contains 0.126 gram of fat, and each cubic centimeter of the separated ether fat 0.233 gram of fat. The total volume of the separated fat, multiplied by 0.233, and the product increased by 0.126, will give the weight of fat in the ten cubic centimeters of milk employed.

Example.—Milk used, ten cubic centimeters of 1.032 specific gravity = 10.32 grams. The observed volume of the saturated ether fat solution is two cubic centimeters. Then the weight of fat is 2 × 0.233 + 0.126 = 0.592 gram. The percentage of fat in the sample is 0.592 × 100 ÷ 10.32 = 5.74.

In the apparatus used in this laboratory the upper division of the graduation is marked 12.6, because this represents the quantity of fat which remains in the ether-alcohol mixture for one liter of milk. From this point the graduation is extended downward to ninety-five, which, for ten cubic centimeters of milk, represents 0.95 gram. After the fat has separated, enough ninety-five per cent alcohol is added to bring the upper surface exactly to the graduation 12.6. The number of grams per liter of milk is then read directly from the scale.

In respect of applicability, the observation made regarding Soxhlet’s areometric method may be repeated.

In practical work in this country the lactobutyrometer is no longer used, but many of the recorded determinations of fat in milk have been made by this method.

=469. Volumetric Methods.=—For practical purposes, the volumetric methods of estimating fat in milk have entirely superseded all the other processes. It has been found that the fat readily separates in a pure state from the other constituents of milk whenever the casein is rendered completely soluble; whereas no process has yet been devised whereby the fat can be easily separated in a pure state from milk which has not been treated with some reagent capable of effecting a solution of the casein. The volumetric methods may be divided into two classes; viz., (1) Those in which the fat is separated by the simple action of gravity, and (2) those in which the natural action of gravity is supplanted by centrifugal motion. Each of these classes embraces a large number of variations and some of the typical ones will be described in the following paragraphs. As solvents for the casein a large number of reagents has been used, including alkalies and single and mixed acids. In practice, preference is given to the least complex and most easily prepared solvents.

=470. Method Of Patrick.=—A typical illustration of the method of collecting the fat after solution of the casein, without the aid of whirling, is found in the process devised by Patrick.

The solvent employed is a mixture of acetic, sulfuric and hydrochloric acids, saturated with sodium sulfate, in the respective volumetric proportions of nine, five and two. The separation is accomplished in a large test tube drawn out near the top into a constricted neck which is graduated to measure the volume of the separated fat or to give direct percentage results.

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, 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. 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.

=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.