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 oleometer. | Per cent fat in milk. | Reading of oleometer. | Per cent fat 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.[458]
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.[459]