The specific gravity is easily secured, and while not of itself decisive, should always be determined. The specific gravity of milk increases for some time after it is drawn and should be made both when fresh and after the lapse of several hours.[420]
Table Showing Specific Gravities Corresponding
to Degrees of the New York Board Of Health
Lactometer. Temperature 60° F.
| Degree. | Sp. gr. | Degree. | Sp. gr. |
|---|---|---|---|
| 90 | 1.02619 | 106 | 1.03074 |
| 91 | 1.02639 | 107 | 1.03103 |
| 92 | 1.02668 | 108 | 1.03132 |
| 93 | 1.02697 | 109 | 1.03161 |
| 94 | 1.02726 | 110 | 1.03190 |
| 95 | 1.02755 | 111 | 1.03219 |
| 96 | 1.02784 | 112 | 1.03248 |
| 97 | 1.02813 | 113 | 1.03277 |
| 98 | 1.02842 | 114 | 1.03306 |
| 99 | 1.02871 | 115 | 1.03335 |
| 100 | 1.02900 | 116 | 1.03364 |
| 101 | 1.02929 | 117 | 1.03393 |
| 102 | 1.02958 | 118 | 1.03422 |
| 103 | 1.02987 | 119 | 1.03451 |
| 104 | 1.03016 | 120 | 1.03480 |
| 105 | 1.03045 |
444. Direct Reading Lactometer.—A more convenient form of lactometer is one which gives the specific gravity directly on the scale. The figures given represent those found in the second and third decimal places of the number expressing the specific gravity. Thus 31 on the scale indicates a specific gravity of 1.031. This instrument is also known as the lactometer of Quévenne. For use with milk, the scale of the instrument does not need to embrace a wider limit than from 25 to 35, and such an instrument is capable of giving more delicate readings than when the scale extends from 14 to 42, as is usually the case with the quévenne instrument.
Langlet has invented a lactoscope with a scale, showing the corrections to be applied for temperatures other than 15°. A detailed description of this instrument, as well as the one proposed by Pinchon, is unnecessary.[421]
445. Density of Sour Milk.—Coagulated milk cannot be used directly for the determination of the specific gravity, both because of its consistence and by reason of the fact that the fat is more or less completely separated. In such a case, the casein may be dissolved by the addition of a measured quantity of a solvent of a known specific gravity, the density of the resulting solution determined and that of the original milk calculated from the observed data. Ammonia is a suitable solvent for this purpose.[422]
446. Density of the Milk Serum.—The specific gravity of the milk serum, after the removal of the fat and casein by precipitation and filtration, may also be determined. For normal cow milk the number is about 1.027.
447. Total Solids.—The direct gravimetric determination of the total solids in milk is attended with many difficulties, and has been the theme of a very extended periodical literature. A mere examination of the many processes which have been proposed would require several pages.
The most direct method of procedure is to dry a small quantity of milk in a flat-bottom dish to constant weight on a steam-bath. The surface of the dish should be very large, even for one or two grams of milk; in fact the relation between the quantity of milk and the surface of the dish should be such that the fluid is just sufficient in amount to moisten the bottom of the dish with the thinnest possible film. The dish, during drying, is kept in a horizontal position at least until its contents will not flow. The water of the sample will be practically all evaporated in about two hours. The operation may be accelerated by drying in vacuo.
The drying may also be accomplished by using a flat-bottom dish containing some absorbent, such as sand, pumice stone, asbestos or crysolite. The milk may also be absorbed by a dried paper coil and dried thereon ([26]).