Among the other lactoscopes which have been used may be mentioned those of Donné, Vogel, Hoppe-Seyler, Trommer, Seidlitz, Reischauer, Mittelstrass, Hénocque, and Heusner.[419] Since the invention of so many quick and accurate methods of fat estimation these instruments have little more than a historical interest.
Fig. 107.—Lactoscope, Lactometer and Creamometer.
441. Creamometry.—The volume of cream which a sample of milk affords under arbitrary conditions of time and temperature is sometimes of value in judging the quality of milk. A convenient creamometer is a small cylinder graduated in such a way that the volume of cream separated in a given time can be easily noted. There are many kinds of apparatus used for this purpose, a typical one being shown in [Fig. 107].
The usual time of setting is twenty-four hours. A quicker determination is secured by placing the milk in strong glass graduated tubes and subjecting these to centrifugal action. The process is not exact and is now rarely practiced as an analytical method, even for valuing the butter making properties of milk.
442. Specific Gravity.—The specific gravity of milk is uniformly referred to a temperature of 15°. Generally no attempt is made to free the milk of dissolved gases beforehand. This should not be done by boiling but by placing the sample in a vacuum for some time. Any of the methods described for determining specific gravity in sugar solutions may be used for milk ([48-59]). The specific gravity of milk varies in general from 1.028 to 1.034. Nearly all good cow milk from herds will show a specific gravity varying from 1.030 to 1.032. In extreme cases from single cows the limits may exceed those first given above, but such milk cannot be regarded as normal.
Increasing quantities of solids not fat in solution, tend to increase the specific gravity, while an excess of fat tends to diminish it. There is a general ratio existing between the solids not fat and the fat in cow milk, which may be expressed as 9: 4. The removal of cream and the addition of water in such a manner as not to affect the specific gravity of the sample disturbs this ratio.
The determination of the specific gravity alone, therefore, cannot be relied upon as an index of the purity of a milk.
443. Lactometry.—A hydrometer especially constructed for use in determining the density of milk is called a lactometer. In this country the one most commonly used is known as the lactometer of the New York Board of Health. It is a hydrometer, delicately constructed, with a large cylindrical air space and a small stem carrying the thermometric and lactometric scales. It is shown held in the creamometer in [Fig. 107]. The milk is brought to a temperature of 60° F. and the reading of the lactometer scale observed. This is converted into a number expressing the specific gravity by means of a table of corresponding values given below. Each mark on the scale of the instrument corresponds to two degrees and these marks extend from 0° to 120°. The numbers of this scale can be converted into those corresponding to the direct reading instrument, described in the next paragraph, by multiplying them by 0.29.
The minimum density for whole milk at 60° F. is fixed by this instrument at 100°, corresponding to a specific gravity of 1.029. The instrument is also constructed without the thermometric scale. The mean density of many thousand samples of pure milk, as observed by the New York authorities, is 1.0319.