Milk
The chief constituents of milk are water, butter, caseine, lactose (milk-sugar), traces of albumen and mineral salts. Butter is present in the form of minute globules, held in suspension; the caseine, for the greater part, is in solution, only a small portion being present in an insoluble suspended condition. In milk only a few days old, the colostrum (the milk secreted during the first few days after parturition) consists largely of rather voluminous cellular conglomerations, containing a sufficient quantity of albumen to coagulate upon heating.
The normal density of milk is 1.030, water being 1.000; the density rising to 1.036, if the fluid has been skimmed.
Good milk contains, on an average, 3.7 per cent. of butter; 5.7 per cent. of lactose, and leaves upon evaporation 12 to 14 per cent. of solid matters.T The most common adulteration of milk consists in the addition of water. This fraud is detected by means of an areometer (lactodensimeter) which gives directly the specific gravity of the fluid under examination. Should the density be much below 1.030, it is certain that water has been added. It does not, however, necessarily follow if it is about 1.030 that the milk is pure, since the gravity of the fluid, which would be increased upon skimming, could be subsequently reduced to 1.030 by the addition of water. The lactodensimeter, therefore, although useful in the detection of a simple admixture, fails to give reliable results if the fraud perpetrated is a double one; and a determination of the proportion of butter present is also usually necessary. Numerous methods have been proposed to accomplish this estimation. The most preferable of these, owing to the rapidity with which the operation is executed, is the use of the lactoscope (galactoscope). This instrument consists of a tube provided with a glass plate fitted at one end, and with a movable glass plate at the other extremity. A few drops of the milk to be tested are placed between the two plates, and the tube lengthened, by screwing out the movable plate, until the fluid no longer transmits the light of a candle placed at a distance of one metre. As the opacity of milk is due to the butter present, it is evident that the proportion of this substance contained in the sample can be estimated by the relative distance which the plates have been separated.
The lactoscope possesses, however, but a limited degree of precision. M. Marchand substitutes to its use the following tests: A test-tube is graduated in three equal divisions, the upper one being subdivided into hundredths extending above, in order to determine accurately the correct volume of the fluid, expanded, as it is, by the temperature of 40°, at which the examination is executed. The first division of the tube is filled with milk, a drop, or two of strong potassa lye added, and the mixture well shaken: the second portion is then filled with ether, and the third with alcohol. The mixture is next again thoroughly agitated, and then exposed to a temperature of 40° in a water-bath. After standing for several hours, a layer of fatty matter becomes sufficiently separated to allow of measurement: but, as it contains some ether and as a small amount of butter may still be retained in the lower aqueous fluid, a correction of the results obtained is necessary. M. Marchand has compiled a table, which facilitates this correction (vide: Journ. de Pharm., Novembre 1854, and Bulletin de l’Académie de Médecine, Paris, 1854, xix., p. 1101).
Previously to the introduction of Marchand’s apparatus, use was made of the lactometer, which consists simply of a graduated glass tube, in which the suspected milk is allowed to remain for 24 hours, at a temperature of 15°. After the lapse of this time, the cream present completely separates as a supernatant layer, the thickness of which indicates the quality of the sample taken.
M. Lacomte recommends the addition of glacial acetic acid, in order to cause the more rapid separation of the cream.
The estimation of the butter being accomplished, it is frequently needful to determine the amount of lactose present. For this purpose, recourse is had to Barreswil’s method, based upon the reduction of cupro-potassic tartrate by milk-sugar in the presence of alkalies. A solution is prepared containing 40 grammes of pure crystallized sulphate of copper, 600 or 700 grammes of caustic soda lye of sp. gr. 1.12, and 160 grammes of neutral tartrate of potassa. The sulphate of copper and tartrate of potassa are previously dissolved separately in a little water, the three solutions united, and water added until the fluid acquires a volume of 1154.4 cubic centimetres. In order to standardize this test solution, a known weight of pure lactose is dissolved in water and the fluid added, drop by drop, from a graduated burette, to a small flask containing 10 cubic centimetres of the copper solution, diluted with 40 cubic centimetres of distilled water, and heated to boiling. At first a yellow precipitate forms, which gradually turns red, and is deposited on the bottom of the flask, leaving the solution colorless. As soon as the test solution is completely decolorized, the addition of the lactose solution is discontinued, and the weight of lactose corresponding to 10 cubic centimetres of the test fluid calculated from the quantity used. The standard of the test solution having been determined, the above operation is repeated, the milk under examination being substituted for the solution of pure lactose. The quantity of milk necessary to decolorize 10 cubic centimetres of the copper solution will evidently contain the same amount of lactose as the quantity of solution used in the preliminary test, and the actual amount of lactose present is very easily calculated. When an estimation of the solid matter contained in the milk is required, a known weight is evaporated to dryness over a water-bath, and the residue weighed. In performing this evaporation, the addition of a known amount of sand, or ground glass, is advisable. The amount of ash present is determined by incinerating the residue left by the evaporation.
Foreign substances are sometimes added to milk, for the purpose of disguising the presence of an abnormal quantity of water, the principal of which are: chalk, bicarbonate of soda, emulsion of almonds, gum tragacanth, gum arabic, starch, flour, decoction of barley or rice, sugar, and cerebral substances. These bodies are detected as follows:
Chalk.—If chalk is contained in the milk, it readily subsides upon allowing the sample to remain at rest for some time in a flask, forming a deposit which effervesces when heated with hydrochloric acid, and dissolves to a solution, in which the characteristic properties of a lime salt can be recognized.
Bicarbonate of soda.—In presence of this compound the milk possesses a strongly alkaline reaction, furnishes a serum having a sharp and bitter taste, and leaves a residue of the salt upon evaporation.
Emulsion of almonds.—The milk has a specific gravity of at least, 1.033. If it is passed through a gauze, small opaque lumps are separated. When examined under the microscope, numerous minute globules, having a diameter of 1/400 of a millimetre, are observed, and, upon adding a few centigrammes of amygdaline to one or two grammes of the milk, the characteristic odor of bitter almonds is produced.
Gum tragacanth.—When shaken in a glass flask and allowed to rest, the milk deposits on the sides small transparent lumps, which usually present a slightly elongated or angular form.
Gum arabic.—The addition of alcohol produces an abundant white opaque precipitate.
Starch, flour, decoction of barley, rice, etc.—Upon boiling the suspected milk, and adding tincture of iodine, the amylaceous substances present produce a blue coloration in the fluid.
Sugar.—If yeast is added, and the mixture allowed to stand for some time at a temperature of 30°, alcoholic fermentation ensues; under these circumstances, lactose does not undergo fermentation.
Cerebral substances.—Adulteration by these substances is probably of much less frequent occurrence than was formerly supposed. The admixture is detected by evaporating the milk to dryness, dissolving the residue in ether, evaporating the etherial solution, and fusing the second residue, which consists of fatty matters, with nitrate of potassa in a platinum crucible. The mass is then taken up with water, and chloride of barium added to the solution. If cerebral substances were contained in the milk, ether will dissolve the fatty matters present, the phosphorus of which is converted into a soluble phosphate by the calcination with nitrate of potassa and is thrown down as a white precipitate, upon the addition of a solution of chloride of barium. This test may be confirmed by a microscopic examination of the milk, when the peculiar appearance of cerebral matter will be detected.U