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.