MILK. Milk is composed of casein, fatty matters, sugar of milk, salts, and water, and when a drop is examined with the microscope, it is seen to contain a great number of small round globules.. Milk is usually a white fluid, but on certain occasions it has been observed to possess a blue colour, which is caused, according to some, by microscopic vegetation, according to others by an infusion of animalculi (Vibrio cyanogenus). The smell, taste, and even colour, of different kinds of fodder, more particularly of turnips, are well known to be communicated to the milk. Not only is milk an article of food fit for keeping up life, but also, by mechanical and chemical decomposition, it yields other kinds of nutritive substances. When milk is allowed to remain undisturbed for some hours in a shallow and wide basin, it divides into two parts; the superior layer, or cream, being more abundant in fatty matters, and the inferior layer containing a greater proportion of casein, and known as skimmed milk. When milk or cream are violently agitated, as by the process of churning, the globules run together, and a more solid substance is formed, known as butter, which differs but very slightly from cream. Curds are merely casein transformed into a solid substance by the action of rennet, or an acid; the mother-liquor is the whey, or serum, and consists only of water, sugar, and salts. Cheese is made either from pure milk, or from milk free from cream (skimmed milk), and consists of casein precipitated by the influence of rennet, and then subjected to great pressure in moulds. Its flavour increases by keeping, from the progressive decomposition which it gradually undergoes. Some kinds of cheese are full of what are called eyes, that is, hollows formed by bubbles of gas; and in these apertures a liquid is sometimes found containing free ammonia, a product of decomposition which greatly heightens the flavour. Little is known of the chemical differences in the processes employed for making the various kinds of cheese. The richest consist almost entirely of cream, as Stilton and Parmesan ; others, as Gruyère, Gloucester, Cheshire, and Dutch cheese, are made with fresh uncreamed milk, or mixtures of this with cream. More or less salt is mixed with cheese, or powdered over it, and various methods of coagulating the casein are used in different places. (Prof. Gregory.) The following analysis of cheese was made by Mr. Jones in Professor Johnston's laboratory:1 The relative proportion of the different constituents of cows' milk depends on the age of the animal, the time after calving, food, temperature, weather, and the time and frequency of milking, &c. Analysis of milk.-Haidlen found 100 parts of milk to contain- With the view of determining approximately the quantity of the constituents of milk, it will be advisable to adopt the following methods:1 1. Determination of the specific gravity.-This may be done conveniently with the hydrometer. Still, if it be expected that the richness of the milk will be proportional to its specific gravity, very erroneous results may be obtained by this test, as it very frequently happens that the quality of milk is in an inverse ratio to its density, so that milk may be of high specific gravity, and yield but little cream. 2. Determination of the cream.-This is effected by the lactometer, invented by the late Sir Joseph Bankes, and consisting of a tube of about eleven inches in height; its inferior end is closed, and the other left open; the upper part is graduated; sometimes the graduations are extended to the whole length of the tube. The tube is filled with milk, and after some hours the number of divisions, included the layer of cream, are read off. According to Dr. Normandy, the thickness of the stratum of cream on pure milk is generally from 8 to 8 per cent. From the observations of the Commission, the average does not exceed 9 per cent. If an hydrometer and lactometer were at hand, it would be advisable to ascertain, first, the amount of cream with the lactometer, and then take the specific gravity of the inferior layer, previously decanted with a pipette. It might be still better to have the lactometer open at the bottom, and connected with a small tube, provided with a stop-cock; by shutting the stop-cock, to determine the cream, and then opening it to let out the skimmed milk, the operation might be considerably simplified. The addition of a small quantity of warm water to milk 1 Dr. Hassall on Food, &c., p. 326. does not increase the quantity of the cream, but facilitates and hastens its formation and separation in a most remarkable manner. Determination of the fatty matters. For this purpose, the casein and fats may be precipitated together, by means of acetic acid; the precipitate being collected on a cloth, dried, and treated with ether, will yield the fat to this fluid; and by evaporating the ethereal extract to dryness, and weighing the residue, the amount of fatty matters may be ascertained. There is another method to effect the same purpose, which is, perhaps, more practical, though less correct than the above; that is, by means of the instrument called the lactoscope, invented by M. Donné, of Paris. It is constructed in such a way that the milk may be examined in it in layers of every thickness; from the thinnest, through which all objects are visible, up to that which allows of nothing to be perceived, it gives at once the richness of the milk, by indicating the degree of opacity, to which the proportion of cream stands in relation. Determination of the cheese, sugar, and saline matters.— This can be effected by weighing the residue left, when the precipitate obtained in milk with acetic acid is treated with ether to ascertain the amount of fatty matters. То determine the sugar, the whey or mother-liquor left after the separation of the casein and fats is evaporated to dryness; the residue weighed, and treated with hot alcohol, will lose an amount of weight equal to that of the sugar it contained. Dr. Hassall proposes to determine the sugar in whey by volumetric measurement with the sulphate of copper and tartrate of potash solution. For the manipulations necessary to perform this experiment, see page 121. The saline or inorganic salts are easily determined by burning the residue, from a known quantity of milk, and ascertaining the weight of the ashes. Adulterations of milk.-Milk may be adulterated with water, flour or starch, milk of almonds, gum, gum tragacanth, chalk, turmeric, sugar, and cerebral matter; it is occasionally mixed with a little carbonate of soda. Finally, it has been found to contain zinc when stored in zinc pans. 1. Water, though a very frequent adulteration, is difficult to detect, the results obtained with the hydrometer (see page 148) being not altogether satisfactory. The general density of pure milk is about 1038; a mixture of 75 parts milk and 25 parts water has a density of 1021; 66 parts of milk mixed with 33 of water has a density of 1020. In some cases, not only is water added, but cream abstracted. The best method of proceeding to detect this adulteration would be to determine first the amount of cream, and then take the specific gravity of the skimmed milk with the apparatus described above (see page 148). A collection of specific gravity beads would be extremely convenient for this purpose. 2. Flour and starch occur in milk as small diaphanous clots. The iodine test (see page 8) will suffice to indicate at once this adulteration; but the milk must be boiled previous to the addition of the tincture of iodine or iodide of potassium. 3. Milk of almonds.—In this case, the addition of a few grains of amygdaline will suffice to develop after a few minutes the odour of bitter almonds. 4. Gum is seldom employed. To detect it, coagulate the milk with a little acetic acid, filter off the whey, and into it a small quantity of alcohol; a dull, opaque, and pour |